Scientists, with the help of a pinch of fossil bone dust, have discovered that modern human beings interbred with Neanderthals tens of thousands of years ago, and that 1 to 4 percent of the genes carried by non-African people are traceable to the much-caricatured, beetle-browed cavemen.
The Neanderthal project, which took four years and involved 57 scientists, is the latest and most astonishing example of the recovery of scientifically useful information from "ancient DNA."
The new data answer a few of the many questions about modern human beings' relationship with their last big hominin competitors, who died out about 30,000 years ago. The data also hint at what Homo sapiens had — but Homo neanderthalensis didn't — that may have made the difference between survival and extinction.
"What this means is that Neanderthals are not totally extinct. In some of us they live on," said Svante Paabo of the Max-Planck Institute for Evolutionary Anthropology in Germany, who led the genome reconstruction described in the Friday's issue of the journal Science.
The findings show that modern humans and Neanderthals, known to occupy the same parts of Europe shortly before the Neanderthals disappeared, interbred with Neanderthals someplace in the Middle East about 80,000 years ago.
The interbreeding occurred after modern humans left Africa but before they diversified, through chance and natural selection, into the ethnic groups that exist today. That's why northern Europeans, the Chinese and Papua New Guineans carry traces of Neanderthal genes, but Africans do not.
Until recently, the recovery of genetic material from objects subjected to the elements for 400 centuries was only the subject of science fiction. It requires high-speed, accurate DNA sequencing and elaborate mathematics and software to assemble the molecular equivalent of million-piece jigsaw puzzle. Even 10 years ago it would have been impossible.
"I really thought until six or seven years ago that it would remain impossible, at least in my lifetime, to get a complete Neanderthal genome," Paabo, who turned 55 last month, told reporters in a telephone news conference this week.
The Neanderthal genes were recovered from three bones excavated in a cave in Croatia about 20 years ago. One is 38,000 years old, another 44,000 years old and one is undated. They appear to be shin bones and all are from females. They also seem to have been intentionally broken, possibly to get at the marrow to eat.
The researchers removed half a gram of bone powder with a dental drill. More than 95 percent of the DNA in the sample belonged to bacteria and fungi, not to the Neanderthals. The scientists used a variety of techniques to eliminate the microbial DNA, recover the non-microbial DNA through polymerase chain reaction amplification, and assure themselves they had Neanderthal material and not modern human contaminants.
Proto-humans and chimpanzees diverged from each other about 6.5 million years ago. Modern humans and Neanderthals diverged about 825,000 years ago. On a genetic level, Neanderthals and modern humans are almost as closely related as today's ethnic groups are to each other.
Nevertheless, there are detectable differences.
The researchers identified 73 genes for which all modern people have the same molecular version but for which Neanderthals have the more ancient, chimpanzee version. In five of the genes, there are two molecular differences between the human and Neanderthal-and-chimpanzee versions, suggesting there may be an especially important difference in the human version.
One of those genes encodes a protein that helps the sperm cell's flagellum beat. Another is for a protein that seems to be involved in the healing of wounds. A third is for a protein abundant in skin, sweat glands and hair roots.
Successful reproduction, survival after injury and the ability to interact optimally with the environment are crucial to a species' survival. Those traits are obvious "targets" for natural selection.
Whether evolutionary changes in those three genes — or in the dozens of others showing hints of natural selection — proved especially beneficial in Homo sapiens' competition with other hominins (including Neanderthals) isn't known. The new research suggests they might be, and evolutionary biologists will be looking at them for further evidence they may hold part of the answer of what makes human beings human.
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