When Homo sapiens first expanded out of Africa, they found a world filled with challenges that our species had never encountered before, including winter. A new study confirms that part of the reason our ancestors were able to conquer the planet so quickly was that related species had forged a path. A few mating encounters were all it took to gift us the hard-won genes that assisted our expansion.
People whose ancestry lies in continents other than Africa have up to 6 percent of their genes from Neanderthals and Denisovans, along with a mysterious fourth species of which we know little. This legacy is particularly common in parts of the genome associated with the immune system. Not all our DNA is equally important, however, and geneticists have debated how much these haplotypes, or genes with a common origin, mattered in our rapid spread to every corner of the planet.
Professor Joshua Akey of the University of Washington claims to have answered this question in Current Biology. "Our work shows that hybridization was not just some curious side note to human history, but had important consequences and contributed to our ancestors' ability to adapt to different environments as they dispersed throughout the world," Akey said in a statement.
Recently, the genome sequences of 1,523 people of European, Asian, and Melanesian descent have been mapped for their Neanderthal and Denisovan genetic inheritance. Akey searched this database for haplotypes that have become so common, their frequency can only be explained if they confer an evolutionary advantage.
Most haplotypes that originate from other species were found to be sufficiently infrequent as to suggest they either made little difference or that the advantages have been balanced by drawbacks. For example, DNA that leads to highly active immune systems but also increases susceptibility to allergies has been proposed as one possibility of such a two-edged sword inheritance.
However, Akey also found 126 haplotypes so common in particular populations, they must be useful in some way. Only seven of these had previously been identified as examples of benefits from interspecies mating. Almost a quarter are associated with the immune system. In most cases, it is not yet clear what benefits a particular haplotype provides.
These advantages don't necessarily apply in all circumstances. The Denisovan genetics that allow Tibetans to thrive at high altitude are of no use to people at sea level, for example, and in 107 cases the legacy was only found in people with ancestors from a particular region.
Most of Akey's haplotypes come from the Neanderthal line. Even among 21 examples found among Melanesians, the population with the highest Denisovan genetic inheritance, only one beneficial gene variation could be definitively identified as originating from that mysterious species. In contrast, 12 came from Neanderthals, five were common to both species, and three have ambiguous origins.
We use the word Neanderthal as an insult, but they were the giant pioneers on whose shoulders we stand.