Sex with Neanderthals helped our ancestors live out of Africa

Inheriting genes from Neanderthals might have conferred a number of advantages to modern humans as they moved out of Africa and adapted to life on other continents, scientists have shown. These include Neanderthal genes linked to the immune system and to skin pigmentation, which are present at a very high frequency in modern humans.

It is now well accepted that when modern humans dispersed out of Africa and journeyed across all continents, they interacted and even bred with the archaic hominins Neanderthals and Denisovans that lived there. In our modern genomes, traces of Neanderthal DNA remain, although it constitutes only a small fraction of it.

Past research suggest that non-African individuals have inherited roughly 2% of their genomes from Neanderthals while those who have Melanesian ancestry have inherited between 2 and 4% of their genomes from Denisovans (Melanesia is a sub-region of Oceania).

Scientists have long been puzzled by the role played by these genes on our traits, biology and evolutionary history.

In the study, published in Current Biology, scientists have studied the genomes of 1,500 people from around the world, comparing them with recently constructed genome-scale maps of Neanderthal and Denisovan sequences.

Immunity and skin colour

They identified 126 high-frequency archaic Neanderthal or Denisovan haplotypes – DNA sequences at different places on the chromosomes and inherited from a single parent. Neanderthal and Denisovan DNA sequences are typically found at relatively low frequencies in modern humans, but on these 126 areas of the genome they were seen here at much higher frequencies, reaching up to 65%.

The scientists also found out that these areas are enriched for genes that influence immunity and skin pigmentation phenotype. These genes would have been advantageous and would have given modern humans useful traits to adapt to life on other continents than Africa.

"The ability to increase to such high population frequencies was most likely facilitated because these sequences were advantageous," lead author Joshua Akey, from the University of Washington School of Medicine in Seattle, explains. "In addition, many of the high-frequency sequences span genes involved in the immune system, which is a frequent target of adaptive evolution."

The evidence presented in this study therefore suggests that mating and hybridisation with archaic humans was an efficient way to was an efficient way for our ancestors to adapt to new environments during the move out of Africa.