Category Archives: Evolution

A reconstruction of Calvapilosa, showing what this primitive mollusc most likely looked like in real life. Photograph: Jakob Vinther/Model made by Esben Horn (10tons.dk)

Lumpy, hairy and with a nail-like horny patch it sounds like a hobbits toe. In fact, its a portrait of what researchers say the common ancestor of slugs, snails and squid might have looked like.

The surmise is based on the discovery of the fossilised remains of a mollusc, thought to have lived about 480 million years ago, which has short spines all over its body and fingernail-like shell over its head which housed a radula a tongue-like structure found only in molluscs with more than 125 rows of teeth.

Believed to be a very early ancestor of a group of marine molluscs known as chitons, the discovery, scientists say, suggests that the common ancestor of all molluscs likely had a similar appearance.

I would say that our animal probably is very close to the spitting image of how the ancestor of all molluscs must have looked like 530 million years ago, said Jakob Vinther, a molecular palaeobiologist from the University of Bristol and co-author of the research published in the journal Nature.

The newly discovered animal is believed to have reached up to 12cm in length although the juvenile found in the Yale collection is less than 2cm long. Its name, Calvapilosa kroegeri, is a reference to the hairy shell that covered its head together with a nod to Bjrn Krger. The palaeontologist spotted a complete version of the fossilised creature among a drawer of recently collected Moroccan rocks at Yale University almost a decade after the first incomplete fossil was found.

We had been looking through those drawers to try and see if there were any specimens and we missed it, said Vinther. [Then Krger said] Why dont you guys use this specimen it is entirely complete, and then he pulled this thing out and it was like dude, that is totally what we needed!

The discovery also sheds light on a previously discovered fossils, revealing that a number of older creatures whose nature had been debated due to a lack of preserved details were also molluscs, due to their similarity in structure to the newly discovered creature. We could bring all these other fossils into the fold of thinking [about] molluscan evolution, said Vinther.

It also reveals that an type of early animal with two shell-like plates, known as Halkieria, was also a mollusc. Despite Halkeria being older, the authors suggest that the number of plates grew during evolution, leading to modern day chitons, which bear a row of eight plates on their back. Basically our animal sits right at the base of the branch that leads to chitons, said Vinther.

With a very early non-molluscan creature called Wiwaxia known to have had scales and spikes, the researchers go further, proposing an evolutionary path in which the common ancestor of all molluscs bore spines, a single plate, and a radula before a variety of branches emerged, eventually giving rise to molluscs as diverse as snails, clams and slugs.

Martin Smith, an invertebrate palaeontologist at the University of Durham who was not involved in the research, described the new find as exciting. This is a really important fossil, he said. Theres been a lot of discussion about the common ancestor of molluscs and of course there is such a wide diversity of body plans of molluscs today ranging from squids to snails to slugs and various other things that it is very hard to work out what their common ancestor looked like.

While it has previously been suggested that the common ancestor was shell-less, the new study, says Smith, points towards a single shell and a radula forming part of the body plan of molluscs, which have since been lost, modified or multiplied in various branches over the course of evolution. It completely transforms how we see the earliest history of molluscs and how we read the fossil record, said Smith of the new find.

But Julia Sigwart of Queens University Belfast, cautioned against such an interpretation, saying that even at 480 million years old, the newly-discovered fossil is too young to draw conclusions about what the common ancestor of all molluscs would have looked like.

This is not a particularly old fossil in the context of molluscan evolution, she said. But, she added, the fossil does show how many different forms existed through the history of molluscs over the last half billion years. Any time we find these exceptionally preserved fossils, they are very important for us to understand what the body plans looked like, because the fossils are so rare.

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Lumpy, hairy, toe-like fossil could reveal the evolution of molluscs - The Guardian

This week, Feb. 6 10, The University of Southern Mississippi will host a series of activities in honor of Charles Darwins birthday.

Many universities and institutions will be celebrating Darwin Day on Feb. 12, which is a day to promote science education and, in particular, Darwins contributions to biology. Southern Miss has extended its day-long celebration to a week. There will be a fossil hunt, lectures, video screenings, socials, a panel discussion and a keynote address through the University Forum by famous paleontologist Neil Shubin, a professor at the University of Chicago and author of Your Inner Fish.

Some of you may be offended that we are having a celebration of Darwin. In fact, in just the last two months, Ive been bombarded on Facebook by accusations of undermining the religious faith of our students by teaching a theory based on little fact with the ultimate desire of implementing a liberal agenda and atheism in our society. That is far from the truth I dont try to undermine anyones faith, Im fairly moderate politically and Im not even an atheist! I would like to challenge our students, particularly Christians who may feel that evolution has implications for their religious faith, to consider a few key points and then join us for some of the activities this week.

First, we are celebrating Darwin because his ideas have had a substantial impact in biology and even in the other sciences. His work did not just resolve the question of whether species were individually created about 6,000 years ago. Rather, his work has impacted science at many levels: Antibiotic resistance, relatedness and migration of human populations, lactose tolerance, emerging diseases and vaccine production, pesticide resistance and even forensic science and software engineering.

Evolution has impacted public policy, too, from fishing regulations to control of invasive species and conservation biology. Evolution is a broad explanation that includes more than just biology. If you reject evolution, you are also rejecting much of geology, chemistry and anthropology. In other words, you are rejecting science.

Second, there are plenty of religious reasons why one can believe in evolution in addition to the overwhelming scientific evidence in its favor. Even St. Augustine, who lived from 354 430 A.D., argued long before Darwin that a literal reading of Genesis was inappropriate. There are plenty of nonliteral texts in the Bible, and one has to determine the intent of those passages based on context and language. Given the words used and the different versions of creation, a literal reading of Genesis 1 2 is not faithful to the text itself.

For example, the Hebrew word adam can also refer to man or mankind, and the passages talk about how God speaks to the waters to bring forth certain kinds of life and speaks to the land to bring forth other kinds of life, poetically paralleling evolution. Man even comes from the dust of the ground. Many major Christian denominations today (e.g., Roman Catholics, Episcopalians, Lutherans, Methodists) have stated that evolution and Christian religious faith are not mutually exclusive. Even the man who gave us the famous quote, Nothing in biology makes sense except in the light of evolution, Theodosius Dobzhansky, was actually a devout Russian Orthodox Christian.

Finally, many people even the loosely religious think that evolution is lacking (or outright wrong) because it seems improbable and reduces us humans to the outcome of chance. Many scientists have actually made this problem worse, because they use words like blind, purposeless and undirected.

Well, science doesnt deal with purpose in this sense (we dont think of stochastic events as either blind or seeing, and chance doesnt do anything), and the use of those words reflects more of the materialistic worldview of the authors. However, we do know that mutations are random with respect to natural selection; two men won the Nobel Prize for demonstrating that in 1969. A Christian, except for those who dont believe in (ontological) chance at all, need not worry: There are plenty of beautiful examples of where randomness or unpredictability at one level is deterministic at another level.

When you flip a (fair) coin, you cant be sure whether it will land on heads or tails. However, if you flip one million coins, you can be quite confident that very close to 50 percent will land on heads and 50 percent will land on tails. The opposite is true, too, where deterministic dynamics at one level lead to unpredictable behavior at another. Famous examples of this are often called chaos, where the tiniest of changes in a predictable framework result in unpredictable patterns. You may have heard of the butterfly effect; a butterfly flapping its wings in Brazil can lead to a tornado in Texas.

Evolution is a fascinating subject. Dont be afraid of it ( reflexive hostility, as Kenneth Miller calls it), and be willing to challenge your (or your pastors or your parents) deep reservations about it, because you want to live a life of integrity, a life of unity, where your faith whatever it be and your knowledge fit together seamlessly. When I teach evolution, I give students a survey at the beginning of the course. One of the questions I ask is, What is the first word that comes to mind when you hear the word evolution? When I first began teaching evolution at USM about 10 years ago, common answers were controversy and religion. These days, the most common answer is dinosaurs. Thats a great sign.

For those with doubts, I have several recommendations. For a book that clearly lays out the scientific evidence for evolution but that also deals with religious (particularly Christian) issues, I highly recommend Kenneth Millers book Finding Darwins God: A Scientists Search for Common Ground between God and Evolution (1999). If youre interested in the improbability, I suggest David Bartholomews God, Chance and Purpose: Can God Have It Both Ways? (2008) and (the non- and even perhaps anti-religious) book by David Hand, The Improbability Principle: Why Coincidences, Miracles, and Rare Events Happen Every Day (2015). All of these books are available in the USM library.

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USM Darwin Day: 'Genesis' a parallel to evolution - The Student Printz

Ivanka Trump 'Feels Terrible' for Insensitive Gala Photo Nordstrom Drops Ivanka Trump's Line Ivanka Trumps Baby Theodore Crawls for First Time in the White House

If we could turn back time actually, we have! Ivanka Trump, the newly minted first daughter of President Donald Trump, has been in the limelight her whole life, which means we've watched her grow up. See her beauty evolution in the video above!

From a rosy-cheeked teen with a rosebud mouth (circa 1998) to a platinum-blonde student and model in the early aughts, to a sleek entrepreneur with a glamorous but understated makeup palette, the former Trump Organization VP has transformed right before our eyes.

Today, Ivanka, who first launched her eponymous brand in 2007 with fine jewelry, is now a mom of three children (with fellow real estate mogul husband Jared Kushner), but she's just as polished as ever.

"I keep my makeup minimal at the office, but that's also because I like to spend my time with my children in the mornings, and that tends to come at the expense of doing my makeup," Ivanka told Who What Wear in 2014. "That being said, I think that bright lipstick can work well, as long as the rest of your makeup is minimal."

Words she clearly lives by! Watch Ivanka's beauty transformation now.

Be a trendsetter! Sign up now for the Stylish by Us Weekly newsletters to get celeb fashion, beauty tips, and more delivered directly to your inbox.

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Ivanka Trump's Beauty Evolution, From 1998 to Today Watch - Us Weekly

Inside Higher Ed

Tracking the Evolution of Student Success Inside Higher Ed College administrators in the field of student success who feel as though their jobs are getting more hectic each day aren't imagining things, according to the EAB. Researchers at the EAB marked the Washington, D.C., based research and consulting firm ...

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Tracking the Evolution of Student Success - Inside Higher Ed

Evolution is the first evolutionin the Pokmon franchisewhen one Pokmon, upon reaching a certain level, using a certain stone, learning a certain move, orbeing traded, evolves into a different kind ofPokmon. In Pokmon Gold, Silver, Crystal, HeartGold and SoulSilver games, it is stated that Professor Elm is an expert on evolution, and discovered that Pikachu evolves from Pichu.

In the anime, during evolution, a Pokmon will become surrounded by a light and slowly change shape. In the Original, Advanced, and Diamond & Pearl series, the Pokmon is surrounded by a white light, while in the Best Wishes and XY series, the Pokmon is surrounded by a golden or blue light.

To some Pokmon, evolution means growing up, while to others, it just refers becoming another species or getting upgraded. However, many of them retain memories during their pre-evolution form.

Pokmon gain experience after battling wild Pokmon and Pokmon Trainers. The more experience a Pokmon gains, the more it levels up. When a Pokmon reaches a particular level, it will evolve into its next form (if it has one). This is very helpful to most Trainers who want their Pokmon to become stronger.

There are ways to share experience, such as allow a Pokmon you wish it to evolve to hold Exp. Share. It is a hold item that allows the user to earn experience even if it did not participate in the battle. This is a convenient tool if you want to level up a new or low-level Pokmon.

You can prevent evolution by simply press the 'B' button on the Game when the Pokmon is attempting to evolve. This is useful as some Pokmon will learn moves that cannot be learnt in a later form or if you want a Pokmon to learn a move earlier than usual. For example, Gabite. If you let Gible evolve into Gabite at level 24 it will learn Dragon Rush at level 49, but if you keep Gible it will learn Dragon Rush at level 37. Sometimes it doesn't matter if you let it evolve or not. The Pokmon can hold an Everstone, which keeps them from evolving, so that you don't have to keep cancelling the evolution every time the Pokmon levels up. However, if your Pokmon has evolved, it might not evolve again (Raticate, Linoone, etc.) Although some Pokmon can evolve more than once (Poliwhirl, Cascoon, etc.), some basic Pokmon just can't evolve, due to undiscovered forms, or if they are really rare (Chatot, Groudon, etc.) There are also Pokmon that can evolve into different things like love, choice, etc. For example, Eevee, Poliwhirl, Wurmple, or Kirlia.

Some Pokmon will not evolve unless you use a special stone called an Evolution Stone. These special Items are linked to the Pokmon's type. Here are a list of Pokmon that can evolve by giving them the stones.

Some items are needed for a Pokmon to evolve. If a Pokmon hold the item allowing it to evolve, let it level up once and it will then evolve into the second form.

A small group of Pokmon refuses to evolve, no matter if you are at the appropriate level to trigger the evolve process, unless you trade them with a friend. Some Pokmon need to have a held item to evolve when trading with a friend. Another group require specific Pokmon to be trade in order to evolve.

Small groups of Pokmon requires a gender to evolve into the next stage.

Certain Pokmon, especially baby Pokmon, requires friendship with its trainer and if they are fond enough, they are willing to evolve. Some Pokmon only evolves in a certain time of the day with friendship.

Special Pokmon evolve at a certain area of location.

Certain Pokmon need to know a move to evolve. This method was introduced in Generation IV, starting with Diamond/Pearl/Platinum.

The current time of day will sometimes affect evolution. This method was introduced in Generation VI, starting with X/Y/OR/AS.

Some Pokmon have certain type of unique way of evolve.

Certain Pokmon will be able to evolve into a Mega form, the final form for one-evolution families and second/true final form for two-evolution families with the use of a Mega Stone, a held object. However this evolution will devolve back into their previous form at the end of the battle.

There's a special evolution that Ash's Greninja can do due to his strong bond with Ash called Ash-Greninja, that was later described as Bond Phenomenon.

Primal Reversion is a similar state to Mega Evolution, but the Pokmon devolve to a previous state in the past.

It is a possible game mechanic in Pokmon Omega Ruby & Alpha Sapphire, most likely because this mechanic was founded during development of Pokmon Omega Ruby & Alpha Sapphire.

