A LONG-awaited rough draft of the Neanderthal genome has revealed that our own DNA contains clear evidence that early humans interbred with Neanderthals.
Such interminglings have been suspected in the past, but there’s more: Neanderthals were probably not the only other Homo species early Homo sapiens mixed with.
These findings call into question the familiar story that modern humans left Africa around 100,000 years ago and swept aside all other Homo species as they made their way around the globe. “It was a very simple story,” says João Zilhão at the University of Bristol, UK. “Its simplicity suggested it would not be true.” A more likely scenario is that as H. sapiens migrated, they met and interbred with other Homo species that have all since died out.
The first definitive evidence of interbreeding comes from Svante Pääbo’s team at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. They reported last week that the genome of humans today is roughly 1 to 4 per cent Neanderthal (Science, vol 328, p 710). This holds true for all non-Africans, suggesting that H. sapiens and Neanderthals interbred sometime between 100,000 and 45,000 years ago, after the first humans left Africa but before they split into regional populations.
Another genetic study confirms this. Jeffrey Long at the University of New Mexico in Albuquerque presented results from nearly 100 modern human populations at a meeting of the American Association for Physical Anthropologists in April. His team found evidence that Eurasians acquired genetic diversity from breeding with other Homo species after they left Africa.
They also noticed a spike in genetic diversity in Indo-Pacific peoples, dating to around 40,000 years ago. This time, it’s unlikely the diversity came from H. sapiens getting it on with Neanderthals, who never travelled that far south. That leaves a number of candidates, including Homo erectus and species related to Homo floresiensis, a small species which lived on an Indonesian island until about 13,000 years ago.
Neither Pääbo nor Long were able to show that when humans arrived in Europe they mixed with resident Neanderthals, but archaeological finds tell a different story, says Zilhão. In Portugal, his team discovered the 25,000-year-old bones of a child they are convinced is a human-Neanderthal hybrid. Zilhão says fossils from Romania and the Czech Republic also bear Neanderthal features, though others dispute this.
Moreover, decorative artefacts characteristic of humans have cropped up at Neanderthal sites, dated to around the time of contact with humans in Africa and the Middle East. Further east, 40,000-year-old human bones from a cave near Beijing, China, have features that recall other Homo species, says Erik Trinkaus of Washington University in St Louis, Missouri.
In March, Pääbo’s team reported the discovery of DNA from a hominin that is probably neither human nor Neanderthal that lived 50,000 to 30,000 years ago in a cave in southern Siberia. They dubbed the creature X-woman, and sequencing machines are already decoding its genome, says Pääbo’s colleague Ed Green of the University of California, Santa Cruz. Could X-woman or its kind have bred with humans, too? “Stay tuned,” Green says.