Researchers at the University of California, Davis have presented a study suggesting that interbreeding with Neanderthals didn’t introduce highly deleterious DNA into the human genome, which is why these genes persisted in modern humans for many thousands of years.
Modern humans and Neanderthals are thought to have done the dirty sometime between 47,000 and 65,000 years ago, shortly after humans first left Africa. While this would have given rise to human-Neanderthal hybrids with roughly equal amounts of DNA from each subspecies, modern Europeans only have between 1.5 and 2.1 percent Neanderthal material in their genomes.
The events that led to the purging of Neanderthal genes are not traceable, but it has been hypothesized that this DNA was eliminated very quickly from the human genome, on the assumption that the hybrids faced major reproductive disadvantages that caused them – and their genes – to die out fairly rapidly.
However, by analyzing the modern human genome and comparing it to that of Neanderthals, the authors of a new study in the journal PLOS Genetics have come up with a formula that allows them to predict the rate at which these genes were eliminated.
According to their findings, alleles – or gene variants – inherited from Neanderthals had only decreased by 56 percent, tens of thousands of years after the initial interbreeding occurred. This suggests that they were eliminated very slowly, indicating that they probably weren’t as detrimental to the reproductive success of hybrids as previous theories claimed.
Instead, the study authors propose that most of the Neanderthal alleles that entered the modern human genome were probably either “neutral” or “weakly deleterious”, and were therefore frequently passed on – especially among small early populations.
The researchers suggest that the elimination of these alleles was then accelerated as human populations grew in size, as this provided more opportunities for natural selection to start weeding out these ever-so-slightly deleterious genes.
Based on this finding, the team concludes that it was population size, rather than reproductive incompatibility, that drove the elimination of Neanderthal alleles from the human genome.