A pair of genes that control how the body burns fat for warmth in the cold, which would have helped prehistoric humans as they spread out of Africa into Siberia and eventually the Americas, came from interbreeding with Denisovans, an extinct species of human, according to reports.
This is the conclusion of new genetic research on Greenland Inuit with low European mixing, living in villages on the coast of Baffin Bay, who almost universally exhibit a highly unusual genetic signature in two specific genes.
The discovery raises the remarkable possibility that the success of Inuit over millennia in the frozen Arctic was not solely due to cultural knowledge and technological skills with insulation and nutrition, but was also supported by newly acquired genetic and biological traits.
The finding, published Tuesday in the journal Molecular Biology and Evolution, is the latest example of “archaic introgression,” or the introduction of genetic material into homo sapiens from extinct species in the genus homo, such as Neanderthals or Denisovans, often to control things like metabolism or immunity. Another major finding in this emerging field, for example, was the discovery that Tibetans have a genetic variant, seemingly taken from Denisovans, that affects the response to hypoxia, or low oxygen due to high altitude.
Denisovans are an extinct species of humans discovered just six years ago, when a few finger bones and teeth in a Siberian cave were shown to have a unique genetic make-up. Both Denisovans and Neanderthals, which are much better known, were established in Eurasia long before homo sapiens, so they had more time to adapt to local conditions.
The interbreeding was likely limited, according to lead author Fernando Racimo of the New York Genome Project. It would have taken place somewhere in Asia, and was probably more than a single mating, but less than a complete blending of groups. Some studies suggest it only takes a few matings for the fragment of a genetic signal to be established.
Much of the genetic material modern humans acquired in this way would have been either neutral or harmful, and so some would be selected out over time. Indeed, ancient humans seem to have had a much higher portion of Neanderthal DNA than modern humans do, 10% versus about 2%.
But some was beneficial, and as humanity pressed ever further north, even past the Arctic Circle, this particular genetic variant might have been crucial, allowing body fat to be more easily converted to heat.
To find it, the researchers used a data bank of global genomic information, and compared the Greenland Inuit to archaic DNA from the Denisovan bones, looking for similarities. They found that the Inuit version of this genetic pair, near the genes WARS2 and TBX15, looks very much like the ancient one, Racimo said, but very different from how the same genes usually appear in other modern humans around the world.
The unusual version is also at high frequencies in Siberian populations, medium frequency in East Asian, low in Europeans, and non-existent in Africans, who are not thought to have interbred with Neanderthals or Denisovans at all.
“So in the genomes of the descendants of non-African modern humans, there still is the remnants of this interbreeding event,” Racimo said. “If you have any non-African ancestry, you probably have some Neanderthal ancestry as well.”
The idea is that ancient homo sapiens would have acquired this advantageous genetic trait by breeding with Denisovans in Asia, and then as they migrated further east over the land bridge to North America, they entered “a temporally and geographically extended period of selection for the archaic haplotype in modern human history,” the paper concludes.
“A likely explanation is that selection on this locus has been acting during the early phases of the peopling of the Americas, perhaps in the Siberian or the Beringian ancestors of both modern Native Americans and Greenlandic Inuit,” it reads.
That same genetic variant likely then moved back westwards to Europe during the Bronze Age, where the people were not as isolated, nor as cold, so the gene did not establish itself quite as strongly.
“So one possible hypothesis is that this gene was beneficial to modern humans as they were crossing Siberia and Beringia, as they were entering the American continent,” Racimo said. “But it is also true that the gene is also involved in a number of other functions like body fat distribution, cranio-facial morphology, bone development. So it’s not clear at the moment if that was exactly the function that led to the rise in frequency of the gene, but it’s our best hypothesis at the moment.”