Ivory carver and historian Nikolai Peristov was searching for mammoth tusks on the banks of Siberia’s Irtysh River in 2008 when he saw a thighbone in the water near the settlement of Ust’-Ishim. Peristov brought it to scientists at the Russian Academy of Sciences who identified it as belonging to a modern human, and not a Neanderthal, based on its teardrop-shaped cross section. Scientists at the University of Oxford subsequently examined samples of the fossilized femur and used radiocarbon dating methods to estimate that it was around 45,000 years old. The oldest modern human fossil ever discovered outside of Africa and the Near East, it is nearly twice as ancient as the next oldest, which comes from a boy who died in another region of Siberia around 24,000 years ago.
In 2012, a team of scientists led by geneticist Svante Paabo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany took samples from the thigh bone fossil to search for DNA. Over the past 30 years, Paabo and his colleagues have worked to develop the tools to extract and sequence fossil DNA, and their techniques have steadily improved. Previously, they were able to map an entire Neanderthal genome extracted from a toe bone; when they compared that genome to human genomes, they found that humans and Neanderthals share a common ancestor, which probably lived around 600,000 years ago.
As it turned out, the 45,000-year-old femur found by Peristov yielded an astonishing amount of genetic material. Paabo’s team, who reported their findings this week in the journal Nature, were able to use the DNA they extracted to make a highly detailed copy of the genome of Ust’-Ishim man, as the fossil was dubbed. (To start with, they found a Y chromosome, which enabled them to identify the thigh bone’s owner as a male.) When they compared that genome with those of ancient and living humans, they found that Ust’-Ishim man’s DNA was more similar to that of non-Africans than Africans, and that he was no more closely related to Europeans than to East Asians. They concluded that instead of being a direct ancestor of any modern people, he was part of an older lineage–one of the early hunter-gatherers who migrated from Africa into Europe and Asia within the past 60,000 years.
The scientists also found that Ust’-Ishim man had pieces of Neanderthal DNA in his genome, making up a total of about 2.3 percent. That Neanderthals and humans interbred at some point isn’t news: Most living people of Eurasian descent are genetically 1.6 percent to 2.1 percent Neanderthal. In a living human, the Neanderthal DNA is broken up into short segments scattered throughout the genome, probably due to the process of cell division over generations. By contrast, the scientists predicted, the genome of Ust’-Ishim man should contain longer strands of Neanderthal DNA, as the strands would not have had time to fragment.
Their predictions proved accurate. By comparing the long stretches of Neanderthal DNA found Ust’-Ishim man’s genome to the shorter strands in living humans, Paabo and his colleagues estimated the rate of fragmentation. They then used this information to determine how long ago Neanderthals and humans interbred. Previous studies based only on the genetic information of living humans suggested that the interbreeding occurred between 37,000 and 86,000 years ago. The new study narrows that window considerably, estimating that Neanderthals and humans interbred between 50,000 and 60,000 years ago.