Modern humans are a little more Neanderthal than we thought.
A highly detailed genetic analysis of a Neanderthal woman who lived about 52,000 years ago suggests that our extinct evolutionary cousins still influence our risk of having a heart attack, developing an eating disorder and suffering from schizophrenia.
Altogether, scientists now estimate that somewhere between 1.8% and 2.6% of the DNA in most people alive today was inherited from Neanderthals, according to a report published Thursday in the journal Science.
The genetic contribution of these archaic hunter-gatherers is highest in people of East Asian descent, accounting for between 2.3% and 2.6% of their DNA.
Neanderthals lived primarily in Europe and western Asia for hundreds of thousands of years before they mysteriously disappeared around 40,000 years ago. However, in humans of western Eurasian heritage, the amount of Neanderthal DNA is a more modest 1.8% to 2.4%, researchers found.
Anthropologists believe that the ancestors of modern humans encountered Neanderthals tens of thousands of years ago, soon after they migrated out of Africa. That would explain why modern people of African descent have little to no Neanderthal DNA.
The new findings emerge from a comprehensive reconstruction of the genome of a Neanderthal woman whose skeletal remains were found in a cave in Vindija, Croatia.
She is the fourth occupant of the Vindija caves to have her DNA sequenced, but only the second Neanderthal whose genetic secrets could be reconstructed with a resolution fine enough to make comparisons to modern humans possible.
The first complete genetic analysis of a Neanderthal focused on an individual who lived in the Siberian province of Altai some 122,000 years ago. That analysis, published in 2013 in the journal Nature, prompted researchers to estimate that Neanderthals' genetic contribution to modern-day non-Africans lay between 1.5% and 2.1%.
The second genome "adds to mounting evidence that Neanderthal ancestry influences disease risk in present-day humans, particularly with respect to neurological, psychiatric, immunological and dermatological" traits, according to the new study led by Svante Pääbo and Kay Prüfer of the Max Planck Institute for Evolutionary Anthropology in Germany and colleagues. (Pääbo and Prüfer led the 2013 Nature study as well.)
Experts in evolutionary genetics say the endurance of Neanderthal DNA in modern humans makes it important to take a long view of acquired traits.
Among the newly discovered gene variants are ones that influence the buildup of LDL cholesterol (the "bad" kind that can lead to heart attacks) and belly fat, as well as the risk of developing rheumatoid arthritis. These may be scourges to modern-day humans, but at least some of the genetic propensities conferred by these stocky homonins probably helped early groups of Homo sapiens to survive, prosper and expand once they migrated out of Africa.
"Neanderthals had been living outside of Africa for hundreds of thousands of years," said Vanderbilt evolutionary geneticist Tony Capra, who was not involved in the current study. "As our closer human ancestors moved into those environments, it's possible that interbreeding with Neanderthals gave ancestral humans benefits."
In a 2009 genetic analysis, Pääbo and colleagues found scant evidence of interbreeding between Homo sapiens and Neanderthals. But as new samples yielded themselves to ever-more-complete analysis, evidence of mating between members of the two distinct peoples has grown.
Such interbreeding gave our human ancestors access to genes that were already adapted to an environment filled with new and unfamiliar challenges, Capra said.
"In general, we know from looking at modern humans that the parts of our bodies influenced the most [by Neanderthal genes] are the parts that interact with the environment — hair, skin, immune system," Capra said. As Homo sapiens made a transition from Africa to a colder, cloudier landscape in which unfamiliar germs threatened and diets shifted, the progeny of interbreeding likely adapted faster and better, he said.
Indeed, some of the newly identified Neanderthal DNA that lives on in humans is associated with blood levels of vitamin D, which is needed for strong bones. Our bodies make vitamin D when our skin is exposed to sunlight, but that commodity became less abundant with the move from Africa to Eurasia.
The new genetic data also provides some insight into the kinds of Neanderthal societies our human ancestors encountered as they moved out of Africa.
By comparing the genetic sequences of the Croatian individual and the much older one from Siberia, the researchers concluded that Neanderthals likely lived in relatively small, isolated groups of around 3,000 adults.
The Siberian Neanderthal appeared to be the product of close inbreeding within the group — scientists inferred that his parents were likely half-siblings. But the Croatian DNA suggests that extreme inbreeding was not ubiquitous.
The study of ancient DNA "really is a time machine," said Dr. Edward M. Rubin, a geneticist at the Lawrence Berkeley National Laboratory who pioneered some of the genetic reconstruction techniques used on ancient DNA samples and wasn't involved in the new study.
The decision by Pääbo's team to post all of their genetic-sequencing data online for others to pore over "has opened a very rich window that will allow large numbers of people to ask what Neanderthals were like and what they contribute to modern humans," Rubin added.
"We're just scratching the surface of what we're going to learn about Neanderthals."
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