Ancient human bone helps date our first sex with Neanderthals
Oldest genome sequence of a modern human suggests Homo sapiens first bred with Neanderthals 50,000-60,000 years ago
An ancient leg bone found by chance on the bank of a Siberian river has helped scientists work out when early humans interbred with our extinct cousins, the Neanderthals.
A local ivory carver spotted the bone sticking out of sediments while fossil hunting in 2008 along the Irtysh river near the settlement of Ust’-Ishim in western Siberia. The bone was later identified as a human femur, but researchers have learned little else about the remains until now.
The importance of the find became clear when a team led by Svante Pääbo and Janet Kelso at the Max Planck Institute for Evolutionary Anthropology in Leipzig ran a series of tests on the fragile material.
Radiocarbon dating of pieces of the leg bone put the remains at around 45,000 years old. The team went on to extract DNA from the bone, which allowed them to reconstruct the oldest modern human genome ever.
The genetic material showed that the thigh bone belonged to a man who carried about 2% Neanderthal DNA, a similar amount to people from Europe and Asia today. The presence of Neanderthal DNA meant that interbreeding between them and modern humans must have taken place at least 45,000 years ago.
But amid the DNA were more clues to when humans and Neanderthals reproduced. Strands of Neanderthal DNA found in modern humans can act like a biological clock, because they are fragmented more and more with each generation since interbreeding happened. The strands of Neanderthal DNA in the Siberian man were on average three times longer than those seen in people alive today. Working backwards, the scientists calculate that Neanderthals contributed to the man’s genetic ancestry somewhere between 7,000 and 13,000 years before he lived.
The findings, published in the journal Nature, suggest that humans and Neanderthals had reproductive sex around 50,000 to 60,000 years ago, though other couplings might well have happened later. Until now, estimates for interbreeding have varied enormously, ranging from 37,000 to 86,000 years ago.
“What we think may be the case is that the ancestors of the Ust’-Ishim man met and interbred with Neanderthals during the initial early admixture event that is shared by all non-Africans at between 50,000 and 60,000 years ago, and perhaps somewhere in the middle East,” Kelso told the Guardian.
But a small number of fragments of Neanderthal DNA in the man’s genome were longer than expected given how many generations had passed. Those might be evidence of his ancestors breeding with Neanderthals closer to the time he was born.
“Everyone outside Africa has about same amount of Neanderthal DNA. It seems to be something early on where one really mixed with Neanderthals in a serious way,” said Pääbo. “Since that happened I wouldn’t be surprised if, now and again, one did it here and there later on too.”
Prior to the latest study, the oldest modern human genome came from the 24,000-year-old remains of a boy buried at Mal’ta near Lake Baikal in easterbn Siberia.
Chris Stringer, head of human origins at the Natural History Museum in London, said the ancient DNA from the Siberian man sheds fresh light on the story of early human migrations out of Africa. In the 1920s and 30s, researchers found 100,000-year-old skeletons of modern humans in caves in Israel. The remains may have belonged to a group of humans that left Africa and ultimately went on to colonise southern Asia, Australia and New Guinea. But an alternative explanation is that they were from a migration that failed to go much further. According to that view, the more successful dispersal of humans out of Africa happened much later, around 60,000 years ago.
The latest findings suggest that the ancestors of modern Australians, who carry a similar amount of Neanderthal DNA to Europeans and Asians, are unlikely to have picked up their own Neanderthal DNA before 60,000 years ago. “The ancestors of Australasians must have been part of a late, rather than early, dispersal through Neanderthal territory,” Stringer said.
“While it is still possible that modern humans did traverse southern Asia before 60,000 years ago, those groups could not have made a significant contribution to the surviving modern populations outside of Africa, which contain evidence of interbreeding with Neanderthals,” he added.