The genomes of many people of non-African descent alive today contain a small percentage of Neanderthal DNA, as a result of interbreeding between modern humans and Neanderthals before the species went extinct around 40,000 years ago. It is estimated that between one and four percent of the modern, non-African genome is Neanderthal.
In one study, researchers used computer simulations to show that the larger effective population size of ancient Homo sapiens compared to Neanderthals – that is, more people who were reproducing – allowed natural selection to push out large swathes of Neanderthal DNA from the human genome. This led to the removal of genes that were harmless to Neanderthals, but may have had small but damaging effects on their human carriers.
'Weakly deleterious variants that could persist in Neanderthals could not persist in [early modern] humans,' said Ivan Juric, a geneticist at 23andMe and first author of the study, published in PLOS Genetics. 'We think that this simple explanation can account for the pattern of Neanderthal ancestry that we see today along the genome of modern humans.'
Findings from a separate study, published in Current Biology, suggested that the remaining Neanderthal DNA in modern humans might have given our ancestors important advantages. The researchers in this second study found ancient sequences of DNA that were present at frequencies higher than expected if they were not proving beneficial. These groups of genes, known as haplotypes, were related to the immune system and skin pigmentation.
Through scouring the genomes of people from Europe, East and South Asia, and Island Melanesia, the team found 126 haplotypes that originated in Neanderthals as well as Denisovans, another ancient hominin cousin.
'Our work shows that hybridisation 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,' said Dr Joshua Akey, a geneticist from the University of Washington School of Medicine, Seattle, and senior author of the second study.
It seems that, while natural selection whittled away the majority of Neanderthal genes, our ancestors kept a handful of advantageous alleles that may have provided, as Dr Akey said, 'an efficient way for modern humans to quickly adapt to the new environments they were encountering'.