Prehistoric people and Neanderthals shared more than caves, shows a new fossil analysis, which finds that Eurasian humans inherited a small amount, about 1-4%, of their genes from the extinct sibling species.

People and Neanderthals likely interbred 50,000 to 80,000 years ago in the Near East, concludes the international genetics team's pair of studies in today's issue of the journal Science. The research was led by German genome researcher Svante Paabo of the Max-Plank Institute for Evolutionary Anthropology. The finding splits the difference in a long-running scholarly debate over whether people are solely African in origin, or spring from "multi-regional" interbreeding of early human species.

"This paper shows that the right theory is 'Mostly Out of Africa'," says population geneticist Henry Harpending of the University of Utah in Salt Lake City. "This study is a spectacular work of (gene) sequencing technology."

For the last decade, past fossil studies of Neanderthal genes produced by the team had downplayed the odds of interbreeding, Paabo acknowledged.But the analysis of three Neanderthal era bones allowed researchers to produce a 60% complete genetic map of Neanderthals, giving them much more statistical power to discern their genetic history."It's a small but very real proportion of ancestry in non-Africans today," says study co-author David Reich of Harvard Medical School. The study is based on bone samples from three bones dated to Neanderthals 38,000 to 45,000 years old, compared to the gene maps of five modern humans worldwide.

Stocky, thick-browed and heavy-boned, the Neanderthals last shared a common ancestor with the African precursors to modern humans some 500,000 years ago. The Neanderthals populated the Near East and Europe until they vanished from the fossil record about 30,000 years ago. Thegene maps produced by the DNA analysis of the bones found Neanderthal genes scattered randomly among non-Africans, Paabo says, indicating they don't account for any racial differences between modern-day Africans and anyone else. Also the study finds no sign of human genes intruding into the Neanderthal lineage.

"From the fossil record, we might have supposed that any interbreeding would have taken place about 100,000 years ago, so this is a bit unexpected," says paleontologist Ian Tattersall of the American Museum of Natural History in New York. Prehistoric humans and Neanderthals shared roughly the same tools and lifestyle at this time, whereas by about 60,000 years ago, modern-looking humans had better tools and made decorations indicating cultures far different than Neanderthals.

The studies also revealed a few dozen genes altered in humans since they genetically diverged from Neanderthals; some related to skull and brain development. But overall, "They were not very genetically distinct from us," Paabo says.

Neanderthals and humans were still distinct species despite the interbreeding, says paleoanthropologist Richard Potts of the Smithsonian's National Museum of Natural History. Some animals "millions of years apart" in terms of splitting into distinct species can interbreed, Potts says. "This isn't so unlikely."