Genetic Ghosts of Hominids Past

Last week I blogged about the strange story of our past encoded in the DNA of lice. We carry two lineages of lice, one of which our Homo sapiens ancestors may have picked up in Asia from another hominid, Homo erectus. I always get a kick imagining human beings, having migrated out of Africa around 50,000 years ago, coming face to face with other species of upright, tool-making, big-brained apes. It’s pretty clear that it happened in Europe, which was occupied by both humans and Neanderthals for several thousand years. But encountering Homo erectus would be even weirder. Studies on DNA suggest humans and Neanderthals share an ancestor dating back half a million years or so. But Homo erectus moved into Asia 1.8 million years ago. These were long-lost cousins, to put it mildly. What’s more, they almost certainly had nothing along the lines of human language. Their brains were very different too; they kept making the same stone tools they had been making since they had left Africa. I can’t help imagining it would have been an awkward encounter, or even a bloody one. Yet it was close enough for us to pick up their lice.

Hot on the heels of the lice study, a new study on human DNA offers some more support to the idea of a very intimate reunion. Until now, most studies of human genes have pointed to Africa as their origin. If you draw a tree of the various versions of a gene, the deepest branches often belong primarily to living Africans. Some genetic markers are shared almost exclusively by Europeans and Asians, which may have evolved as humans moved out of Africa. These patterns suggested that humans sweeping out of Africa did not interbreed with Neanderthals or Homo erectus. Or, if they did, none of the DNA of those other hominids is around today. But in a paper in press in Molecular Biology and Evolution, University of Arizona scientists report the discovery of a gene that flouts the pattern.

Known as RRM2P4, this gene has its roots in Asia. Over half of people sampled from South China had the oldest version of the gene, while only 1 out of 177 Africans who were surveyed had it. And by studying the variation in different versions of the gene, the researchers concluded that the most recent common ancestor of them existed 2 million years ago. The simplest explanation for this pattern is that at least a few humans and Homo erectus came face to face in Asia and had kids.

The authors point out that the gene they looked at isn’t big enough to offer a huge amount of statistical confidence. That will have to wait for other genes with Asian roots, if they’re out there. But if RRM2P4 is any guide, humans and Homo erectus didn’t just trade lice. Our hominid cousins may not have been able to survive as a species with us in the neighborhood, but all was not war between the species.