Oldest human DNA narrows time of Neandertal hookups

DNA of a 45,000-year-old Siberian man, the oldest modern human genetic material retrieved to date, indicates that he lived a short time after Homo sapiens interbred with Neandertals, a new report finds.

This ancient man belonged to a population that was related to earlier people who left Africa and split into European and Central Asian lines, paleogeneticist Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and colleagues report in the Oct. 23 Nature.

An amateur fossil hunter found the ancient man’s leg bone sticking out of an eroding river bank near the Siberian settlement of Ust’-Ishim in 2008. Pääbo’s team estimated how long ago the man lived from measurements of the decay rate of radioactive carbon in the bone.

It’s possible that the Stone Age man belonged to a group that left cutting tools at several Siberian sites dating to as early as 47,000 years ago, the scientists say. No fossils have been found at those sites, raising questions about whether modern humans or Neandertals made the tools.

The ancient man’s DNA “shows that there were indeed modern humans in the area of those Siberian sites who could have made stone tools,” says Max Planck paleogeneticist and study coauthor Janet Kelso.

Nine samples of crushed bone from the fossil yielded genetic sequences covering about two-thirds of the ancient man’s genome, which allowed for comparisons with the DNA of modern people. The Siberian man displays a genetic link to Andaman Islanders, who are thought to descend from an ancient human migration along the coast of South Asia, the researchers found. The Andaman Islands lie between India and Southeast Asia. The man is also related to East Asians and Native Americans — considered to be descendants of a northern migration through Asia. Perhaps the Ust’-Ishim individual hailed from a third wave of Asian colonists that contributed some genes to ancestors of present-day Asians before dying out, the investigators suggest.

Comparisons with Neandertal DNA reveal that he shared 2.3 percent of his genes with Neandertals. Present-day East Asians carry a 1.7 to 2.1 percent genetic contribution from Neandertals, a figure that falls slightly, to between 1.6 and 1.8 percent, in living Europeans.

Based on this new data, modern humans likely interbred with Neandertals between about 50,000 and 60,000 years ago, the scientists estimate. The bulk of mating between the two hominid species occurred during that time, they say, rather than over a longer period extending further back in the Stone Age (SN: 11/3/12, p. 8).

No signs of interbreeding with Neandertal cousins called Denisovans appeared in the Siberian man’s DNA. That makes sense, remarks Stanford University paleogeneticist Morten Rasmussen, since Denisovan genes cluster among present-day Southeast Asians (SN: 9/22/12, p. 5).

Evidence that Neandertal interbreeding probably began no more than 60,000 years ago supports the idea that a wave of H. sapiens dispersed out of Africa around that time, occasionally mated with Neandertals and passed Neandertal DNA to human populations that led to all non-Africans today, says paleoanthropologist Chris Stringer of the Natural History Museum in London.

Paleogeneticist Mattias Jakobsson of Uppsala University in Sweden finds it intriguing that the Ust’-Ishim man displays equal relatedness to present-day Asians, a 24,000-year-old Siberian child and a 7,000-year-old Spanish hunter-gatherer (SN: 5/17/14, p. 26). Unlike Pääbo’s team, Jakobsson suspects the Ust’-Ishim man belonged to a population that has yet to be pinned down with ancient DNA but nonetheless helped give rise to current East Asians and Europeans.