Neanderthal-Human Sex Bred Light Skins and Infertility
IT IS surprising what a little hanky-panky can do. A handful of sexual encounters between humans and Neanderthals made many of us what we are today, affecting both our appearance and our vulnerability to disease. But the genetic legacy left by the Neanderthals also highlights just how different we are from our sister species.
Neanderthals lived in Europe and Asia between about 200,000 and 30,000 years ago. Our species – sometimes dubbed "modern humans" – made it to Eurasia about 65,000 years ago, and so the two species had plenty of time to cosy up. In 2010, geneticists discovered that they had been very close neighbours indeed. They sequenced a Neanderthal genome and discovered it carried genes that also appear in the genomes of people of European and Asian descent: our species must have interbred with Neanderthals.
Now, by studying Neanderthal genes in people alive today, researchers are beginning to appreciate how that interbreeding influenced our species.
In one new study of 1000 human genomes, Sriram Sankararaman and David Reich of Harvard Medical School and colleagues found that Neanderthal DNA is most common in regions of the genome with the greatest genetic variability, making them a prime target for natural selection. While Neanderthal DNA may make up only 1.6 to 1.8 per cent of the Eurasian genome, it punches above its weight in terms of biological impact, says Reich (Nature, DOI: 10.1038/nature12961).
Joshua Akey and Ben Vernot of the University of Washington in Seattle have analysed the Neanderthal DNA in a further 665 humans (Science, DOI: 10.1126/science.1245938). Both their study and the Harvard one found a hotspot of Neanderthal ancestry in genes relating to keratin, a fibrous protein found in our hair, skin and nails.
One of the genes, BNC2, is involved in skin pigmentation. That implies that Eurasians owe their paler skins partly to Neanderthals. Light skin is an advantage at higher latitudes because it is more efficient at generating vitamin D from sunlight, so Neanderthal DNA may have helped modern humans to adapt to life outside Africa.
If so, the adaptation took thousands of years to become universal. A third study published this week describes a DNA analysis of one person who lived in Stone Age Europe about 7000 years ago – 40,000 years after any Neanderthal interbreeding. His genes suggest his skin was dark (Nature, doi.org/q74). It may be that the Neanderthal keratin affected early Eurasians' hair instead, perhaps straightening it.
Not all of the Neanderthal genes are beneficial. Sankararaman and Reich found that our Neanderthal inheritance includes several genes that make us susceptible to diseases including type 2 diabetes, lupus and Crohn's disease.
Some of the genes, meanwhile, appear to have led to fertility problems. For instance, Sankararaman found that the X chromosome is almost devoid of Neanderthal DNA. This suggests that most Neanderthal DNA that wound up on the X chromosome made the bearer less fertile – a common occurrence when related but distinct species interbreed – and so it quickly disappeared from the human gene pool. "Neanderthal alleles were swept away," says Sankararaman.
"This underlines that modern humans and Neanderthals are indeed different species," says Fred Spoor of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who was not involved in any of the studies.
The genetic evidence further backs this up. Neanderthal DNA is irregularly spaced through the modern human genome rather than being fully mixed. That implies that interbreeding occurred very rarely. Sankararaman estimates it may have happened just four times.
"But these relatively few matings obviously were an important event in the history of non-Africans," says Reich.Filed under: Science
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