Devolution is a form of reverse evolution when a Pokemon reverts to a previous state. Devolution does not exist in the games but is common in the Manga and TCG. Devolution is achieved mainly unnaturally - through experimentation, or Devolution Spray. Though certain Pokemon, such as Isamu's Clefairy and Pikachu are capable of devolving themselves. The TCG also has it that Eeveelutions can revert into Eevee and that Mew has twospecial attacks named the Devolution Beam and Devo Crush - both of which which devolves the Pokemon hit by the attack. Golurk and Jirachi also possess special attacks which allow them to devolve an opposing Pokemon.

In the Anime Episode, Electric Shock Showdown, Misty points out that once Pikachu evolves with the use of the Thunder Stone, Pikachu won't revert back.

In An Epic Defense Force, Luke's Golett seemingly evolves to Golurk in a Movie then Devolves back to Golett, repairing the Golurk Statue, as it turns out to be fictitious.

Another instance of the non-existence of Devolution, is when Bonnie requested Ash's Frogadier to devolve back to Froakie, Clemont said that it would be impossible.

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Evolution | Pokmon Wiki | Fandom powered by Wikia

There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.

Evolution refers to change in a biological population's inherited traits from generation to generation. Some believe that: all species on Earth originated by the mechanism of evolution, through descent from common ancestors. Evolution occurs as changes accumulate over generations. Charles Darwin recognized evolution by natural selection, also called "descent with modification", as the fundamental process underlying all of life, whether viewed at a large scale above the level of species (macroevolution), in terms of formation of new species, changes within lineages, and extinction, or at a small scale within a species (microevolution), in terms of change in gene frequency. (Keep in mind that this entire page is based on the idea that something came from nothing. If any of this were true, we would be continually shifting in shape and "breed". Humans were created separately when life began. (Genesis 1.)) In a nutshell, evolution by natural selection can be simplified to the following principles:

In modern genetic terminology, variability of traits in a population is the expression (phenotype) of heritable traits (genes), which at least on Earth are stored in DNA (or sometimes RNA or proteins). Variability of traits ultimately originates from mutation, and new combinations of genes are continually produced via recombination as part of sexual reproduction. The result of natural selection is adaptation, like a "hand in glove" fit between organism and environment. Evolution, defined in population genetics as change in gene frequency in a population, can be influenced by other processes besides natural selection, including genetic drift (random changes, especially in small populations) and gene flow (wherein new genes come into a population from other populations). In a sense, mutation is a creative process of expansion in which new possibilities come into existence (most of which don't work so well), and this is balanced by natural selection, another creative process of contraction that reduces the possibilities to those that work best in a particular environment.

It sure felt good when Earth was flat, Earth was the center of the universe, Earth was only about 6,000 years old, life on Earth originated as the handiwork of a supernatural creator, and when species were fixed and didn't evolve. And it felt really good when humans were special, created separately from other life on Earth, not descended from a common ancestor. So what if understanding evolution revolutionized our entire worldview. Reality is overrated.

The word evolution (from the Latin e, meaning "from, out of," and volvo, "to roll," thus "to unroll [like a scroll]") was initially used in 1662, and was variously used, including with respect to physical movement, describing tactical wheeling maneuvers for realignment of troops or ships. In medicine, mathematics, and general writing early use of the term referred to growth and development within individuals.[2][3]; its first use in relation to biological change over generations came in 1762, when Charles Bonnet used it for his now outdated concept of "pre-formation", in which females carried a miniature form (homunculus) of all future generations. The term gradually gained more general meaning of progressive change. In 1832 Scottish geologist Charles Lyell referred to gradual change over long periods of time. Charles Darwin only used the word in print once, in the closing paragraph of The Origin of Species (1859), and rather favored the phrases "transmutation by means of natural selection" and "descent with modification". In the subsequent modern synthesis of evolution, Julian Huxley and others adopted the term, which thereby became the accepted technical term used by scientists.[4][5]Although in contemporary usage the term "evolution" most commonly refers to biological evolution, usage has evolved, and the word also refers more generally to "accumulation of change", including in many disciplines besides biology.

The idea that life has evolved over time is not a recent one, and Charles Darwin did not, in fact, come up with the idea of evolution in general. For example, ancient Greek philosophers, like Aristotle, had ideas about biological development.[6] Later, in Medieval times, Augustine used evolution as a basis for the philosophy of history.[6]

The first significant step in the theory of evolution was made by Carl Linnaeus.[7] His leading contribution to science was his creation of the binomial system of nomenclature in lay terms, the two-part name given to species, such as Homo sapiens for humans. He, like other biologists of his time, believed in the fixity of the species, and in the scala naturae, or the scale of life. His ideas were consistent with the Judeo-Christian teachings of his time.

Erasmus Darwin, the grandfather of Charles Darwin, was the first scientist to whom credit can be given for something starting to approach modern concepts of evolution, as noted in his contributions to botany and zoology. His writings contained many comments (mostly in footnotes and side writings) that suggested his beliefs in common descent. He concluded that vestigial organs (such as the appendix in humans) are leftovers from previous generations. The elder Darwin, however, offered no mechanism by which he believed evolution could occur.

Georges Cuvier proposed a mechanism by which the fossil record could develop over time without evolution - which by now had come into usage as a term.[8] His hypothesis, catastrophism, was that a series of disasters destroy all life within a limited area, and that living organisms move in to this newly opened area. This idea prefigures in some respects the 1970s hypothesis of 'punctuated equilibrium'.

Lamarck was the first scientist to whom credit can be given for a theory of evolution.[9] His idea centered on use and disuse, the concept being that the more an organism used a particular part of its body, the more developed that organ became within a species. It is sound only for individuals (e.g. a weightlifter will develop larger muscles over time, but will not pass this trait on to any children.) Nevertheless, modern research into epigenetics suggests that parents can induce some traits into their offspring by non-genetic inheritance, and that Lamarck was therefore not completely wrong.

By the first half of the 19th century, scientists had gathered a great deal of information on species, and had inferred that life on Earth had existed for a very long time, and that some species had become extinct.[10] Natural selection was the first theory to provide a mechanism to explain those observations. Prior to the theory of natural selection, the concept that species could change over time had been proposed, but without a satisfactory explanation.[Who?]Alfred Russel Wallace and Charles Darwin came to the conclusion, independently, that competition for resources and the struggle for survival helped determine which changes became permanent and which traits were discarded.

The theory of evolution by natural selection, as we know it today, was published in a joint paper by Wallace and Darwin on 20 August, 1858, based on Wallace's observations in the Malay Archipelago and Darwin's observations over many years including those made during his voyage on HMS Beagle. Charles Lyell's Principles of Geology, which suggested slow changes over very long periods of time, also contributed to the nascent theory.[11] Darwin drew heavily on his knowledge of human experience in breeding domestic animals (artificial selection), particularly the varieties produced by pigeon breeders (Darwin was one himself), for his understanding of how variations could develop within a population over time. Darwin set out his theory (at the time, a hypothesis) of natural selection in his books On the Origin of Species and The Descent of Man.[12]

For more information, see Non-Darwinian evolution.

Although natural selection was the first mechanism proposed in evolutionary theory (and remains the most common), other forms of selection play a part as well. The most notable of these is sexual selection, which occurs due to some heritable preference for a trait in breeding partners. Derivation of traits through this mechanism is driven by (usually) the female's choice in mating partner rather than direct impact on fitness. Sexual selection often leads to the rise of features which would likely not occur under natural selection, such as the tail of a peacock or the long necks of giraffes.[13]

It should be noted that sexual selection can be divided into two forms, distinguishable by who actually "makes" mating decisions. The first of these is intersexual selection, and in this form of selection the limiting sex (which is usually female) will choose a partner. The other form is intrasexual selection, or mate competition. In this form of selection, one sex (usually males) competes for "mating rights" to members of the other sex.

In addition to selection, other mechanisms have been proposed, most notably genetic drift. More controversial is the importance of symbiosis (which has been recognized in the case of the origins of eukaryotes). Universally rejected is Lamarckism or directed (rather than random) variations.

The eclipse of Darwinism is a phrase to describe the state of affairs prior to the modern synthesis when evolution was widely accepted in scientific circles but relatively few biologists believed that natural selection was its primary mechanism. Instead non-Darwinian mechanisms of evolution such as neo-Lamarckism, saltationism, or orthogenesis were advocated. These mechanisms were included in most textbooks until the 1930's but were rejected by the neo-Darwinian synthesis theorists in the 1940's as evidence had proven the role of natural selection in evolution.[14]

The modern evolutionary synthesis is a union of ideas from several biological specialties, which attempts to explain how evolution proceeds. It has been accepted by many scientists. It is also referred to as the new synthesis, the evolutionary synthesis, the neo-Darwinian synthesis, or the synthetic theory of evolution. The synthesis was produced between 1936 and 1947 due to the reconciliation of Mendelian genetics with natural selection into a gradual framework of evolution. The synthesis of Darwinian natural selection (1859) and Mendelian inheritance (1865) is the cornerstone of neo-Darwinism.[15]

Julian Huxley (1887 1975) invented the term, when he produced his book, Evolution: The Modern Synthesis (1942). Other major figures in the modern synthesis included R. A. Fisher (1890 - 1962), Theodosius Dobzhansky (1900 - 1975), Ernst Mayr (1904 - 2005), George Gaylord Simpson (1902 1984), and G. Ledyard Stebbins (1906 - 2000).

Over the past decade, new conceptions of evolutionary theory have emerged going under the umbrella term of the "Extended Synthesis," which is intended to modify the existing Modern Synthesis. This proposed extended synthesis incorporates new possibilities for integration and expansion in evolutionary theory, such as Evo-devo, Epigenetic Inheritance and Niche Construction. Its proponents include Massimo Pigliucci, Gerd Mller, and Eva Jablonka.[16] In 2008 sixteen scientists met at the Konrad Lorenz Institute in Altenberg, Austria, to propose an extended synthesis.[17]

Evolutionary theory has at its core three main tenets, observations of patterns within nature. These three patterns were observed by both Darwin and Wallace, and they eventually gave rise to the modern theory of evolution by natural selection.[18]

Darwin and Wallace both noted that populations display natural variability in form, physiology, and behaviour (phenotypic variability). For example, within a population, some members may be very large, some may be very small, and most may be somewhere in the middle. This natural variability is the fundamental source upon which natural selection acts.

Having observed that natural variability exists, early evolutionary biologists also noted that some of these variants endowed their possessor with some competitive edge over other members of the species, conferring greater survival or reproduction. Although at first the implications of this fact were unclear, the writings of Thomas Malthus spurred Darwin and Wallace to recognize that individuals that have traits that enhance their ability to survive and reproduce pass on these traits to subsequent generations. Differential fitness, also known as differential reproductive success, in essence, is the process by which traits that enhance survival and reproduction gain greater representation in subsequent generations.

Only if variation is heritable, will it confer an advantage into future generations. Although early evolutionary scientists did not have the benefit of modern molecular tools, they surmised that the source of variation must in part have a heritable basis, in contrast with variation expressed solely in response to different environmental conditions. In fact, one of the first predictions made by evolutionary theory was the existence of a heritable factor, now known to be DNA!

Thus the combination of phenotypic variability, differential fitness, and heritability of fitness define evolution by natural selection. Darwin and Wallace independently came to the conclusion that those organisms best suited to their environment would survive to produce more offspring. Therefore, the heritable factor responsible would increase in frequency within the population.[19]

Evolutionary biology seeks to explain the following three broad patterns observable in all life.

Diversity is fundamental to life at all levels of organization: ecosystems, communities, species, populations, individuals, organs, and molecules.

According to the Genetic Variation Program arm of the National Human Genome Research Institute, about 99.5% of human DNA is the same from person to person. The other 0.5% accounts for a number of simple and complex traits we possess.[20] There is tremendous genetic diversity within almost all species, including humans. No two individuals have an identical DNA sequence, with the exception of identical twins or clones. This genetic variation contributes to phenotypic variation - that is, diversity in the outward appearance and behavior of individuals of the same species.

Populations must adapt to their environment to survive.

Living organisms have morphological, biochemical, and behavioral features that make them well adapted for life in the environments in which they are usually found. For example, consider the hollow bones and feathers of birds that enable them to fly, or the cryptic coloration that allows many organisms to hide from their predators or prey. These features may give the superficial appearance that organisms were designed by a creator (or engineer) to live in a particular environment. Evolutionary biology has demonstrated that adaptations arise through selection acting on a population through genetic variation.

Species evolved along different paths from a common ancestor.

All living species differ from one another. In some cases, these differences are subtle, while in other cases the differences are dramatic. Carl Linnaeus (1707-1778) proposed a classification that is still used today with slight changes. In the modern scheme, related species are grouped into genera, related genera into families, and so on. This hierarchical pattern of relationship produces a tree-like pattern, which implies a process of splitting and divergence from a common ancestor. While Linnaeus classified species using similar physical characteristics, modern evolutionary biologists also base classification on DNA analysis, which can distinguish between superficial resemblances between species and those which are due to common ancestry.

Biological evolution results from changes over time in the genetic constitution of species. The accumulation of genetic variations often, but not always, produces noticeable changes in the appearance or behavior of organisms. Evolution requires both the production of variation and the spread of some variants that replace others.[21]

Genetic variation arises through two processes, mutation and recombination. Mutation occurs when DNA is imperfectly copied during replication, or by changes in genetic material caused by such mutagens as radiation, leading to a difference between a parent's gene and that of its offspring. Some mutations affect only one bit in the DNA; others produce rearrangements of, or changes in, large blocks of DNA.

Recombination occurs when genes from two parents are shuffled to produce an offspring, as happens in every instance of sexual reproduction. Usually the two parents belong to the same species, but sometimes (especially in bacteria) genes move between more distantly related organisms.

The fate of any particular genetic variant depends on two processes, drift and selection. Drift refers to random fluctuations in gene frequency, and its effects are usually seen at the level of DNA. Ten flips of a coin do not always (or even usually) produce exactly five heads and five tails; drift refers to the same statistical issue applied to the transmission of genetic variants across generations. Genetic drift is inverse to population size; that is, genetic drift has a greater effect on small populations than larger ones. For example, if a small part of a population becomes geographically isolated its members will develop new traits faster.

