THE ancestors of today鈥檚 Europeans interbred with another early human species. That鈥檚 one explanation being proposed to explain the discovery of a peculiar stretch of DNA in some modern Europeans.
As part of an extensive gene-mapping programme, researchers at deCODE Genetics in Reykjavik, Iceland, have been searching for places in the human genome where chunks of DNA containing many genes get turned back-to-front. One of these inversions, on chromosome 17, caught their eye because it turns up in about 20 per cent of Europeans, yet is rare in Africans and almost absent in Asians.
When the researchers looked at the families of nearly 30,000 Icelanders, they found that women who have the inversion had, on average, 3.5 per cent more children than women who did not (Nature Genetics, DOI: 10.1038/ng1508). 鈥淭hat is a very significant impact when you think of an evolutionary timescale,鈥� says Kari Stefansson, deCODE鈥檚 chief executive. However, the inversion is almost a million DNA letters long, and the researchers do not know which gene or genes within it are responsible for the effect.
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Because stretches of DNA like this one remain back-to-front down the generations, it is possible to roughly date the origin of a particular inversion by counting the number of genetic differences that have accumulated in it compared to a normal DNA sequence. This particular inversion has so many differences that it must have occurred about 3 million years ago, well before modern humans evolved.
鈥淔inding any genetic variant that affects fertility is really startling, and raises lots of questions 鈥� like why doesn鈥檛 everybody have this?鈥� says David Reich, a geneticist at Harvard University. After all, 3 million years is plenty of time for such an advantageous trait to spread. One possibility is that the gene鈥檚 fertility advantage may be offset by some unknown disadvantage that prevents it from sweeping through the entire population.
But Stefansson offers another intriguing possibility 鈥� that the inversion was native to some other species of early human, and came to our own species only about 50,000 years ago. 鈥淭here aren鈥檛 all that many ways you can explain it except by the reintroduction into the modern human population,鈥� he says. 鈥淭hat raises the possibility it was reintroduced by cross-breeding with earlier species.鈥�
Such a relatively recent arrival would explain why the inversion has not spread to everyone. It would also explain why, despite its age, copies of the inversion in different individuals are remarkably similar. Rare interbreeding events would have brought only one or a few variants into the population. In the absence of DNA samples from these other hominid species, however, Stefansson admits his conjecture will be hard to prove.
