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Researchers have long debated what happened when the indigenous Neanderthals of Europe met “modern humans” arriving from Africa starting some 40,000 years ago. The end result was the disappearance of the Neanderthals, but what happened during the roughly 10,000 years that the two human species shared a land?
A new review of the fossil record from that period has come up with a provocative conclusion: The two groups saw each other as kindred spirits and, when conditions were right, they mated.
How often this happened will never be known, but paleoanthropologist Erik Trinkaus says it probably occurred more often than is generally imagined.
In his latest work, published last week in the journal Proceedings of the National Academy of Sciences, Trinkaus, of Washington University in St. Louis, analyzed prehistoric fossil remains from various parts of Europe. He concluded that a significant number have attributes associated with both Neanderthals and the modern humans who replaced them.
“Given the data we now have, it would be highly improbable to argue there is no Neanderthal contribution to the early European population that came out of Africa,” Trinkaus said. “I believe there was continuous breeding between the two for some period of time.
“Both groups would seem to us dirty and smelly but, cleaned up, we would understand both to be human. There’s good reason to think that they did as well.”
The conclusion, one of the strongest to date in this debate, remains controversial, and it has potentially broad implications. It suggests, for instance, that humans today should still have some Neanderthal genes. It also means that the unanswered question of why the Neanderthals died out is even more puzzling — because under this scenario they were quite capable of living successfully alongside the more modern newcomers.
As with all theories regarding the Neanderthals, there are problems with the one Trinkaus and others are advancing. So far, analysis of modern humans’ DNA has turned up no identifiable Neanderthal genetic material. Instead, it points to a common East African male ancestor from about 100,000 years ago and a common East African female from 170,000 years ago. Because the sampling remains limited, evolutionary geneticists generally do not say their findings settle the matter — although an ongoing mapping of the Neanderthal genome by European researchers may change the equation.
Chris Stringer, a top paleontology researcher at the Natural History Museum in London, said the genetic evidence has kept him “on the fence” regarding Trinkaus’s theory of more widespread interbreeding. He said Neanderthals and modern humans from Africa would be considered distinct “homo” species, making interbreeding less likely but not impossible. Under stressed conditions, he said, zebras and horses will mate, as will lions and tigers, so related humans might have done the same.
But one genetic trait of modern Europeans makes him doubt there was any major Neanderthal input — the fact that most humans today are genetically ill-adapted to cold weather. Only some native Indian populations, as well as people in the north of Eurasia and aborigines in Australia (who experience deep cold at night), have good genetic defenses to cold. Since Neanderthals lived in Europe for hundreds of thousands of years, through ice ages and frigid conditions, they would have become genetically suited to such conditions, Stringer said. The fact that Europeans are not, he added, suggests that any Neanderthal contribution to their makeup is limited.
Trinkaus says that while there is no evidence that they were any less intelligent or capable than the newcomers, they seem to have had a less evolved social structure and less ability to develop new technologies. As the number of migrants from the south increased, he said, Neanderthals were to some extent absorbed into the arriving population and to some extent were outcompeted for resources.
By the time the Neanderthals were dying about 30,000 years ago, the fossil record suggests that about 10 to 20 percent of the genetic material in European humans was from Neanderthals, he says.
(Posted on April 30, 2007)