A Neanderthal man

New Delhi, May 7: Scientists have sequenced the Neanderthal genome and discovered evidence for gene flow between modern humans and their extinct human relative, resolving a long-standing controversy over whether the two species had ever interbred.

The draft sequence of the Neanderthal genome released by an international research team yesterday suggests all modern Europeans and Asians carry genetic signatures from the Neanderthals, a consequence of the interbreeding believed to have occurred tens of thousands of years ago in West Asia.

The Neanderthals (Homo neanderthalensis) appear to have emerged about 400,000 years ago and disappeared some 30,000 years ago. Modern humans (Homo sapiens) began trudging out of Africa about 80,000 years ago. The data from the Neanderthal genome shows that between 1 per cent and 4 per cent of the genomes of people in Eurasia are derived from Neanderthals.

The genome study, which appeared in the US journal Science today, suggests that the Neanderthal genome is closer to the genomes of modern humans from outside Africa than to genomes of Africans.

“The most likely explanation is that the Neanderthals had interbred with (modern) humans in West Asia before human populations split in Europe and Asia,” Hernan Burbano, a team member at the Max Planck Institute of Evolutionary Anthropology in Germany, told The Telegraph.

The researchers said it was “striking” that the Neanderthals appeared as closely related to a Chinese and a Papuan individual as to a French individual. This would suggest that the gene flow between Neanderthals and modern humans had occurred before modern humans had diverged into Europeans, East Asians, or Papuans.

“The simplest explanation is the gene flow occurred before these populations diverged,” said David Reich, a senior scientist and team member at Harvard University. “The main surprise is that the gene flow did not occur specifically in Europe, even though some archaeologists had suggested that was the most likely place for gene flow,” Reich said.

Most of the genetic material for the study came from the bone fragments of three Neanderthal women excavated earlier from a cave in Croatia. But the scientists also used Neanderthal remains from Spain and Russia for comparisons.

The data suggest that the gene flow from Neanderthals to humans is likely to have occurred between 50,000 and 80,000 years ago in West Asia — a zone where humans just out of Africa passed through before moving into Europe or Asia.

Even a small number of instances of interbreeding could account for these observations, said Richard Green, an assistant professor at the University of California, Santa Cruz, and lead author of the paper.

Scientists believe that the Neanderthal sequence makes it possible to learn more about modern human evolution. “A genetic sequence common to chimpanzees and orangutans and Neanderthals but different in modern humans would mean it was a human-specific (genetic) substitution,” Burbano said. “This study has helped us create a catalogue of human-specific substitutions.”

The researchers have identified specific genes related to the development of cranial features, the collar bone and the rib cage that appear to have played an important role in the evolution of modern humans.

A change in the gene RUNX2, for instance, appears to have played a role in the unique upper body and cranial features in modern humans — which are different from these features in Neanderthals.

The genome study is consistent with fossil records. The results show that the ancestors of Neanderthals and modern humans diverged about 400,000 years ago. The Neanderthals had lived in Europe and the western Asia until they disappeared about 30,000 years ago.

“How (Neanderthals and modern humans) interacted culturally is not something we can speculate on in any meaningful way,” Green said in a statement issued through the University of California, Santa Cruz. “But knowing there was gene flow is important. It is fascinating to think about how that may have happened.”