The principle of natural selection was discovered by Charles Darwin (1809-1882), and it is the process by which organisms become adapted to their environments. Selection occurs when some individual organisms have genes that encode physical or behavioral features that allow them to better harvest resources, avoid predators, reproduce successfully, and so forth, relative to other individuals that do not carry those genes. The individuals that have more useful (adaptive) features will tend to leave more offspring than other individuals, so the responsible genes will become more common over time, leading the population as a whole to become better adapted.

Through a variety of mechanisms, gene duplication can occur which gives rise to two identical genes in the genome. Since only one of these genes is necessary, the other gene can undergo mutations without having an adverse effect on the original function of the gene. These duplicated genes called paralogs can give rise to protein families with similar yet distinctly different functions. For example, the olfactory protein family consists of around 900 different smell receptors that all arose via gene duplication followed by unimpeded mutation.

The process that many people find most confusing about evolution is speciation, which is not a separate mechanism at all, but rather a consequence of the preceding mechanisms played out in time and space. Speciation occurs when a population changes sufficiently over time that it becomes convenient to refer to the early and late forms by different names. Speciation also occurs when one population splits into two distinct forms that can no longer interbreed. Reproductive isolation does not generally happen in one generation; it may require many thousands of generations when, for example, one part of a population becomes geographically separated from the rest and adapts to a new environment. Given time, it is inevitable that two populations that live apart will diverge by mutation, drift, and selection until eventually their genes are no longer compatible for successful reproduction.

Working alongside with natural selection (death and survival pressure), spatial evolution is caused by individuals with random variation that are selected nonrandomly by how fast they travel away from home populations. The faster the individuals, the faster the individual she or he mates with, leading to fast offspring. This is both behavioral and morphological. The individuals 'race' their way to become a distinct species. Examples of Spatial evolution are new. For example, Australian researchers have detailed a new mechanism of evolution that is not based on natural selection but rather on how populations of organisms, such as cane toads, move around.[22][23]

Common descent explains the many shared features (homologies) of the majority of the organisms on the planet. There is an enormous amount of evidence that suggests all living organisms derived from a common ancestor long ago. For instance, all vertebrate embryos have the same body plan and look very similar in early development. We have the genetic code, which is all but identical in every known organism, from bacteria to humans. We have the shared presence of pseudogenes in similar species. All simians, including us humans, have an inactive gene, L-gulonolactone oxidase, which was originally used to synthesize Vitamin C. Then, we have the evidence for convergence, which explains relationships for all species, from fungal slime you find in shower stalls to sequoia. The tree of life between simple anatomical similarities is strikingly similar to a tree constructed from genetic molecular similarities. Then, there are others, including cool stuff like chromosome fusion, endosymbotic theory, retroviruses, Hox genes, and deep homology, oh my.

Considering all of this, evolution has the intricacy and the reality of quantum mechanics. But you don't see unqualified people running around and decrying quantum mechanics, do you? Well actually you do, but opposition to quantum mechanics is widely considered fringe kookery, while opposition to evolution is treated by many people as a reasonable position.

So yes, in other words, evolution is a theory.

Evolutionary concepts can also be applied to non-biological processes. Universe formation, evolutionary algorithms in computer science and the development of languages are three such subjects. The study of etymology is one component of analyzing how languages have evolved, and parallels biological evolution (for example) in the way the same language diverges over time into two different languages when two populations that speak the same language become geographically isolated.

Another example of non-biological evolution is the evolution of technology and innovation, which, while being (mostly) intelligently-designed,[24] is (mostly) not random. James Burke studied, authored books, and hosted television programmes on the evolution of technology through a historical context.

Models of cultural evolution, such as memetics, have been devised and applied over the years with varying degrees of success.[25]

Somewhat confusingly, the word "evolution" is also used in some sciences in a way that has no relation to the biological concept whatsoever. When an astronomer speaks of "stellar evolution", (s)he is taking about the changes that happen to a star over very long periods of time, as it progresses from gas cloud to protostar to main sequence star to post-main-sequence giant to stellar remnant. When a cosmologist speaks of "cosmic evolution", (s)he is talking about the changes in the size/shape/nature of the universe over time, sometimes on very long time scales, and sometimes at very brief time scales (such as fractions of a second after the Big Bang). Neither of these uses of the word "evolution" has anything to do with populations, heritable traits, selection criteria, descent, or any of the other hallmarks of "evolution" as the term is used in biology.

Creationists consequently confuse the biological and non-biological meanings of the word "evolution" and they claim that the Theory of Evolution includes the origin of the universe and the origin of life. The biological theory of evolution as proposed by Darwin and others has nothing to say about either the origin of the universe or the origin of life on Earth, though some biologists have extended the theory to the very beginning of life.[26]

There are a number of broad arguments creationists/anti-evolutionists make. Specific claims are examined at our common descent page. They're mostly arguments born of a lack of understanding what evolution by mutation and natural selection actually is, though rarely they're advanced by more savvy creationists as direct misrepresentations and distortions of the theory of evolution.

Often creationists ask how likely it is that all this complex life could have come about by random chance. They suggest that since individual events, such as the abiogenetic formation of proteins, emergence of RNA, organization of unicellular into multicellular organisms, etc., are purportedly so highly improbable that the entire chain events culminating in the existence of even a single complex organism could not have happened as described. Therefore, God did it. As creationism is largely a program of negative apologetics (e.g. an attempt to show a claim that is viewed as contrary to Christian faith is internally inconsistent or irrational according to the Christian perspective), arguments such as this are in essence arguments from incredulity with the proponent denying a fact (in this case the statistical probability that such and such essential event will have occurred) in order to draw the unsupported conclusion that some other cause (the Christian God) was at work.

The implied argument that a god or "designer" was at work is itself fraught with more untenable problems. Putting aside that the illusion of design is itself problematic, and assuming for the sake of argument that "design" is even identifiable in biological systems, if "random chance" is inadequate to account for some outcome, one is simply making unsupported assertions to contend that it is more probable that a designer was at work. If the causes are "designers" about which nothing is known, if they are capable of doing anything, if it is not known how or why they act, if it is not known when they acted (or will act), or if it is not known what they did (or did not, or could, or would), the causes are not enough to account for the results. If so, "design" in this sense is indistinguishable from random chance.

Nonetheless, evolution by natural selection isn't a random process. While genetic mutations may appear randomly, the natural selection of specific traits to produce a statistically significant allele (gene variation) frequency in a discrete population of organisms is highly deterministic. If a gene aids survival with respect to any particular environmental stressor, then it is selected by means of the survival and reproduction of the individuals carrying that gene and perpetuates in the population of organisms. If the trait is detrimental to survival, it will leave organisms vulnerable to a particular environmental stressor and through attrition lower the frequency of the allele(s) contributing to that trait in the subject population.

Many creationists hold erroneous beliefs about evolution such as that which is expressed by the statement "I accept microevolution, but not macroevolution." (This is the position of YEC nincompoop Kent Hovind.) Microevolution is supposed to be evolution that doesn't result in a new species, and macroevolution is supposed to be evolution that does lead to a new species. This argument is akin to someone saying that while one believes that wind can sometimes erode rock, one doesn't believe it can change the rock's shape. Micro- and macroevolution describe the same process, but with a difference in operational time. If one accepts microevolution, they must also accept macroevolution, since the former inevitably leads to the latter if given a long enough time period and the separation of breeding isolates. One cannot simply accept one and not the other. In biology, macroevolution is a broad subject of which speciation is only one part. This argument against speciation may be an attempt by creationists to reserve the power to produce a species for God alone.

Some creationists have abandoned the attempt to deny that new species can appear (and disappear) by natural means, in favor of drawing a barrier, not between species, but between baramins (also known as "kinds"), some sort of collection larger than species. To date, there has not been given any indication of just what sort of a thing a baramin is, what is the nature of the barrier between baramins, or how one might detect the barrier (or suspect its non-existence) in any particular case, other than the uninformative "baramins are those things that present a barrier to evolution."

Irreducible complexity is a fancy name for the "watchmaker" argument. In a nutshell, irreducible complexity describes an organ (or other facet of a living thing) which the ideology's supporters claim could not have evolved in small gradual steps. It is claimed to be so complex that it cannot be reduced into other parts. In fact, every example of irreducible complexity Behe and others have come up with has been shown to not be irreducibly complex (for example, the incremental stages towards the "irreducibly complex" human eye that are found in the sight organs of other living organisms).[27]

For any theory to be accepted as scientific it must be falsifiable. In other words, it must be capable of making statements which could theoretically be disproved. Evolution's opponents claim that the theory of evolution does not have this property, although this claim can be easily rejected. Theoretically, evolution could be falsified if scientists discovered an organism so complex and unique, with absolutely no explainable path as to how it could have evolved. Such an organism has not been found. Similarlyand ironicallythere are the demands made by some creationists that they be shown, say, a dog giving birth to a cat before they'll accept evolution. Such an event, if it occurred, would falsify (or at least strongly challenge) evolution, since speciation doesn't happen in a single generation and modern animals don't evolve into other modern animals.

Sometimes the phrase "evolution is only a theory" will be heard. This phrase rests on the common use of "theory" to mean what scientists call a "hypothesis," i.e., is something that is possible but not proven. Science, however, uses "theory" in a much different sense, namely as a testable model of the manner of interaction of a set of natural phenomena, capable of predicting future occurrences or observations of the same kind, and capable of being tested through experiment or observation. This sets it at a significantly higher level of reasoning than "wild and unproven guess," which is what is implied when this argument is mentioned. Also unlike "wild guesses", scientific theory is among the best explanations for phenomena, and scientists who successfully create new theories will often be famous. As Sheldon Cooper once said, "Evolution isn't an opinion, it's fact."[28] Note that creationists don't say that gravity is "only a theory." And if anyone says you can't directly observe evolution, send them to Professor Lenski.

Strictly speaking, evolution is something that happens in the world of life, and should be distinguished from a theory of evolution, which is (according to the above definition) a model of how evolution occurs. Thus evolution bears the same relationship with a theory of evolution as flight with a theory of flight, or sound with a theory of sound, or planetary motion with a theory of planetary motion. This is often expressed in the saying that "Evolution is both a theory and a fact", that is to say, the word "evolution" can refer not only to the process (the "something that happens"), but also to a fact that it is observed under such-and-such circumstances, and to a theory that is involved with the process ("how it happens", "what the consequences are of it happening").[29]

One creationist claim is that there is a lack of support for evolution among scientists. This claim has for example been articulated, "Interestingly, ever since Charles Darwin's book The Origin of Species was published in 1859, various aspects of the theory have been a matter of considerable disagreement even among top evolutionary scientists."[30] To counter this claim one need only note that scientists' disagreements are about details over the way that evolution functions - and not about the historical fact of it.

Many simulations of evolution (of digital creatures) towards some goal exist. Some of the best are documented here:

In which creatures made of nodes and muscles frantically try to run to the right. Code publicly available; run it online![31]

In which randomly generated octagons with wheels frantically try to drive to the right. Run it online![32] Code not publicly available; explanation available.[33]

Or, "Evolution IS a Blind Watchmaker". Watch a bunch of gears, ratchets, clock hands, and springs frantically try to accurately tell time, and simultaneously disprove the watchmaker analogy. Code publicly available.[34]

Read the original here:

Evolution - RationalWiki

The web today is a growing universe of interlinked web pages and web apps, teeming with videos, photos, and interactive content. What the average user doesn't see is the interplay of web technologies and browsers that makes all this possible.

Over time web technologies have evolved to give web developers the ability to create new generations of useful and immersive web experiences. Today's web is a result of the ongoing efforts of an open web community that helps define these web technologies, like HTML5, CSS3 and WebGL and ensure that they're supported in all web browsers.

The color bands in this visualization represent the interaction between web technologies and browsers, which brings to life the many powerful web apps that we use daily.

Made with some friends from the Google Chrome team

2011 & 2012 versions by Hyperakt and Vizzuality

2010 version by mgmt design and GOOD

Wikipedia, CanIUse.com, W3C, HTML5rocks.com and Mozilla Developer Network

Browser screenshots used in this infographic were sourced with best efforts from the web community.

More here:

Evolution of the Web

How a Communicating System Can Change the Way You Think About Comfort

Evolution systems might look similar to other heating and cooling systems, but dont be deceived by whats on the outside. Inside these dependable, high-efficiency products is technology that allows two-way communication between unit and control. For you, that means no longer having to be the exclusive manager of comfort in your home by dictating the rise and fall of temperature and accounting for every moment of being home, away or asleep through programming and regular changes.

The Evolution system has the smarts to do some of that thinking for you to help you be more comfortable, but more importantly, to help you save money. And the brain behind it all is the Evolution Connex control. When installed as part of a complete Evolution system, it can manage temperature, humidity, air quality, fan speeds, ventilation and zoning whatever system components you want to throw at it. And it makes it easier for you to create comfort settings and connect with your system at home or away.

Because it can communicate with the Evolution system, the Connex control always knows whats going on with the system and how to ramp it up or down to save money. It knows the temperature outside and what the Evolution furnace, air conditioner or heat pump has done each time it comes on to warm or cool your home. And it takes that information to adjust the system to save money while staying within your comfort settings. So even when its 75 degrees outside and a perfectly comfortable temperature inside, if the humidity in your house is still greater than you like it, the control will tell the outdoor unit to cool at lower, longer stages so that humidity can be reduced.

Once youve set your schedule and comfort preferences for the Evolution system, the Connex control will actively manage your homes temperatures whenever youre away or asleep. For instance, instead of trying to quickly ramp up heating or cooling to meet your waking or at home settings, the control monitors the outdoor temperature and system output so it can adjust the time and level of ramp up to save you the most money. And, when installed as part of an Evolution Hybrid Heat system, which consists of a gas furnace paired with a heat pump in place of an air conditioner, it can also save by choosing between electric heat pump and gas furnace to heat with the most economical energy source for the condition.

A Wi-Fi enabled Connex control can let you make changes to your system from anywhere at any time for ultimate convenience.* Because even when your normally predictable schedule becomes unpredictable you still want to come home to comfort. And because the thing you may have forgotten as you headed down the road to a vacation hideaway was setting a vacation schedule.

