'Evolution purged many Neanderthal genes from human genome'

 Neanderthal genetic material is found in only small amounts in the genomes of modern humans because, after inter-breeding, natural selection removed large numbers of weakly deleterious Neanderthal gene variants, according to a new study.

Humans and Neanderthals interbred tens of thousands of years ago, but today, Neanderthal DNA makes up only 1-4 per cent of the genomes of modern non-African people.

To understand how modern humans lost their Neanderthal genetic material and how humans and Neanderthals remained distinct, researchers from University of California in the US, developed a novel method for estimating the average strength of natural selection against Neanderthal genetic material.

They found that natural selection removed many Neanderthal alleles from the genome that might have had mildly negative effects.

The scientists estimate that these gene variations were able to persist in Neanderthals because they had a much smaller population size than humans.

Once transferred into the human genome, however, these alleles became subject to natural selection, which was more effective in the larger human populations and has removed these gene variants over time.

The study is one of the first attempts to quantify the strength of natural selection against Neanderthal genes.

It enhances the understanding of how Neanderthals contributed to human genomes. It also confirms previous reports that East Asian people had somewhat higher initial levels of Neanderthal ancestry than Europeans.

These findings shed new light on the role of population size on losing or maintaining Neanderthal ancestry in humans, and add to our understanding of our close relatives.

"The key finding of our study is that the current levels of Neanderthal ancestry in modern humans are in part due to long-term differences in human and Neanderthal population sizes," said Ivan Juric from University of California.

"The human population size has historically been much larger, and this is important since selection is more efficient at removing deleterious variants in large populations," said Juric.

Therefore, a weakly deleterious variants that could persist in Neanderthals could not persist in humans.

The researchers think that this simple explanation can account for the pattern of Neanderthal ancestry that we see today along the genome of modern humans.
The study appears in the journal PLOS Genetics.