After decades of digging, paleoanthropologists say modern humans arose in Africa some 200,000 years ago and all archaic species of humans then disappeared, surviving only outside Africa, as did the Neanderthals in Europe.

Geneticists studying DNA now say that, to the contrary, a previously unknown archaic species of human, a cousin of the Neanderthals, may have lingered in Africa until perhaps 25,000 years ago, coexisting with the modern humans and on occasion interbreeding with them.

The geneticists reached this conclusion, reported on Thursday, July 26, in the journal Cell, after decoding the entire genome of three isolated hunter-gatherer peoples in Africa, hoping to cast light on the origins of modern human evolution.

But the finding is regarded skeptically by some paleoanthropologists because of the absence in the fossil record of anything that would support the geneticists' statistical calculations.

The geneticists, led by Joseph Lachance and Sarah Tishkoff of the University of Pennsylvania, decoded the entire genomes of five men each from two Tanzanian tribes and the forest-dwelling pygmies of Cameroon. The genomes of the pygmies and the Tanzanians contained many short stretches of DNA with highly unusual sequences. Through mutation, the genomes of species that once had a common ancestor grow increasingly unlike one another.

Tishkoff's team interprets these divergent DNA sequences as genetic remnants of an interbreeding with an archaic species of

Richard Klein, a paleoanthropologist at Stanford University, said the new claim of archaic and modern human interbreeding "is a further example of the tendency for geneticists to ignore fossil and archaeological evidence, perhaps because they think it can always be molded to fit the genetics after the fact."

Tishkoff said she agreed on the need for caution in making statistical inferences, and that there are other events besides interbreeding that can make a single DNA sequence look ancient. "But when you see it at a genomewide level, it's harder to explain away," she said.

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DNA and fossils tell differing tales of human origins