For these reasons and many more, remote connectivity can be so helpful. But the Connex control is also designed with other conveniences in mind, like a Touch-N-Go feature that lets you change from sleep or away settings to wake or home with ultimate ease. And you wont even have to think about when a filter needs changing the control will remind you so you can go right on with your busy life.

*when connected to a Wi-Fi network with the included wireless router

Originally posted here:

MyEvolution // About Evolution

Evolution is change in the heritable characteristics of biological populations over successive generations.[1][2] Evolutionary processes give rise to biodiversity at every level of biological organisation, including the levels of species, individual organisms, and molecules.[3]

All life on Earth shares a common ancestor known as the last universal common ancestor (LUCA),[4][5][6] which lived approximately 3.53.8 billion years ago,[7] although a study in 2015 found "remains of biotic life" from 4.1 billion years ago in ancient rocks in Western Australia.[8][9] In July 2016, scientists reported identifying a set of 355 genes from the LUCA of all organisms living on Earth.[10]

Repeated formation of new species (speciation), change within species (anagenesis), and loss of species (extinction) throughout the evolutionary history of life on Earth are demonstrated by shared sets of morphological and biochemical traits, including shared DNA sequences.[11] These shared traits are more similar among species that share a more recent common ancestor, and can be used to reconstruct a biological "tree of life" based on evolutionary relationships (phylogenetics), using both existing species and fossils. The fossil record includes a progression from early biogenic graphite,[12] to microbial mat fossils,[13][14][15] to fossilized multicellular organisms. Existing patterns of biodiversity have been shaped both by speciation and by extinction.[16] More than 99 percent of all species that ever lived on Earth are estimated to be extinct.[17][18] Estimates of Earth's current species range from 10 to 14 million,[19] of which about 1.2 million have been documented.[20] More recently, in May 2016, scientists reported that 1 trillion species are estimated to be on Earth currently with only one-thousandth of one percent described.[21]

In the mid-19th century, Charles Darwin formulated the scientific theory of evolution by natural selection, published in his book On the Origin of Species (1859). Evolution by natural selection is a process demonstrated by the observation that more offspring are produced than can possibly survive, along with three facts about populations: 1) traits vary among individuals with respect to morphology, physiology, and behaviour (phenotypic variation), 2) different traits confer different rates of survival and reproduction (differential fitness), and 3) traits can be passed from generation to generation (heritability of fitness).[22] Thus, in successive generations members of a population are replaced by progeny of parents better adapted to survive and reproduce in the biophysical environment in which natural selection takes place. This teleonomy is the quality whereby the process of natural selection creates and preserves traits that are seemingly fitted for the functional roles they perform.[23] Natural selection, including sexual selection, is the only known cause of adaptation but not the only known cause of evolution. Other, nonadaptive evolutionary processes include mutation, genetic drift and gene migration.[24]

In the early 20th century the modern evolutionary synthesis integrated classical genetics with Darwin's theory of evolution by natural selection through the discipline of population genetics. The importance of natural selection as a cause of evolution was accepted into other branches of biology. Moreover, previously held notions about evolution, such as orthogenesis, evolutionism, and other beliefs about innate "progress" within the largest-scale trends in evolution, became obsolete scientific theories.[25] Scientists continue to study various aspects of evolutionary biology by forming and testing hypotheses, constructing mathematical models of theoretical biology and biological theories, using observational data, and performing experiments in both the field and the laboratory.

In terms of practical application, an understanding of evolution has been instrumental to developments in numerous scientific and industrial fields, including agriculture, human and veterinary medicine, and the life sciences in general.[26][27][28] Discoveries in evolutionary biology have made a significant impact not just in the traditional branches of biology but also in other academic disciplines, including biological anthropology, and evolutionary psychology.[29][30]Evolutionary computation, a sub-field of artificial intelligence, involves the application of Darwinian principles to problems in computer science.

The proposal that one type of organism could descend from another type goes back to some of the first pre-Socratic Greek philosophers, such as Anaximander and Empedocles.[32] Such proposals survived into Roman times. The poet and philosopher Lucretius followed Empedocles in his masterwork De rerum natura (On the Nature of Things).[33][34] In contrast to these materialistic views, Aristotle considered all natural things, not only living things, as being imperfect actualisations of different fixed natural possibilities, known as "forms," "ideas," or (in Latin translations) "species."[35][36] This was part of his teleological understanding of nature in which all things have an intended role to play in a divine cosmic order. Variations of this idea became the standard understanding of the Middle Ages and were integrated into Christian learning, but Aristotle did not demand that real types of organisms always correspond one-for-one with exact metaphysical forms and specifically gave examples of how new types of living things could come to be.[37]

In the 17th century, the new method of modern science rejected Aristotle's approach. It sought explanations of natural phenomena in terms of physical laws that were the same for all visible things and that did not require the existence of any fixed natural categories or divine cosmic order. However, this new approach was slow to take root in the biological sciences, the last bastion of the concept of fixed natural types. John Ray applied one of the previously more general terms for fixed natural types, "species," to plant and animal types, but he strictly identified each type of living thing as a species and proposed that each species could be defined by the features that perpetuated themselves generation after generation.[38] The biological classification introduced by Carl Linnaeus in 1735 explicitly recognized the hierarchical nature of species relationships, but still viewed species as fixed according to a divine plan.[39]

Other naturalists of this time speculated on the evolutionary change of species over time according to natural laws. In 1751, Pierre Louis Maupertuis wrote of natural modifications occurring during reproduction and accumulating over many generations to produce new species.[40]Georges-Louis Leclerc, Comte de Buffon suggested that species could degenerate into different organisms, and Erasmus Darwin proposed that all warm-blooded animals could have descended from a single microorganism (or "filament").[41] The first full-fledged evolutionary scheme was Jean-Baptiste Lamarck's "transmutation" theory of 1809,[42] which envisaged spontaneous generation continually producing simple forms of life that developed greater complexity in parallel lineages with an inherent progressive tendency, and postulated that on a local level these lineages adapted to the environment by inheriting changes caused by their use or disuse in parents.[43][44] (The latter process was later called Lamarckism.)[43][45][46][47] These ideas were condemned by established naturalists as speculation lacking empirical support. In particular, Georges Cuvier insisted that species were unrelated and fixed, their similarities reflecting divine design for functional needs. In the meantime, Ray's ideas of benevolent design had been developed by William Paley into the Natural Theology or Evidences of the Existence and Attributes of the Deity (1802), which proposed complex adaptations as evidence of divine design and which was admired by Charles Darwin.[48][49][50]

The crucial break from the concept of constant typological classes or types in biology came with the theory of evolution through natural selection, which was formulated by Charles Darwin in terms of variable populations. Partly influenced by An Essay on the Principle of Population (1798) by Thomas Robert Malthus, Darwin noted that population growth would lead to a "struggle for existence" in which favorable variations prevailed as others perished. In each generation, many offspring fail to survive to an age of reproduction because of limited resources. This could explain the diversity of plants and animals from a common ancestry through the working of natural laws in the same way for all types of organism.[51][52][53][54] Darwin developed his theory of "natural selection" from 1838 onwards and was writing up his "big book" on the subject when Alfred Russel Wallace sent him a version of virtually the same theory in 1858. Their separate papers were presented together at a 1858 meeting of the Linnean Society of London.[55] At the end of 1859, Darwin's publication of his "abstract" as On the Origin of Species explained natural selection in detail and in a way that led to an increasingly wide acceptance of concepts of evolution. Thomas Henry Huxley applied Darwin's ideas to humans, using paleontology and comparative anatomy to provide strong evidence that humans and apes shared a common ancestry. Some were disturbed by this since it implied that humans did not have a special place in the universe.[56]

Precise mechanisms of reproductive heritability and the origin of new traits remained a mystery. Towards this end, Darwin developed his provisional theory of pangenesis.[57] In 1865, Gregor Mendel reported that traits were inherited in a predictable manner through the independent assortment and segregation of elements (later known as genes). Mendel's laws of inheritance eventually supplanted most of Darwin's pangenesis theory.[58]August Weismann made the important distinction between germ cells that give rise to gametes (such as sperm and egg cells) and the somatic cells of the body, demonstrating that heredity passes through the germ line only. Hugo de Vries connected Darwin's pangenesis theory to Weismann's germ/soma cell distinction and proposed that Darwin's pangenes were concentrated in the cell nucleus and when expressed they could move into the cytoplasm to change the cells structure. De Vries was also one of the researchers who made Mendel's work well-known, believing that Mendelian traits corresponded to the transfer of heritable variations along the germline.[59] To explain how new variants originate, de Vries developed a mutation theory that led to a temporary rift between those who accepted Darwinian evolution and biometricians who allied with de Vries.[44][60][61] In the 1930s, pioneers in the field of population genetics, such as Ronald Fisher, Sewall Wright and J. B. S. Haldane set the foundations of evolution onto a robust statistical philosophy. The false contradiction between Darwin's theory, genetic mutations, and Mendelian inheritance was thus reconciled.[62]

In the 1920s and 1930s a modern evolutionary synthesis connected natural selection, mutation theory, and Mendelian inheritance into a unified theory that applied generally to any branch of biology. The modern synthesis was able to explain patterns observed across species in populations, through fossil transitions in palaeontology, and even complex cellular mechanisms in developmental biology.[44][63] The publication of the structure of DNA by James Watson and Francis Crick in 1953 demonstrated a physical mechanism for inheritance.[64]Molecular biology improved our understanding of the relationship between genotype and phenotype. Advancements were also made in phylogenetic systematics, mapping the transition of traits into a comparative and testable framework through the publication and use of evolutionary trees.[65][66] In 1973, evolutionary biologist Theodosius Dobzhansky penned that "nothing in biology makes sense except in the light of evolution," because it has brought to light the relations of what first seemed disjointed facts in natural history into a coherent explanatory body of knowledge that describes and predicts many observable facts about life on this planet.[67]

Since then, the modern synthesis has been further extended to explain biological phenomena across the full and integrative scale of the biological hierarchy, from genes to species. This extension, known as evolutionary developmental biology and informally called "evo-devo," emphasises how changes between generations (evolution) acts on patterns of change within individual organisms (development).[68][69][70]

Evolution in organisms occurs through changes in heritable traitsthe inherited characteristics of an organism. In humans, for example, eye colour is an inherited characteristic and an individual might inherit the "brown-eye trait" from one of their parents.[71] Inherited traits are controlled by genes and the complete set of genes within an organism's genome (genetic material) is called its genotype.[72]

The complete set of observable traits that make up the structure and behaviour of an organism is called its phenotype. These traits come from the interaction of its genotype with the environment.[73] As a result, many aspects of an organism's phenotype are not inherited. For example, suntanned skin comes from the interaction between a person's genotype and sunlight; thus, suntans are not passed on to people's children. However, some people tan more easily than others, due to differences in genotypic variation; a striking example are people with the inherited trait of albinism, who do not tan at all and are very sensitive to sunburn.[74]

Heritable traits are passed from one generation to the next via DNA, a molecule that encodes genetic information.[72] DNA is a long biopolymer composed of four types of bases. The sequence of bases along a particular DNA molecule specify the genetic information, in a manner similar to a sequence of letters spelling out a sentence. Before a cell divides, the DNA is copied, so that each of the resulting two cells will inherit the DNA sequence. Portions of a DNA molecule that specify a single functional unit are called genes; different genes have different sequences of bases. Within cells, the long strands of DNA form condensed structures called chromosomes. The specific location of a DNA sequence within a chromosome is known as a locus. If the DNA sequence at a locus varies between individuals, the different forms of this sequence are called alleles. DNA sequences can change through mutations, producing new alleles. If a mutation occurs within a gene, the new allele may affect the trait that the gene controls, altering the phenotype of the organism.[75] However, while this simple correspondence between an allele and a trait works in some cases, most traits are more complex and are controlled by quantitative trait loci (multiple interacting genes).[76][77]

Recent findings have confirmed important examples of heritable changes that cannot be explained by changes to the sequence of nucleotides in the DNA. These phenomena are classed as epigenetic inheritance systems.[78]DNA methylation marking chromatin, self-sustaining metabolic loops, gene silencing by RNA interference and the three-dimensional conformation of proteins (such as prions) are areas where epigenetic inheritance systems have been discovered at the organismic level.[79][80] Developmental biologists suggest that complex interactions in genetic networks and communication among cells can lead to heritable variations that may underlay some of the mechanics in developmental plasticity and canalisation.[81] Heritability may also occur at even larger scales. For example, ecological inheritance through the process of niche construction is defined by the regular and repeated activities of organisms in their environment. This generates a legacy of effects that modify and feed back into the selection regime of subsequent generations. Descendants inherit genes plus environmental characteristics generated by the ecological actions of ancestors.[82] Other examples of heritability in evolution that are not under the direct control of genes include the inheritance of cultural traits and symbiogenesis.[83][84]

An individual organism's phenotype results from both its genotype and the influence from the environment it has lived in. A substantial part of the phenotypic variation in a population is caused by genotypic variation.[77] The modern evolutionary synthesis defines evolution as the change over time in this genetic variation. The frequency of one particular allele will become more or less prevalent relative to other forms of that gene. Variation disappears when a new allele reaches the point of fixationwhen it either disappears from the population or replaces the ancestral allele entirely.[85]

Natural selection will only cause evolution if there is enough genetic variation in a population. Before the discovery of Mendelian genetics, one common hypothesis was blending inheritance. But with blending inheritance, genetic variance would be rapidly lost, making evolution by natural selection implausible. The HardyWeinberg principle provides the solution to how variation is maintained in a population with Mendelian inheritance. The frequencies of alleles (variations in a gene) will remain constant in the absence of selection, mutation, migration and genetic drift.[86]

Variation comes from mutations in the genome, reshuffling of genes through sexual reproduction and migration between populations (gene flow). Despite the constant introduction of new variation through mutation and gene flow, most of the genome of a species is identical in all individuals of that species.[87] However, even relatively small differences in genotype can lead to dramatic differences in phenotype: for example, chimpanzees and humans differ in only about 5% of their genomes.[88]

