Neanderthals in Our Midst

/ by Lee Billings /

Genetic and fossil evidence comes together to reveal a hidden chapter of human history.

The Muierii fossils exhibit several Neanderthal-like features, including a bump at the back of the head. Photo courtesy of Erik Trinkaus.

They were first identified in 1856, when scientists unearthed ancient bones in Germany’s Neander Valley. Since then, numerous archaeological finds have revealed that Neanderthals, like early modern humans, buried their dead, skinned animals, built fires, and fashioned sophisticated tools from wood and stone. But there were differences, too: Neanderthals were stockier and had bigger noses, brows, muscles, and, curiously, brains.

Fossil evidence places the Neanderthals in Europe and western Asia roughly 230,000 years ago, where they apparently thrived for thousands of years. But about 40,000 years ago, early Homo sapiens began migrating northward, out of Africa and into Europe and Asia. Within 10,000 years, the Neanderthals had vanished from their old haunts; small, isolated Neanderthal populations hung on for a few more millennia in the Iberian Peninsula and then disappeared for good.

The specifics of the Neanderthal demise is one of the greatest mysteries in anthropology. For decades, many experts have maintained that humans completely replaced the Neanderthals, consistently out-doing them and slaughtering them when they got in the way. Other anthropologists, however, believe that rather than dying out, the Neanderthals assimilated into early human populations through interbreeding, also known as admixture.

In last week’s online edition of the Proceedings of the National Academy of Sciences, Erik Trinkaus, an anthropologist at Washington University in St. Louis, and his colleagues in Romania, published a new analysis of early modern human fossils.  These 30,000-year-old bones, discovered in the Romanian cave Pestera Muierii in 1952, provide a tantalizingly close glimpse into the transition between Neanderthals and humans. The human remains display some distinctly Neanderthal-like features, suggesting that early humans and Neanderthals may have mated with each other and reproduced.

“The Muierii fossils are just one of several examples from Romania, France, and the Czech Republic, where we have basically modern humans with three or four characteristics that are very reminiscent of Neanderthals,” such as a bulge at the back of the cranium, Trinkaus said. “Either they re-evolved these characteristics from their African ancestors, or, more likely, they acquired them through descent from the Neanderthals.”

Trinkaus is no stranger to controversial claims. Seven years ago in Portugal, he and several collaborators identified the 24,500-year-old skeleton of a young boy as another likely hybrid. The claim was hotly and publicly disputed by other top anthropologists, most notably Jeff Schwartz of the University of Pittsburgh and Ian Tattersall, curator of anthropology at the American Museum of Natural History, both co-authors of a 1999 commentary dismissing the Portugal fossils as those of a “chunky” human child. Then, as now, Tattersall and Schwartz maintain that Trinkaus’s fossils are products of normal variation, not interbreeding between species.

“This stuff conflates the way organisms, especially ones closely related to each other, can vary in similar ways—like being chunky or slender, tall or short—from the things that make them distinct species,” Schwartz said, adding that the most reliable defining characteristics of Neanderthal fossils are their large, wedge-shaped snouts and a depression, not a bulge, in the back of their heads.

Via e-mail, Tattersall agreed that the fossils are not evidence of interbreeding: “Though there may have been a bit of Pleistocene hanky-panky, it evidently did not add up to a biologically significant exchange of genes.”

To date, the strongest argument against interbreeding between humans and Neanderthals has been genetic evidence.

Researchers frequently track evolutionary changes with mitochondrial DNA (mtDNA), which is useful for tracing lineages because it is passed down directly from mother to offspring, largely without mixing with the father’s DNA. Additionally, mtDNA is easy to sequence and highly variable—allowing researchers to make fine distinctions between different populations and pinpoint when mutations occurred. In 1997, a University of Munich team headed by molecular biologist Svante Pääbo successfully compared mtDNA from Neanderthal fossils with mtDNA in modern humans. They found such great differences that Neanderthals were declared evolutionary “dead ends” that had little impact on the human genome. Several subsequent mtDNA studies have supported those results.

“It is totally clear and indisputable that the Neanderthals did not contribute any mitochondrial DNA to people living today,” said Pääbo, who is now at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. “But it is also true that mitochondrial DNA is only one tiny part of our genome—the possibility is there that they could have contributed other parts.”

Scientists have recently begun analyzing other genetic material from the human nuclear genome to determine whether it could have come from Neanderthals. Researchers believe that if interbreeding did occur, any beneficial traits from Neanderthal genes should have spread via positive natural selections in human populations, while deleterious Neanderthal traits would quickly fade away.

“The question is: Can we find genes in living humans that have some unique signature that would be consistent with them having come from a Neanderthal?” said John Hawks, an anthropologist at the University of Wisconsin-Madison. “If we picked up something useful from them, we ought to see it increasing in frequency within the past 30,000 years or so in places where we know Neanderthals and early modern humans interacted, like Europe and western Asia.”

One evolutionary biologist may have found such a signature. Last September, Bruce Lahn and his colleagues at the University of Chicago published a paper in the journal Science, in which they tracked the worldwide distribution of variants of a gene crucial for regulating brain size. These variants, called haplogroup D, proved to be prevalent, found in 70 percent of the global human population, as well as young, spreading from what may have been just a single copy introduced into the human genome roughly 37,000 years ago.

Today, Lahn’s lab will publish a follow-up paper online in The Proceedings of the National Academy of Sciences, proposing that the haplogroup D variant found in modern populations comes from Neanderthals.

The paper presents three reasons that Neanderthals are the likely source. First, fossil evidence shows that humans and Neanderthals coexisted at about the same time the variant entered the human genome. Second, by counting mutations between the D and non-D variants, Lahn’s group estimates that the two variants were isolated in separate populations for about a million years, which is roughly consistent with other estimates of Neanderthals’ isolation from early modern humans. Finally, haplotype D is more widespread in Europe and Asia than in Africa, suggesting an origin in Eurasia, the home of the Neanderthals.

“I hope our paper will trigger a major rethinking, at least in the genetics community,” Lahn said. “The idea of mixing between humans and other archaic relative species may become more accepted, shifting the discussion from whether there was admixture, to how much admixture occurred.”

At last, anthropology and genetics have a point of agreement in terms of the fate of Neanderthals. Henry Harpending, an anthropologist at the University of Utah who edited Lahn’s paper, described the analysis supporting its conclusions as “quite bulletproof.” Pääbo, made famous for his mtDNA evidence against interbreeding, hailed it as “the most compelling case to date showing a genetic contribution of Neanderthals to modern humans,” and said he plans to to seek confirmation of Lahn’s findings in his own work on the Neanderthal genome.

Originally published November 7, 2006

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