Be careful whom you call a Neanderthal. You may be one yourself. Or at least you may have Neanderthal ancestors.
That's the conclusion of a study being released Thursday that examined DNA extracted from Neanderthal bones more than 35,000 years old.
More Than A Handshake?
There's little question that modern humans and Neanderthals bumped into each other once upon a time.
"The archaeological record shows they overlapped between about 30,000 and 80,000 years ago," says David Reich, a geneticist at Harvard Medical School.
There was some fossil evidence that they may have done more than shake hands in passing, but the initial genetic evidence suggested otherwise.
"And so the big open question was whether they exchanged genes during that time when they overlapped in the Middle East and in Europe," he says.
To find out whether the two human species interbred, scientists from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, launched an audacious effort to sequence all 3 billion letters of Neanderthal DNA. As hard as it may be to believe, fragments of DNA can survive in bones that are more than 30,000 years old. New techniques can sequence those fragments, and new software can line the fragments up.
The main strategy for finding whether modern humans and Neanderthals exchanged genes was to compare the Neanderthal DNA with DNA from modern humans -- some whose ancestors had remained in Africa, and others who had migrated to Europe, Asia and the New World. If the Neanderthal DNA was more similar to the non-African DNA, it would indicate that Neanderthals contributed their genes after the first modern humans migrated out of Africa.
Reich says that's what they found.
"The non-African is more closely related to the Neanderthal," Reich says. "Quite definitely. Highly statistically significantly overwhelming evidence that they're more closely related on average to Neanderthals."
In other words, Reich and his Leipzig co-authors are totally sure that Neanderthal genes found their way into modern humans when the two species intersected. They report their findings in the journal Science.
"The simplest possible explanation is that it occurred once," Reich says. "But it very easily could have occurred on multiple occasions. Perhaps it's likely that it occurred on multiple occasions."
Reich says it's hard to pin down exactly how much DNA Neanderthals contributed to modern humans. "We estimate about 1 to 4 percent of the genetic ancestry of non-Africans is from Neanderthals," he says.
'Neanderthal' Is No Insult
And just to be clear: Reich says there's no stigma to have a bit of Neanderthal heritage.
"Absolutely not," he says. "In fact, people who have Neanderthal DNA have done just fine in our society."
Now that scientists have the sequence, they can start looking for regions of DNA that differ between Neanderthals and humans.
"And then, once the regions are identified, [we can] look to see what genes are here, what biological sense can we make of this," geneticist Richard Green say.
Green recently left Leipzig to join the faculty of the University of California, Santa Cruz. He says they've already begun to identify genes that have changed -- for example, genes that control the tails on sperm, genes that are involved in wound healing, genes that affect the shape of our hair.
Learning About Ourselves
Along with the chimpanzee genome completed a few years ago, the Neanderthal genome will help scientists like Green pin down the genetic changes that made modern humans human.
"Which one of these happened very recently, since we split even from Neanderthals? And which ones of these are much older and happened in the 6 million years or so since we diverged from chimpanzees?" Green says.
Green says for the moment, the real value of the Neanderthal genome is what it tells us about how modern human evolved. John Hawks, an anthropologist at the University of Wisconsin, agrees.
Hawks compares this finding to the first time humans were able to look back at the Earth from beyond Earth's orbit.
"We can say, 'Here's a picture of what we are from the outside -- from the distant past,' " says Hawks. "And we can use that to get a new picture of what we are. It's amazing. I can't express how I feel this morning. It's really cool."
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