Mutations are changes in the DNA sequence of a cell's genome. When mutations occur, they may alter the product of a gene, or prevent the gene from functioning, or have no effect. Based on studies in the fly Drosophila melanogaster, it has been suggested that if a mutation changes a protein produced by a gene, this will probably be harmful, with about 70% of these mutations having damaging effects, and the remainder being either neutral or weakly beneficial.[89]

Mutations can involve large sections of a chromosome becoming duplicated (usually by genetic recombination), which can introduce extra copies of a gene into a genome.[90] Extra copies of genes are a major source of the raw material needed for new genes to evolve.[91] This is important because most new genes evolve within gene families from pre-existing genes that share common ancestors.[92] For example, the human eye uses four genes to make structures that sense light: three for colour vision and one for night vision; all four are descended from a single ancestral gene.[93]

New genes can be generated from an ancestral gene when a duplicate copy mutates and acquires a new function. This process is easier once a gene has been duplicated because it increases the redundancy of the system; one gene in the pair can acquire a new function while the other copy continues to perform its original function.[94][95] Other types of mutations can even generate entirely new genes from previously noncoding DNA.[96][97]

The generation of new genes can also involve small parts of several genes being duplicated, with these fragments then recombining to form new combinations with new functions.[98][99] When new genes are assembled from shuffling pre-existing parts, domains act as modules with simple independent functions, which can be mixed together to produce new combinations with new and complex functions.[100] For example, polyketide synthases are large enzymes that make antibiotics; they contain up to one hundred independent domains that each catalyse one step in the overall process, like a step in an assembly line.[101]

In asexual organisms, genes are inherited together, or linked, as they cannot mix with genes of other organisms during reproduction. In contrast, the offspring of sexual organisms contain random mixtures of their parents' chromosomes that are produced through independent assortment. In a related process called homologous recombination, sexual organisms exchange DNA between two matching chromosomes.[102] Recombination and reassortment do not alter allele frequencies, but instead change which alleles are associated with each other, producing offspring with new combinations of alleles.[103] Sex usually increases genetic variation and may increase the rate of evolution.[104][105]

The two-fold cost of sex was first described by John Maynard Smith.[106] The first cost is that in sexually dimorphic species only one of the two sexes can bear young. (This cost does not apply to hermaphroditic species, like most plants and many invertebrates.) The second cost is that any individual who reproduces sexually can only pass on 50% of its genes to any individual offspring, with even less passed on as each new generation passes.[107] Yet sexual reproduction is the more common means of reproduction among eukaryotes and multicellular organisms. The Red Queen hypothesis has been used to explain the significance of sexual reproduction as a means to enable continual evolution and adaptation in response to coevolution with other species in an ever-changing environment.[107][108][109][110]

Gene flow is the exchange of genes between populations and between species.[111] It can therefore be a source of variation that is new to a population or to a species. Gene flow can be caused by the movement of individuals between separate populations of organisms, as might be caused by the movement of mice between inland and coastal populations, or the movement of pollen between heavy metal tolerant and heavy metal sensitive populations of grasses.

Gene transfer between species includes the formation of hybrid organisms and horizontal gene transfer. Horizontal gene transfer is the transfer of genetic material from one organism to another organism that is not its offspring; this is most common among bacteria.[112] In medicine, this contributes to the spread of antibiotic resistance, as when one bacteria acquires resistance genes it can rapidly transfer them to other species.[113] Horizontal transfer of genes from bacteria to eukaryotes such as the yeast Saccharomyces cerevisiae and the adzuki bean weevil Callosobruchus chinensis has occurred.[114][115] An example of larger-scale transfers are the eukaryotic bdelloid rotifers, which have received a range of genes from bacteria, fungi and plants.[116]Viruses can also carry DNA between organisms, allowing transfer of genes even across biological domains.[117]

Large-scale gene transfer has also occurred between the ancestors of eukaryotic cells and bacteria, during the acquisition of chloroplasts and mitochondria. It is possible that eukaryotes themselves originated from horizontal gene transfers between bacteria and archaea.[118]

From a Neo-Darwinian perspective, evolution occurs when there are changes in the frequencies of alleles within a population of interbreeding organisms.[86] For example, the allele for black colour in a population of moths becoming more common. Mechanisms that can lead to changes in allele frequencies include natural selection, genetic drift, genetic hitchhiking, mutation and gene flow.

Evolution by means of natural selection is the process by which traits that enhance survival and reproduction become more common in successive generations of a population. It has often been called a "self-evident" mechanism because it necessarily follows from three simple facts:[22]

More offspring are produced than can possibly survive, and these conditions produce competition between organisms for survival and reproduction. Consequently, organisms with traits that give them an advantage over their competitors are more likely to pass on their traits to the next generation than those with traits that do not confer an advantage.[119]

The central concept of natural selection is the evolutionary fitness of an organism.[120] Fitness is measured by an organism's ability to survive and reproduce, which determines the size of its genetic contribution to the next generation.[120] However, fitness is not the same as the total number of offspring: instead fitness is indicated by the proportion of subsequent generations that carry an organism's genes.[121] For example, if an organism could survive well and reproduce rapidly, but its offspring were all too small and weak to survive, this organism would make little genetic contribution to future generations and would thus have low fitness.[120]

If an allele increases fitness more than the other alleles of that gene, then with each generation this allele will become more common within the population. These traits are said to be "selected for." Examples of traits that can increase fitness are enhanced survival and increased fecundity. Conversely, the lower fitness caused by having a less beneficial or deleterious allele results in this allele becoming rarerthey are "selected against."[122] Importantly, the fitness of an allele is not a fixed characteristic; if the environment changes, previously neutral or harmful traits may become beneficial and previously beneficial traits become harmful.[75] However, even if the direction of selection does reverse in this way, traits that were lost in the past may not re-evolve in an identical form (see Dollo's law).[123][124]

Natural selection within a population for a trait that can vary across a range of values, such as height, can be categorised into three different types. The first is directional selection, which is a shift in the average value of a trait over timefor example, organisms slowly getting taller.[125] Secondly, disruptive selection is selection for extreme trait values and often results in two different values becoming most common, with selection against the average value. This would be when either short or tall organisms had an advantage, but not those of medium height. Finally, in stabilising selection there is selection against extreme trait values on both ends, which causes a decrease in variance around the average value and less diversity.[119][126] This would, for example, cause organisms to slowly become all the same height.

A special case of natural selection is sexual selection, which is selection for any trait that increases mating success by increasing the attractiveness of an organism to potential mates.[127] Traits that evolved through sexual selection are particularly prominent among males of several animal species. Although sexually favoured, traits such as cumbersome antlers, mating calls, large body size and bright colours often attract predation, which compromises the survival of individual males.[128][129] This survival disadvantage is balanced by higher reproductive success in males that show these hard-to-fake, sexually selected traits.[130]

Natural selection most generally makes nature the measure against which individuals and individual traits, are more or less likely to survive. "Nature" in this sense refers to an ecosystem, that is, a system in which organisms interact with every other element, physical as well as biological, in their local environment. Eugene Odum, a founder of ecology, defined an ecosystem as: "Any unit that includes all of the organisms...in a given area interacting with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity and material cycles (ie: exchange of materials between living and nonliving parts) within the system."[131] Each population within an ecosystem occupies a distinct niche, or position, with distinct relationships to other parts of the system. These relationships involve the life history of the organism, its position in the food chain and its geographic range. This broad understanding of nature enables scientists to delineate specific forces which, together, comprise natural selection.

Natural selection can act at different levels of organisation, such as genes, cells, individual organisms, groups of organisms and species.[132][133][134] Selection can act at multiple levels simultaneously.[135] An example of selection occurring below the level of the individual organism are genes called transposons, which can replicate and spread throughout a genome.[136] Selection at a level above the individual, such as group selection, may allow the evolution of cooperation, as discussed below.[137]

In addition to being a major source of variation, mutation may also function as a mechanism of evolution when there are different probabilities at the molecular level for different mutations to occur, a process known as mutation bias.[138] If two genotypes, for example one with the nucleotide G and another with the nucleotide A in the same position, have the same fitness, but mutation from G to A happens more often than mutation from A to G, then genotypes with A will tend to evolve.[139] Different insertion vs. deletion mutation biases in different taxa can lead to the evolution of different genome sizes.[140][141] Developmental or mutational biases have also been observed in morphological evolution.[142][143] For example, according to the phenotype-first theory of evolution, mutations can eventually cause the genetic assimilation of traits that were previously induced by the environment.[144][145]

Mutation bias effects are superimposed on other processes. If selection would favor either one out of two mutations, but there is no extra advantage to having both, then the mutation that occurs the most frequently is the one that is most likely to become fixed in a population.[146][147] Mutations leading to the loss of function of a gene are much more common than mutations that produce a new, fully functional gene. Most loss of function mutations are selected against. But when selection is weak, mutation bias towards loss of function can affect evolution.[148] For example, pigments are no longer useful when animals live in the darkness of caves, and tend to be lost.[149] This kind of loss of function can occur because of mutation bias, and/or because the function had a cost, and once the benefit of the function disappeared, natural selection leads to the loss. Loss of sporulation ability in Bacillus subtilis during laboratory evolution appears to have been caused by mutation bias, rather than natural selection against the cost of maintaining sporulation ability.[150] When there is no selection for loss of function, the speed at which loss evolves depends more on the mutation rate than it does on the effective population size,[151] indicating that it is driven more by mutation bias than by genetic drift. In parasitic organisms, mutation bias leads to selection pressures as seen in Ehrlichia. Mutations are biased towards antigenic variants in outer-membrane proteins.

Genetic drift is the change in allele frequency from one generation to the next that occurs because alleles are subject to sampling error.[152] As a result, when selective forces are absent or relatively weak, allele frequencies tend to "drift" upward or downward randomly (in a random walk). This drift halts when an allele eventually becomes fixed, either by disappearing from the population, or replacing the other alleles entirely. Genetic drift may therefore eliminate some alleles from a population due to chance alone. Even in the absence of selective forces, genetic drift can cause two separate populations that began with the same genetic structure to drift apart into two divergent populations with different sets of alleles.[153]

It is usually difficult to measure the relative importance of selection and neutral processes, including drift.[154] The comparative importance of adaptive and non-adaptive forces in driving evolutionary change is an area of current research.[155]

The neutral theory of molecular evolution proposed that most evolutionary changes are the result of the fixation of neutral mutations by genetic drift.[156] Hence, in this model, most genetic changes in a population are the result of constant mutation pressure and genetic drift.[157] This form of the neutral theory is now largely abandoned, since it does not seem to fit the genetic variation seen in nature.[158][159] However, a more recent and better-supported version of this model is the nearly neutral theory, where a mutation that would be effectively neutral in a small population is not necessarily neutral in a large population.[119] Other alternative theories propose that genetic drift is dwarfed by other stochastic forces in evolution, such as genetic hitchhiking, also known as genetic draft.[152][160][161]

The time for a neutral allele to become fixed by genetic drift depends on population size, with fixation occurring more rapidly in smaller populations.[162] The number of individuals in a population is not critical, but instead a measure known as the effective population size.[163] The effective population is usually smaller than the total population since it takes into account factors such as the level of inbreeding and the stage of the lifecycle in which the population is the smallest.[163] The effective population size may not be the same for every gene in the same population.[164]

Recombination allows alleles on the same strand of DNA to become separated. However, the rate of recombination is low (approximately two events per chromosome per generation). As a result, genes close together on a chromosome may not always be shuffled away from each other and genes that are close together tend to be inherited together, a phenomenon known as linkage.[165] This tendency is measured by finding how often two alleles occur together on a single chromosome compared to expectations, which is called their linkage disequilibrium. A set of alleles that is usually inherited in a group is called a haplotype. This can be important when one allele in a particular haplotype is strongly beneficial: natural selection can drive a selective sweep that will also cause the other alleles in the haplotype to become more common in the population; this effect is called genetic hitchhiking or genetic draft.[166] Genetic draft caused by the fact that some neutral genes are genetically linked to others that are under selection can be partially captured by an appropriate effective population size.[160]

Gene flow involves the exchange of genes between populations and between species.[111] The presence or absence of gene flow fundamentally changes the course of evolution. Due to the complexity of organisms, any two completely isolated populations will eventually evolve genetic incompatibilities through neutral processes, as in the Bateson-Dobzhansky-Muller model, even if both populations remain essentially identical in terms of their adaptation to the environment.

If genetic differentiation between populations develops, gene flow between populations can introduce traits or alleles which are disadvantageous in the local population and this may lead to organisms within these populations evolving mechanisms that prevent mating with genetically distant populations, eventually resulting in the appearance of new species. Thus, exchange of genetic information between individuals is fundamentally important for the development of the biological species concept.

During the development of the modern synthesis, Sewall Wright developed his shifting balance theory, which regarded gene flow between partially isolated populations as an important aspect of adaptive evolution.[167] However, recently there has been substantial criticism of the importance of the shifting balance theory.[168]

Evolution influences every aspect of the form and behaviour of organisms. Most prominent are the specific behavioural and physical adaptations that are the outcome of natural selection. These adaptations increase fitness by aiding activities such as finding food, avoiding predators or attracting mates. Organisms can also respond to selection by cooperating with each other, usually by aiding their relatives or engaging in mutually beneficial symbiosis. In the longer term, evolution produces new species through splitting ancestral populations of organisms into new groups that cannot or will not interbreed.

