Neanderthals may have died out 30,000 years ago, but new analyses from the University of Washington and Harvard confirm that genetic traces of the stocky cave-dwellers live on in most of us.
Working separately, two research teams probed the genomes of nearly 1,700 people from Europe and east Asia in the most comprehensive survey for DNA passed down from Neanderthals who mated with early Homo sapiens.
Their results, published Wednesday, agree with previous estimates that 1 to 3 percent of the genome of living Europeans and Asians came from Neanderthal ancestors.
The UW team estimates that the average person carries at least 300 to 400 Neanderthal genes, out of a total of about 25,000. The scientists were able to zero in more precisely than ever before on where those ancestral genes are located.
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“You can think of it as finding fossils in the human genome,” said University of Pennsylvania evolutionary geneticist Sarah Tishkoff, who was not involved in the research. “I think it’s really a breakthrough.”
The approach means it could be possible to mine the genomes of living people for DNA from even more ancient and distant ancestors — including those whose fossil remains are as yet undiscovered.
“We don’t necessarily have to find ancient DNA from fossils,” said UW geneticist Joshua Akey, co-author of the study published in Science Express. “We can find ancient DNA in the genomes of modern individuals.”
Most of the Neanderthal genes appear to be relegated to parts of the genome that aren’t vital to body functions, Akey said.
But both the UW and Harvard scientists found a high concentration of Neanderthal DNA in chromosome regions that influence hair and skin. That means early humans must have picked up some valuable traits from interbreeding with Neanderthals, such as a tough skin resistant to infection or better adapted to the cold.
“It’s tempting to speculate that … these genes were important in helping humans adapt to non-African environments,” said Harvard geneticist David Reich, co-author of the second study, published in Nature.
Scientists believe modern humans first migrated out of Africa about 100,000 years ago, encountering Neanderthals who had been living throughout Eurasia for hundreds of thousands of years.
The fact that the two closely related species interbred was revealed when scientists were able to sequence Neanderthal DNA extracted from fossil bones and compare it with modern human DNA. (Neanderthals never lived in Africa, so modern Africans don’t carry their genes.)
For the new analyses, researchers used statistical methods to scan genomes collected as part of what’s called the 1000 Genomes Project and identify the DNA sequences most likely to have come from Neanderthals, said UW doctoral student and study lead author Benjamin Vernot.
Because Neanderthals and humans evolved separately for so long, their DNA has accumulated a different mix of mutations and other features that help in distinguishing between the two species, Vernot explained.
Human genomes can offer a richer source of Neanderthal DNA than fossils, because the latter represent only a few individuals. The human gene pool contains contributions from all Neanderthal ancestors.
The fact that so little Neanderthal DNA survives in modern humans suggests that interbreeding carried evolutionary costs. The near total absence of Neanderthal DNA on the X-chromosome and in the testes of modern humans hints that hybrid males might have been infertile, Reich said.
The UW team noted the marked absence of Neanderthal DNA in chromosome regions that control speech and language. That supports the theory that the beetle-browed hominids lacked the sophisticated communication abilities of humans, so Neanderthal genes related to communication would have been weeded out by evolution.
Focusing on the portions of the human genome where Neanderthal DNA has been eliminated may be one way to get a better handle on the characteristics that are uniquely human, Vernot said.
“We’ve identified all these Neanderthal genes,” he said. “Now we have to do the hard work of figuring out why we have them — and why we don’t have others.”
Sandi Doughton at: 206-464-2491 or firstname.lastname@example.org