Evolution Is Killing Off the Neanderthal’s Leftover Genes

Although the Neanderthals went extinct 30,000 years ago, their genes live on in human beings. A new study at the University of California, Davis, has found, however, that those strands of DNA are systematically being erased through natural selection.

“On average, there has been weak but widespread selection against Neanderthal genes,” said Graham Coop, professor at the universities’ department of Evolution and Ecology. The selection may have come about as a consequence of a small number of Neanderthals interacting with a substantially larger modern human population.

Before disappearing, Neanderthals chiefly lived in Europe and Central Asia until after a break away from our African ancestors around half a million years ago.

Discoveries by archaeologists suggest they had adopted a relatively sophisticated culture, according to Coop, who is also senior author on a paper describing the work, published in the journal PLOS Genetics. Due to DNA sampling from Neanderthal fossils, scientists have enough genetic information to locate their genes among our own.

As modern humans left Africa, they spread to Europe and Asia, interbreeding with Neanderthals around 50,000 to 80,000 years ago. These hybrid offspring would be equal parts Homosapien and Neanderthal, capable of breeding with Homosapiens, Neanderthals and other hybrids.

This begs the question: what happened to the Neanderthal DNA? The modern genes of people of European lineage contain roughly 2-3 percent. It’s slightly more in East Asians, while being largely absent in those of African ancestry.

Weak but widespread genes

Coop’s team came up with a system to assess the level of natural selection which acts upon Neanderthal DNA in the human genome. They Neanderthals may have become genetically incompatible with Homosapiens. Due to this incompatibility, the hybrid offspring were unable to thrive or were not actively fertile – they were in evolutionary terms not ‘fit’ enough.

What the researchers discovered, though, was something else. There wasn’t a strong selection against a small number of their genes but rather they weak and widespread selection opposing Neanderthal DNA and gradually bringing about its removal from the modern human genome.

Coop asserted that that is in keeping with the understanding that a minor Neanderthal population mixed with a much greater human one. Whereas inbreeding within small populations causes genetic variants to be relatively common, even if harmful, when mixed with a significantly larger population, the process of natural selection goes against the variants – rooting them out.

“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,” Juric said. “Weakly deleterious variants that could persist in Neanderthals could not persist in humans. We think that this simple explanation can account for the pattern of Neanderthal ancestry that we see today along the genome of modern humans.”

The conclusions are in keeping with similar work that has been recently published. Juric notes that if the Neanderthal population had been greater when first coming into contact with modern humans, the genetic mix could be different.

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Image credit: Michael Himbeault via Flickr

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