These outcomes of evolution are distinguished based on time scale as macroevolution versus microevolution. Macroevolution refers to evolution that occurs at or above the level of species, in particular speciation and extinction; whereas microevolution refers to smaller evolutionary changes within a species or population, in particular shifts in gene frequency and adaptation.[170] In general, macroevolution is regarded as the outcome of long periods of microevolution.[171] Thus, the distinction between micro- and macroevolution is not a fundamental onethe difference is simply the time involved.[172] However, in macroevolution, the traits of the entire species may be important. For instance, a large amount of variation among individuals allows a species to rapidly adapt to new habitats, lessening the chance of it going extinct, while a wide geographic range increases the chance of speciation, by making it more likely that part of the population will become isolated. In this sense, microevolution and macroevolution might involve selection at different levelswith microevolution acting on genes and organisms, versus macroevolutionary processes such as species selection acting on entire species and affecting their rates of speciation and extinction.[174][175]

A common misconception is that evolution has goals, long-term plans, or an innate tendency for "progress," as expressed in beliefs such as orthogenesis and evolutionism; realistically however, evolution has no long-term goal and does not necessarily produce greater complexity.[176][177][178] Although complex species have evolved, they occur as a side effect of the overall number of organisms increasing and simple forms of life still remain more common in the biosphere.[179] For example, the overwhelming majority of species are microscopic prokaryotes, which form about half the world's biomass despite their small size,[180] and constitute the vast majority of Earth's biodiversity.[181] Simple organisms have therefore been the dominant form of life on Earth throughout its history and continue to be the main form of life up to the present day, with complex life only appearing more diverse because it is more noticeable.[182] Indeed, the evolution of microorganisms is particularly important to modern evolutionary research, since their rapid reproduction allows the study of experimental evolution and the observation of evolution and adaptation in real time.[183][184]

Adaptation is the process that makes organisms better suited to their habitat.[185][186] Also, the term adaptation may refer to a trait that is important for an organism's survival. For example, the adaptation of horses' teeth to the grinding of grass. By using the term adaptation for the evolutionary process and adaptive trait for the product (the bodily part or function), the two senses of the word may be distinguished. Adaptations are produced by natural selection.[187] The following definitions are due to Theodosius Dobzhansky:

Adaptation may cause either the gain of a new feature, or the loss of an ancestral feature. An example that shows both types of change is bacterial adaptation to antibiotic selection, with genetic changes causing antibiotic resistance by both modifying the target of the drug, or increasing the activity of transporters that pump the drug out of the cell.[191] Other striking examples are the bacteria Escherichia coli evolving the ability to use citric acid as a nutrient in a long-term laboratory experiment,[192]Flavobacterium evolving a novel enzyme that allows these bacteria to grow on the by-products of nylon manufacturing,[193][194] and the soil bacterium Sphingobium evolving an entirely new metabolic pathway that degrades the synthetic pesticide pentachlorophenol.[195][196] An interesting but still controversial idea is that some adaptations might increase the ability of organisms to generate genetic diversity and adapt by natural selection (increasing organisms' evolvability).[197][198][199][200][201]

Adaptation occurs through the gradual modification of existing structures. Consequently, structures with similar internal organisation may have different functions in related organisms. This is the result of a single ancestral structure being adapted to function in different ways. The bones within bat wings, for example, are very similar to those in mice feet and primate hands, due to the descent of all these structures from a common mammalian ancestor.[203] However, since all living organisms are related to some extent,[204] even organs that appear to have little or no structural similarity, such as arthropod, squid and vertebrate eyes, or the limbs and wings of arthropods and vertebrates, can depend on a common set of homologous genes that control their assembly and function; this is called deep homology.[205][206]

During evolution, some structures may lose their original function and become vestigial structures.[207] Such structures may have little or no function in a current species, yet have a clear function in ancestral species, or other closely related species. Examples include pseudogenes,[208] the non-functional remains of eyes in blind cave-dwelling fish,[209] wings in flightless birds,[210] the presence of hip bones in whales and snakes,[202] and sexual traits in organisms that reproduce via asexual reproduction.[211] Examples of vestigial structures in humans include wisdom teeth,[212] the coccyx,[207] the vermiform appendix,[207] and other behavioural vestiges such as goose bumps[213][214] and primitive reflexes.[215][216][217]

However, many traits that appear to be simple adaptations are in fact exaptations: structures originally adapted for one function, but which coincidentally became somewhat useful for some other function in the process. One example is the African lizard Holaspis guentheri, which developed an extremely flat head for hiding in crevices, as can be seen by looking at its near relatives. However, in this species, the head has become so flattened that it assists in gliding from tree to treean exaptation. Within cells, molecular machines such as the bacterial flagella[219] and protein sorting machinery[220] evolved by the recruitment of several pre-existing proteins that previously had different functions.[170] Another example is the recruitment of enzymes from glycolysis and xenobiotic metabolism to serve as structural proteins called crystallins within the lenses of organisms' eyes.[221][222]

An area of current investigation in evolutionary developmental biology is the developmental basis of adaptations and exaptations.[223] This research addresses the origin and evolution of embryonic development and how modifications of development and developmental processes produce novel features.[224] These studies have shown that evolution can alter development to produce new structures, such as embryonic bone structures that develop into the jaw in other animals instead forming part of the middle ear in mammals.[225] It is also possible for structures that have been lost in evolution to reappear due to changes in developmental genes, such as a mutation in chickens causing embryos to grow teeth similar to those of crocodiles.[226] It is now becoming clear that most alterations in the form of organisms are due to changes in a small set of conserved genes.[227]

Interactions between organisms can produce both conflict and cooperation. When the interaction is between pairs of species, such as a pathogen and a host, or a predator and its prey, these species can develop matched sets of adaptations. Here, the evolution of one species causes adaptations in a second species. These changes in the second species then, in turn, cause new adaptations in the first species. This cycle of selection and response is called coevolution.[228] An example is the production of tetrodotoxin in the rough-skinned newt and the evolution of tetrodotoxin resistance in its predator, the common garter snake. In this predator-prey pair, an evolutionary arms race has produced high levels of toxin in the newt and correspondingly high levels of toxin resistance in the snake.[229]

Not all co-evolved interactions between species involve conflict.[230] Many cases of mutually beneficial interactions have evolved. For instance, an extreme cooperation exists between plants and the mycorrhizal fungi that grow on their roots and aid the plant in absorbing nutrients from the soil.[231] This is a reciprocal relationship as the plants provide the fungi with sugars from photosynthesis. Here, the fungi actually grow inside plant cells, allowing them to exchange nutrients with their hosts, while sending signals that suppress the plant immune system.[232]

Coalitions between organisms of the same species have also evolved. An extreme case is the eusociality found in social insects, such as bees, termites and ants, where sterile insects feed and guard the small number of organisms in a colony that are able to reproduce. On an even smaller scale, the somatic cells that make up the body of an animal limit their reproduction so they can maintain a stable organism, which then supports a small number of the animal's germ cells to produce offspring. Here, somatic cells respond to specific signals that instruct them whether to grow, remain as they are, or die. If cells ignore these signals and multiply inappropriately, their uncontrolled growth causes cancer.[233]

Such cooperation within species may have evolved through the process of kin selection, which is where one organism acts to help raise a relative's offspring.[234] This activity is selected for because if the helping individual contains alleles which promote the helping activity, it is likely that its kin will also contain these alleles and thus those alleles will be passed on.[235] Other processes that may promote cooperation include group selection, where cooperation provides benefits to a group of organisms.[236]

Speciation is the process where a species diverges into two or more descendant species.[237]

There are multiple ways to define the concept of "species." The choice of definition is dependent on the particularities of the species concerned.[238] For example, some species concepts apply more readily toward sexually reproducing organisms while others lend themselves better toward asexual organisms. Despite the diversity of various species concepts, these various concepts can be placed into one of three broad philosophical approaches: interbreeding, ecological and phylogenetic.[239] The Biological Species Concept (BSC) is a classic example of the interbreeding approach. Defined by Ernst Mayr in 1942, the BSC states that "species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups."[240] Despite its wide and long-term use, the BSC like others is not without controversy, for example because these concepts cannot be applied to prokaryotes,[241] and this is called the species problem.[238] Some researchers have attempted a unifying monistic definition of species, while others adopt a pluralistic approach and suggest that there may be different ways to logically interpret the definition of a species.[238][239]

Barriers to reproduction between two diverging sexual populations are required for the populations to become new species. Gene flow may slow this process by spreading the new genetic variants also to the other populations. Depending on how far two species have diverged since their most recent common ancestor, it may still be possible for them to produce offspring, as with horses and donkeys mating to produce mules.[242] Such hybrids are generally infertile. In this case, closely related species may regularly interbreed, but hybrids will be selected against and the species will remain distinct. However, viable hybrids are occasionally formed and these new species can either have properties intermediate between their parent species, or possess a totally new phenotype.[243] The importance of hybridisation in producing new species of animals is unclear, although cases have been seen in many types of animals,[244] with the gray tree frog being a particularly well-studied example.[245]

Speciation has been observed multiple times under both controlled laboratory conditions and in nature.[246] In sexually reproducing organisms, speciation results from reproductive isolation followed by genealogical divergence. There are four mechanisms for speciation. The most common in animals is allopatric speciation, which occurs in populations initially isolated geographically, such as by habitat fragmentation or migration. Selection under these conditions can produce very rapid changes in the appearance and behaviour of organisms.[247][248] As selection and drift act independently on populations isolated from the rest of their species, separation may eventually produce organisms that cannot interbreed.[249]

The second mechanism of speciation is peripatric speciation, which occurs when small populations of organisms become isolated in a new environment. This differs from allopatric speciation in that the isolated populations are numerically much smaller than the parental population. Here, the founder effect causes rapid speciation after an increase in inbreeding increases selection on homozygotes, leading to rapid genetic change.[250]

The third mechanism of speciation is parapatric speciation. This is similar to peripatric speciation in that a small population enters a new habitat, but differs in that there is no physical separation between these two populations. Instead, speciation results from the evolution of mechanisms that reduce gene flow between the two populations.[237] Generally this occurs when there has been a drastic change in the environment within the parental species' habitat. One example is the grass Anthoxanthum odoratum, which can undergo parapatric speciation in response to localised metal pollution from mines.[251] Here, plants evolve that have resistance to high levels of metals in the soil. Selection against interbreeding with the metal-sensitive parental population produced a gradual change in the flowering time of the metal-resistant plants, which eventually produced complete reproductive isolation. Selection against hybrids between the two populations may cause reinforcement, which is the evolution of traits that promote mating within a species, as well as character displacement, which is when two species become more distinct in appearance.[252]

Finally, in sympatric speciation species diverge without geographic isolation or changes in habitat. This form is rare since even a small amount of gene flow may remove genetic differences between parts of a population.[253] Generally, sympatric speciation in animals requires the evolution of both genetic differences and non-random mating, to allow reproductive isolation to evolve.[254]

One type of sympatric speciation involves crossbreeding of two related species to produce a new hybrid species. This is not common in animals as animal hybrids are usually sterile. This is because during meiosis the homologous chromosomes from each parent are from different species and cannot successfully pair. However, it is more common in plants because plants often double their number of chromosomes, to form polyploids.[255] This allows the chromosomes from each parental species to form matching pairs during meiosis, since each parent's chromosomes are represented by a pair already.[256] An example of such a speciation event is when the plant species Arabidopsis thaliana and Arabidopsis arenosa crossbred to give the new species Arabidopsis suecica.[257] This happened about 20,000 years ago,[258] and the speciation process has been repeated in the laboratory, which allows the study of the genetic mechanisms involved in this process.[259] Indeed, chromosome doubling within a species may be a common cause of reproductive isolation, as half the doubled chromosomes will be unmatched when breeding with undoubled organisms.[260]

Speciation events are important in the theory of punctuated equilibrium, which accounts for the pattern in the fossil record of short "bursts" of evolution interspersed with relatively long periods of stasis, where species remain relatively unchanged.[261] In this theory, speciation and rapid evolution are linked, with natural selection and genetic drift acting most strongly on organisms undergoing speciation in novel habitats or small populations. As a result, the periods of stasis in the fossil record correspond to the parental population and the organisms undergoing speciation and rapid evolution are found in small populations or geographically restricted habitats and therefore rarely being preserved as fossils.[174]

Extinction is the disappearance of an entire species. Extinction is not an unusual event, as species regularly appear through speciation and disappear through extinction.[262] Nearly all animal and plant species that have lived on Earth are now extinct,[263] and extinction appears to be the ultimate fate of all species.[264] These extinctions have happened continuously throughout the history of life, although the rate of extinction spikes in occasional mass extinction events.[265] The CretaceousPaleogene extinction event, during which the non-avian dinosaurs became extinct, is the most well-known, but the earlier PermianTriassic extinction event was even more severe, with approximately 96% of all marine species driven to extinction.[265] The Holocene extinction event is an ongoing mass extinction associated with humanity's expansion across the globe over the past few thousand years. Present-day extinction rates are 1001000 times greater than the background rate and up to 30% of current species may be extinct by the mid 21st century.[266] Human activities are now the primary cause of the ongoing extinction event;[267]global warming may further accelerate it in the future.[268]

The role of extinction in evolution is not very well understood and may depend on which type of extinction is considered.[265] The causes of the continuous "low-level" extinction events, which form the majority of extinctions, may be the result of competition between species for limited resources (the competitive exclusion principle).[68] If one species can out-compete another, this could produce species selection, with the fitter species surviving and the other species being driven to extinction.[133] The intermittent mass extinctions are also important, but instead of acting as a selective force, they drastically reduce diversity in a nonspecific manner and promote bursts of rapid evolution and speciation in survivors.[269]

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Evolution - Wikipedia

Human evolution is the evolutionary process that led to the emergence of anatomically modern humans. The topic typically focuses on the evolutionary history of the primatesin particular the genus Homo, and the emergence of Homo sapiens as a distinct species of the hominids (or "great apes")rather than studying the earlier history that led to the primates. The study of human evolution involves many scientific disciplines, including physical anthropology, primatology, archaeology, paleontology, neurobiology, ethology, linguistics, evolutionary psychology, embryology and genetics.[1] Genetic studies show that primates diverged from other mammals about 85 million years ago, in the Late Cretaceous period, and the earliest fossils appear in the Paleocene, around 55 million years ago.[2] Within the Hominoidea (apes) superfamily, the Hominidae family diverged from the Hylobatidae (gibbon) family some 1520 million years ago; African great apes (subfamily Homininae) diverged from orangutans (Ponginae) about 14 million years ago; the Hominini tribe (humans, Australopithecines and other extinct biped genera, and chimpanzees) parted from the Gorillini tribe (gorillas) about 8 million years ago; and, in turn, the subtribes Hominina (humans and biped ancestors) and Panina (chimps) separated about 7.5 million years ago to 5.6 million years ago.[3]

The basic adaptation of the hominin line is bipedalism. The earliest bipedal hominin is considered to be either Sahelanthropus or Orrorin; alternatively, either Sahelanthropus or Orrorin may instead be the last shared ancestor between chimps and humans. Ardipithecus, a full biped, arose somewhat later, and the early bipeds eventually evolved into the australopithecines, and later into the genus Homo.

