New Scientist

Image: Riebart

Until about five years ago, one feature united the ancient human species that once walked the Earth: all were well and truly extinct. The Denisovans vanished from Eurasia around 50,000 years ago and the Neanderthals some 10,000 years later, leaving only Homo sapiens. Others went the same way much earlier, leaving just a few fossils – if that – to tell their story. But we now know these species are not entirely gone. Traces of them are buried within my cells and yours.

By having sex with our direct ancestors, ancient human species made sure they left a genetic legacy that survives to this day, one with a greater significance than previously suspected. People of non-African descent inherit between 2 and 4 per cent of their DNA from Neanderthals; indigenous Melanesians get 3 to 4 per cent of theirs from Denisovans; and some hunter-gatherer groups in central Africa get a small proportion from species we haven’t even identified yet – we just know they existed. Crucially, recent studies have revealed that if you combine all the ancient DNA in living humans, you could recover a sizeable chunk of the original genomes. A study published this year suggests about 10 per cent of the Denisovan genome is still “alive”, mainly in people from Papua New Guinea. It also suggests that about 40 per cent of the Neanderthal genome can be put together from the bits living people carry. Joshua Akey of the University of Washington in Seattle thinks that figure may creep up with more research.

The last few years of genetic decoding have revealed the surprising ways in which we express this legacy. It is partly responsible for the physical variation in modern humans – red hair and freckles have links with Neanderthal DNA, for instance – and it affects our health (see “The 4 genetic traits that helped humans conquer the world“).

And hidden in all this is a far bigger story: genes from our extinct cousins helped us conquer the planet. Without them, our ancestors may not have coped with unfamiliar diseases, thrived in the thin air of the high Tibetan plateau, or withstood the chilly winds of the Arctic. Read more on newscientist.com…