The earliest documented representative of the genus Homo is Homo habilis, which evolved around 2.8 million years ago,[4] and is arguably the earliest species for which there is positive evidence of the use of stone tools. The brains of these early hominins were about the same size as that of a chimpanzee, although it has been suggested that this was the time in which the human SRGAP2 gene doubled, producing a more rapid wiring of the frontal cortex. During the next million years a process of rapid encephalization occurred, and with the arrival of Homo erectus and Homo ergaster in the fossil record, cranial capacity had doubled to 850cm3.[5] (Such an increase in human brain size is equivalent to each generation having 125,000 more neurons than their parents.) It is believed that Homo erectus and Homo ergaster were the first to use fire and complex tools, and were the first of the hominin line to leave Africa, spreading throughout Africa, Asia, and Europe between 1.3to1.8 million years ago.

According to the recent African origin of modern humans theory, modern humans evolved in Africa possibly from Homo heidelbergensis, Homo rhodesiensis or Homo antecessor and migrated out of the continent some 50,000 to 100,000 years ago, gradually replacing local populations of Homo erectus, Denisova hominins, Homo floresiensis and Homo neanderthalensis.[6][7][8][9][10]Archaic Homo sapiens, the forerunner of anatomically modern humans, evolved in the Middle Paleolithic between 400,000 and 250,000 years ago.[11][12][13] Recent DNA evidence suggests that several haplotypes of Neanderthal origin are present among all non-African populations, and Neanderthals and other hominins, such as Denisovans, may have contributed up to 6% of their genome to present-day humans, suggestive of a limited inter-breeding between these species.[14][15][16] The transition to behavioral modernity with the development of symbolic culture, language, and specialized lithic technology happened around 50,000 years ago according to many anthropologists[17] although some suggest a gradual change in behavior over a longer time span.[18]

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The word homo, the name of the biological genus to which humans belong, is Latin for "human". It was chosen originally by Carl Linnaeus in his classification system. The word "human" is from the Latin humanus, the adjectival form of homo. The Latin "homo" derives from the Indo-European root *dhghem, or "earth".[19] Linnaeus and other scientists of his time also considered the great apes to be the closest relatives of humans based on morphological and anatomical similarities.

The possibility of linking humans with earlier apes by descent became clear only after 1859 with the publication of Charles Darwin's On the Origin of Species, in which he argued for the idea of the evolution of new species from earlier ones. Darwin's book did not address the question of human evolution, saying only that "Light will be thrown on the origin of man and his history."

The first debates about the nature of human evolution arose between Thomas Henry Huxley and Richard Owen. Huxley argued for human evolution from apes by illustrating many of the similarities and differences between humans and apes, and did so particularly in his 1863 book Evidence as to Man's Place in Nature. However, many of Darwin's early supporters (such as Alfred Russel Wallace and Charles Lyell) did not initially agree that the origin of the mental capacities and the moral sensibilities of humans could be explained by natural selection, though this later changed. Darwin applied the theory of evolution and sexual selection to humans when he published The Descent of Man in 1871.[20]

A major problem at that time was the lack of fossil intermediaries. Neanderthal remains were discovered in a limestone quarry in 1856, three years before the publication of On the Origin of Species, and Neanderthal fossils had been discovered in Gibraltar even earlier, but it was originally claimed that these were human remains of a creature suffering some kind of illness.[21] Despite the 1891 discovery by Eugne Dubois of what is now called Homo erectus at Trinil, Java, it was only in the 1920s when such fossils were discovered in Africa, that intermediate species began to accumulate.[citation needed] In 1925, Raymond Dart described Australopithecus africanus.[22] The type specimen was the Taung Child, an australopithecine infant which was discovered in a cave. The child's remains were a remarkably well-preserved tiny skull and an endocast of the brain.

Although the brain was small (410cm3), its shape was rounded, unlike that of chimpanzees and gorillas, and more like a modern human brain. Also, the specimen showed short canine teeth, and the position of the foramen magnum (the hole in the skull where the spine enters) was evidence of bipedal locomotion. All of these traits convinced Dart that the Taung Child was a bipedal human ancestor, a transitional form between apes and humans.

During the 1960s and 1970s, hundreds of fossils were found in East Africa in the regions of the Olduvai Gorge and Lake Turkana. The driving force of these searches was the Leakey family, with Louis Leakey and his wife Mary Leakey, and later their son Richard and daughter-in-law Meaveall successful and world-renowned fossil hunters and palaeoanthropologists. From the fossil beds of Olduvai and Lake Turkana they amassed specimens of the early hominins: the australopithecines and Homo species, and even Homo erectus.

These finds cemented Africa as the cradle of humankind. In the late 1970s and the 1980s, Ethiopia emerged as the new hot spot of palaeoanthropology after "Lucy", the most complete fossil member of the species Australopithecus afarensis, was found in 1974 by Donald Johanson near Hadar in the desertic Afar Triangle region of northern Ethiopia. Although the specimen had a small brain, the pelvis and leg bones were almost identical in function to those of modern humans, showing with certainty that these hominins had walked erect.[23] Lucy was classified as a new species, Australopithecus afarensis, which is thought to be more closely related to the genus Homo as a direct ancestor, or as a close relative of an unknown ancestor, than any other known hominid or hominin from this early time range; see terms "hominid" and "hominin".[24] (The specimen was nicknamed "Lucy" after the Beatles' song "Lucy in the Sky with Diamonds", which was played loudly and repeatedly in the camp during the excavations.[25]) The Afar Triangle area would later yield discovery of many more hominin fossils, particularly those uncovered or described by teams headed by Tim D. White in the 1990s, including Ardipithecus ramidus and Ardipithecus kadabba.[26]

In 2013, fossil skeletons of Homo naledi, an extinct species of hominin assigned (provisionally) to the genus Homo, were found in the Rising Star Cave system, a site in South Africa's Cradle of Humankind region in Gauteng province near Johannesburg.[27][28] As of September 2015[update], fossils of at least fifteen individuals, amounting to 1550 specimens, have been excavated from the cave.[28] The species is characterized by a body mass and stature similar to small-bodied human populations, a smaller endocranial volume similar to Australopithecus, and a cranial morphology (skull shape) similar to early Homo species. The skeletal anatomy combines primitive features known from australopithecines with features known from early hominins. The individuals show signs of having been deliberately disposed of within the cave near the time of death. The fossils have not yet been dated.[29]

The genetic revolution in studies of human evolution started when Vincent Sarich and Allan Wilson measured the strength of immunological cross-reactions of blood serum albumin between pairs of creatures, including humans and African apes (chimpanzees and gorillas).[30] The strength of the reaction could be expressed numerically as an immunological distance, which was in turn proportional to the number of amino acid differences between homologous proteins in different species. By constructing a calibration curve of the ID of species' pairs with known divergence times in the fossil record, the data could be used as a molecular clock to estimate the times of divergence of pairs with poorer or unknown fossil records.

In their seminal 1967 paper in Science, Sarich and Wilson estimated the divergence time of humans and apes as four to five million years ago,[30] at a time when standard interpretations of the fossil record gave this divergence as at least 10 to as much as 30 million years. Subsequent fossil discoveries, notably "Lucy", and reinterpretation of older fossil materials, notably Ramapithecus, showed the younger estimates to be correct and validated the albumin method.

Progress in DNA sequencing, specifically mitochondrial DNA (mtDNA) and then Y-chromosome DNA (Y-DNA) advanced the understanding of human origins.[31][32][33] Application of the molecular clock principle revolutionized the study of molecular evolution.

On the basis of a separation from the orangutan between 10 and 20 million years ago, earlier studies of the molecular clock suggested that there were about 76 mutations per generation that were not inherited by human children from their parents; this evidence supported the divergence time between hominins and chimps noted above. However, a 2012 study in Iceland of 78 children and their parents suggests a mutation rate of only 36 mutations per generation; this datum extends the separation between humans and chimps to an earlier period greater than 7 million years ago (Ma). Additional research with 226 offspring of wild chimp populations in 8 locations suggests that chimps reproduce at age 26.5 years, on average; which suggests the human divergence from chimps occurred between 7 and 13 million years ago. And these data suggest that Ardipithecus (4.5 Ma), Orrorin (6 Ma) and Sahelanthropus (7 Ma) all may be on the hominin lineage, and even that the separation may have occurred outside the East African Rift region.

Furthermore, analysis of the two species' genes in 2006 provides evidence that after human ancestors had started to diverge from chimpanzees, interspecies mating between "proto-human" and "proto-chimps" nonetheless occurred regularly enough to change certain genes in the new gene pool:

The research suggests:

In the 1990s, several teams of paleoanthropologists were working throughout Africa looking for evidence of the earliest divergence of the hominin lineage from the great apes. In 1994, Meave Leakey discovered Australopithecus anamensis. The find was overshadowed by Tim D. White's 1995 discovery of Ardipithecus ramidus, which pushed back the fossil record to 4.2 million years ago.

In 2000, Martin Pickford and Brigitte Senut discovered, in the Tugen Hills of Kenya, a 6-million-year-old bipedal hominin which they named Orrorin tugenensis. And in 2001, a team led by Michel Brunet discovered the skull of Sahelanthropus tchadensis which was dated as 7.2 million years ago, and which Brunet argued was a bipedal, and therefore a hominidthat is, a hominin (cf Hominidae; terms "hominids" and hominins).

Different models for the beginning of the present human species.

Anthropologists in the 1980s were divided regarding some details of reproductive barriers and migratory dispersals of the Homo genus. Subsequently, genetics has been used to investigate and resolve these issues. According to the Sahara pump theory evidence suggests that genus Homo have migrated out of Africa at least three and possibly four times (e.g. Homo erectus, Homo heidelbergensis and two or three times for Homo sapiens).

Recent evidence suggests that humans may have left Africa half a million years earlier than previously thought. A joint Franco-Indian team has found human artefacts in the Siwalk Hills north of New Delhi dating back at least 2.6 million years. This is earlier than the previous earliest finding of genus Homo at Dmanisi, in Georgia, dating to 1.85 million years. Although controversial, this strengthens the case that human tools have been found at a Chinese cave 2.48 million years ago.[37] This suggests that the Asian "Chopper" tool tradition, found in Java and northern China may have left Africa before the appearance of the Acheulian hand axe.

The "out of Africa" model proposed that modern H. sapiens speciated in Africa recently (that is, approximately 200,000 years ago) and the subsequent migration through Eurasia resulted in nearly complete replacement of other Homo species. This model has been developed by Chris B. Stringer and Peter Andrews.[38][39] In contrast, the multiregional hypothesis proposed that Homo genus contained only a single interconnected population as it does today (not separate species), and that its evolution took place worldwide continuously over the last couple million years. This model was proposed in 1988 by Milford H. Wolpoff.[40][41]

Sequencing mtDNA and Y-DNA sampled from a wide range of indigenous populations revealed ancestral information relating to both male and female genetic heritage.[42] Aligned in genetic tree differences were interpreted as supportive of a recent single origin.[43] Analyses have shown a greater diversity of DNA patterns throughout Africa, consistent with the idea that Africa is the ancestral home of mitochondrial Eve and Y-chromosomal Adam.[44]

"Out of Africa" has gained support from research using female mitochondrial DNA and the male Y chromosome. After analysing genealogy trees constructed using 133 types of mtDNA, researchers concluded that all were descended from a female African progenitor, dubbed Mitochondrial Eve. "Out of Africa" is also supported by the fact that mitochondrial genetic diversity is highest among African populations.[45]

A broad study of African genetic diversity, headed by Sarah Tishkoff, found the San people had the greatest genetic diversity among the 113 distinct populations sampled, making them one of 14 "ancestral population clusters". The research also located the origin of modern human migration in south-western Africa, near the coastal border of Namibia and Angola.[46] The fossil evidence was insufficient for Richard Leakey to resolve this debate.[47] Studies of haplogroups in Y-chromosomal DNA and mitochondrial DNA have largely supported a recent African origin.[48] Evidence from autosomal DNA also predominantly supports a Recent African origin. However, evidence for archaic admixture in modern humans had been suggested by some studies.[49]

Recent sequencing of Neanderthal[50] and Denisovan[14] genomes shows that some admixture occurred. Modern humans outside Africa have 24% Neanderthal alleles in their genome, and some Melanesians have an additional 46% of Denisovan alleles. These new results do not contradict the "out of Africa" model, except in its strictest interpretation. After recovery from a genetic bottleneck that might be due to the Toba supervolcano catastrophe, a fairly small group left Africa and briefly interbred with Neanderthals, probably in the middle-east or even North Africa before their departure. Their still predominantly African descendants spread to populate the world. A fraction in turn interbred with Denisovans, probably in south-east Asia, before populating Melanesia.[51]HLA haplotypes of Neanderthal and Denisova origin have been identified in modern Eurasian and Oceanian populations.[16]

There are still differing theories on whether there was a single exodus from Africa or several. A multiple dispersal model involves the Southern Dispersal theory,[52] which has gained support in recent years from genetic, linguistic and archaeological evidence. In this theory, there was a coastal dispersal of modern humans from the Horn of Africa around 70,000 years ago. This group helped to populate Southeast Asia and Oceania, explaining the discovery of early human sites in these areas much earlier than those in the Levant.[52]

A second wave of humans may have dispersed across the Sinai Peninsula into Asia, resulting in the bulk of human population for Eurasia. This second group possibly possessed a more sophisticated tool technology and was less dependent on coastal food sources than the original group. Much of the evidence for the first group's expansion would have been destroyed by the rising sea levels at the end of each glacial maximum.[52] The multiple dispersal model is contradicted by studies indicating that the populations of Eurasia and the populations of Southeast Asia and Oceania are all descended from the same mitochondrial DNA lineages, which support a single migration out of Africa that gave rise to all non-African populations.[53]

Stephen Oppenheimer, on the basis of the early date of Badoshan Iranian Aurignacian, suggests that this second dispersal, may have occurred with a pluvial period about 50,000 years before the present, with modern human big-game hunting cultures spreading up the Zagros Mountains, carrying modern human genomes from Oman, throughout the Persian Gulf, northward into Armenia and Anatolia, with a variant travelling south into Israel and to Cyrenicia.[54]

Human evolution is characterized by a number of morphological, developmental, physiological, and behavioral changes that have taken place since the split between the last common ancestor of humans and chimpanzees. The most significant of these adaptations are bipedalism, increased brain size, lengthened ontogeny (gestation and infancy), and decreased sexual dimorphism. The relationship between these changes is the subject of ongoing debate.[55][pageneeded] Other significant morphological changes included the evolution of a power and precision grip, a change first occurring in H. erectus.[56]

Bipedalism is the basic adaptation of the hominin and is considered the main cause behind a suite of skeletal changes shared by all bipedal hominins. The earliest hominin, of presumably primitive bipedalism, is considered to be either Sahelanthropus[57] or Orrorin, both of which arose some 6 to 7 million years ago. The non-bipedal knuckle-walkers, the gorilla and chimpanzee, diverged from the hominin line over a period covering the same time, so either of Sahelanthropus or Orrorin may be our last shared ancestor. Ardipithecus, a full biped, arose somewhat later.[citation needed]

The early bipeds eventually evolved into the australopithecines and later the genus Homo. There are several theories of the adaptation value of bipedalism. It is possible that bipedalism was favored because it freed the hands for reaching and carrying food, saved energy during locomotion,[58] enabled long distance running and hunting, provided an enhanced field of vision, and helped avoid hyperthermia by reducing the surface area exposed to direct sun; features all advantageous for thriving in the new savanna environment versus the previous forest habitat.[32][58][59] A new study provides support for the hypothesis that walking on two legs, or bipedalism, evolved because it used less energy than quadrupedal knuckle-walking.[60][61] However, recent studies suggest that bipedality without the ability to use fire would not have allowed global dispersal.[62]

Anatomically, the evolution of bipedalism has been accompanied by a large number of skeletal changes, not just to the legs and pelvis, but also to the vertebral column, feet and ankles, and skull.[63] The femur evolved into a slightly more angular position to move the center of gravity toward the geometric center of the body. The knee and ankle joints became increasingly robust to better support increased weight. To support the increased weight on each vertebra in the upright position, the human vertebral column became S-shaped and the lumbar vertebrae became shorter and wider. In the feet the big toe moved into alignment with the other toes to help in forward locomotion. The arms and forearms shortened relative to the legs making it easier to run. The foramen magnum migrated under the skull and more anterior.[64]

The most significant changes occurred in the pelvic region, where the long downward facing iliac blade was shortened and widened as a requirement for keeping the center of gravity stable while walking;[65] bipedal hominids have a shorter but broader, bowl-like pelvis due to this. A drawback is that the birth canal of bipedal apes is smaller than in knuckle-walking apes, though there has been a widening of it in comparison to that of australopithecine and modern humans, permitting the passage of newborns due to the increase in cranial size but this is limited to the upper portion, since further increase can hinder normal bipedal movement.[66]

The shortening of the pelvis and smaller birth canal evolved as a requirement for bipedalism and had significant effects on the process of human birth which is much more difficult in modern humans than in other primates. During human birth, because of the variation in size of the pelvic region, the fetal head must be in a transverse position (compared to the mother) during entry into the birth canal and rotate about 90 degrees upon exit.[67] The smaller birth canal became a limiting factor to brain size increases in early humans and prompted a shorter gestation period leading to the relative immaturity of human offspring, who are unable to walk much before 12 months and have greater neoteny, compared to other primates, who are mobile at a much earlier age.[59] The increased brain growth after birth and the increased dependency of children on mothers had a big effect upon the female reproductive cycle,[68] and the more frequent appearance of alloparenting in humans when compared with other hominids.[69] Delayed human sexual maturity also led to the evolution of menopause with one explanation providing that elderly women could better pass on their genes by taking care of their daughter's offspring, as compared to having more of their own.[70]

The human species developed a much larger brain than that of other primatestypically 1,330 cm3 in modern humans, over twice the size of that of a chimpanzee or gorilla.[71] The pattern of encephalization started with Homo habilis,[72] which at approximately 600cm3 had a brain slightly larger than that of chimpanzees, and continued with Homo erectus (8001,100cm3), reaching a maximum in Neanderthals with an average size of (1,2001,900cm3), larger even than Homo sapiens. The pattern of human postnatal brain growth differs from that of other apes (heterochrony) and allows for extended periods of social learning and language acquisition in juvenile humans. However, the differences between the structure of human brains and those of other apes may be even more significant than differences in size.[73][74][75][76]

The increase in volume over time has affected areas within the brain unequallythe temporal lobes, which contain centers for language processing, have increased disproportionately, and seems to favor a belief that there was evolution after leaving Africa, as has the prefrontal cortex which has been related to complex decision-making and moderating social behavior.[71] Encephalization has been tied to an increasing emphasis on meat in the diet,[77][78][79] or with the development of cooking,[80] and it has been proposed that intelligence increased as a response to an increased necessity for solving social problems as human society became more complex.[81] The human brain was able to expand because of the changes in the morphology of smaller mandibles and mandible muscle attachments to the skull into allowing more room for the brain to grow.[82]

The increase in volume of the neocortex also included a rapid increase in size of the cerebellum. Traditionally the cerebellum has been associated with a paleocerebellum and archicerebellum as well as a neocerebellum. Its function has also traditionally been associated with balance, fine motor control but more recently speech and cognition. The great apes including humans and its antecessors had a more pronounced development of the cerebellum relative to the neocortex than other primates. It has been suggested that because of its function of sensory-motor control and assisting in learning complex muscular action sequences, the cerebellum may have underpinned the evolution of human's technological adaptations including the preadaptation of speech.[83][84][85][86]

The reason for this encephalization is difficult to discern, as the major changes from Homo erectus to Homo heidelbergensis were not associated with major changes in technology. It has been suggested that the changes have been associated with social changes, increased empathic abilities[87][88] and increases in size of social groupings[89][90][91]

The reduced degree of sexual dimorphism is visible primarily in the reduction of the male canine tooth relative to other ape species (except gibbons) and reduced brow ridges and general robustness of males. Another important physiological change related to sexuality in humans was the evolution of hidden estrus. Humans and bonobos are the only apes in which the female is fertile year round and in which no special signals of fertility are produced by the body (such as genital swelling during estrus).

Nonetheless, humans retain a degree of sexual dimorphism in the distribution of body hair and subcutaneous fat, and in the overall size, males being around 15% larger than females. These changes taken together have been interpreted as a result of an increased emphasis on pair bonding as a possible solution to the requirement for increased parental investment due to the prolonged infancy of offspring.

A number of other changes have also characterized the evolution of humans, among them an increased importance on vision rather than smell; a smaller gut; loss of body hair; evolution of sweat glands; a change in the shape of the dental arcade from being u-shaped to being parabolic; development of a chin (found in Homo sapiens alone); development of styloid processes; and the development of a descended larynx.

The evidence on which scientific accounts of human evolution are based comes from many fields of natural science. The main source of knowledge about the evolutionary process has traditionally been the fossil record, but since the development of genetics beginning in the 1970s, DNA analysis has come to occupy a place of comparable importance. The studies of ontogeny, phylogeny and especially evolutionary developmental biology of both vertebrates and invertebrates offer considerable insight into the evolution of all life, including how humans evolved. The specific study of the origin and life of humans is anthropology, particularly paleoanthropology which focuses on the study of human prehistory.[92]

The closest living relatives of humans are bonobos and chimpanzees (both genus Pan) and gorillas (genus Gorilla).[93] With the sequencing of both the human and chimpanzee genome, current estimates of the similarity between their DNA sequences range between 95% and 99%.[93][94][95] By using the technique called the molecular clock which estimates the time required for the number of divergent mutations to accumulate between two lineages, the approximate date for the split between lineages can be calculated.

The gibbons (family Hylobatidae) and then orangutans (genus Pongo) were the first groups to split from the line leading to the hominins, including humansfollowed by gorillas, and, ultimately, by the chimpanzees (genus Pan). The splitting date between hominin and chimpanzee lineages is placed by some between 4to8 million years ago, that is, during the Late Miocene.[3][96][97]Speciation, however, appears to have been unusually drawn-out. Initial divergence occurred sometime between 7to13 million years ago, but ongoing hybridization blurred the separation and delayed complete separation during several millions of years. Patterson (2006) dated the final divergence at 5to6 million years ago.[98]

Genetic evidence has also been employed to resolve the question of whether there was any gene flow between early modern humans and Neanderthals, and to enhance our understanding of the early human migration patterns and splitting dates. By comparing the parts of the genome that are not under natural selection and which therefore accumulate mutations at a fairly steady rate, it is possible to reconstruct a genetic tree incorporating the entire human species since the last shared ancestor.

Each time a certain mutation (Single-nucleotide polymorphism) appears in an individual and is passed on to his or her descendants a haplogroup is formed including all of the descendants of the individual who will also carry that mutation. By comparing mitochondrial DNA which is inherited only from the mother, geneticists have concluded that the last female common ancestor whose genetic marker is found in all modern humans, the so-called mitochondrial Eve, must have lived around 200,000 years ago.

Human evolutionary genetics studies how one human genome differs from the other, the evolutionary past that gave rise to it, and its current effects. Differences between genomes have anthropological, medical and forensic implications and applications. Genetic data can provide important insight into human evolution.

There is little fossil evidence for the divergence of the gorilla, chimpanzee and hominin lineages.[99] The earliest fossils that have been proposed as members of the hominin lineage are Sahelanthropus tchadensis dating from 7 million years ago, Orrorin tugenensis dating from 5.7 million years ago, and Ardipithecus kadabba dating to 5.6 million years ago. Each of these have been argued to be a bipedal ancestor of later hominins but, in each case, the claims have been contested. It is also possible that one or more of these species are ancestors of another branch of African apes, or that they represent a shared ancestor between hominins and other apes.

The question then of the relationship between these early fossil species and the hominin lineage is still to be resolved. From these early species, the australopithecines arose around 4 million years ago and diverged into robust (also called Paranthropus) and gracile branches, one of which (possibly A. garhi) probably went on to become ancestors of the genus Homo. The australopithecine species that is best represented in the fossil record is Australopithecus afarensis with more than one hundred fossil individuals represented, found from Northern Ethiopia (such as the famous "Lucy"), to Kenya, and South Africa. Fossils of robust australopithecines such as Au. robustus (or alternatively Paranthropus robustus) and Au./P. boisei are particularly abundant in South Africa at sites such as Kromdraai and Swartkrans, and around Lake Turkana in Kenya.

The earliest member of the genus Homo is Homo habilis which evolved around 2.8 million years ago.[4]Homo habilis is the first species for which we have positive evidence of the use of stone tools. They developed the Oldowan lithic technology, named after the Olduvai Gorge in which the first specimens were found. Some scientists consider Homo rudolfensis, a larger bodied group of fossils with similar morphology to the original H. habilis fossils, to be a separate species while others consider them to be part of H. habilissimply representing intraspecies variation, or perhaps even sexual dimorphism. The brains of these early hominins were about the same size as that of a chimpanzee, and their main adaptation was bipedalism as an adaptation to terrestrial living.

During the next million years, a process of encephalization began and, by the arrival (about 1.9 million years ago) of Homo erectus in the fossil record, cranial capacity had doubled. Homo erectus were the first of the hominins to emigrate from Africa, and, from 1.8to1.3 million years ago, this species spread through Africa, Asia, and Europe. One population of H. erectus, also sometimes classified as a separate species Homo ergaster, remained in Africa and evolved into Homo sapiens. It is believed that these species, H. erectus and H. ergaster, were the first to use fire and complex tools.

The earliest transitional fossils between H. ergaster/erectus and archaic H. sapiens are from Africa, such as Homo rhodesiensis, but seemingly transitional forms were also found at Dmanisi, Georgia. These descendants of African H. erectus spread through Eurasia from ca. 500,000 years ago evolving into H. antecessor, H. heidelbergensis and H. neanderthalensis. The earliest fossils of anatomically modern humans are from the Middle Paleolithic, about 200,000 years ago such as the Omo remains of Ethiopia; later fossils from Es Skhul cave in Israel and Southern Europe begin around 90,000 years ago (0.09 million years ago).

As modern humans spread out from Africa, they encountered other hominins such as Homo neanderthalensis and the so-called Denisovans, who may have evolved from populations of Homo erectus that had left Africa around 2 million years ago. The nature of interaction between early humans and these sister species has been a long-standing source of controversy, the question being whether humans replaced these earlier species or whether they were in fact similar enough to interbreed, in which case these earlier populations may have contributed genetic material to modern humans.[100][101]

This migration out of Africa is estimated to have begun about 70,000 years BP (Before Present) and modern humans subsequently spread globally, replacing earlier hominins either through competition or hybridization. They inhabited Eurasia and Oceania by 40,000 years BP, and the Americas by at least 14,500 years BP.[102]

Evolutionary history of the primates can be traced back 65 million years.[103] One of the oldest known primate-like mammal species, the Plesiadapis, came from North America;[104] another, Archicebus, came from China.[105] Other similar basal primates were widespread in Eurasia and Africa during the tropical conditions of the Paleocene and Eocene.

David R. Begun [106] concluded that early primates flourished in Eurasia and that a lineage leading to the African apes and humans, including to Dryopithecus, migrated south from Europe or Western Asia into Africa. The surviving tropical population of primateswhich is seen most completely in the Upper Eocene and lowermost Oligocene fossil beds of the Faiyum depression southwest of Cairogave rise to all extant primate species, including the lemurs of Madagascar, lorises of Southeast Asia, galagos or "bush babies" of Africa, and to the anthropoids, which are the Platyrrhines or New World monkeys, the Catarrhines or Old World monkeys, and the great apes, including humans and other hominids.

The earliest known catarrhine is Kamoyapithecus from uppermost Oligocene at Eragaleit in the northern Great Rift Valley in Kenya, dated to 24 million years ago.[107] Its ancestry is thought to be species related to Aegyptopithecus, Propliopithecus, and Parapithecus from the Faiyum, at around 35 million years ago.[108] In 2010, Saadanius was described as a close relative of the last common ancestor of the crown catarrhines, and tentatively dated to 2928 million years ago, helping to fill an 11-million-year gap in the fossil record.[109]

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