Why we have so few Neanderthal genes

Human evolution may not simply be a case of “survival of the fittest”, but also “survival of the many”, according to a new genetic study of early human origins.

A trio of evolutionary geneticist from the University of California, Davis in the US, led by Ivan Juric, found even though early Homo sapiens and Neanderthals (Homo neanderthalensis) interbred, the smaller population size of the latter meant many of their genetic traits were, over the millennia, weeded out.

The work – one of the first attempts to quantify natural selection against Neanderthal genetic material – was published in PLOS Genetics.

Currently, Neanderthal genes comprise only 1 to 4% of the human genome – very low considering interbreeding started a mere 100,000 years ago, which isn’t all that long in evolutionary terms.

The process by which genes are retained or rubbed out is called natural selection. Genes that allow an individual to better survive and reproduce are more likely passed onto offspring. Those that don’t, though, can disappear.

To understand how Neanderthal genetic material was whittled away, Juric and his team modelled the strength of natural selection against Neanderthal genes.

They estimated Neanderthal allele frequency – where an allele is a variant form of a gene – in the modern-day human genome.

They found that after hybridisation, Neanderthal alleles were slowly purged – even those with only mildly negative effect – thanks in part to the larger Homo sapiens population, Juric says.

Colin Groves, an anthropologist from the Australian National University in Canberra, suggests that these less desirable traits could have included anything from practical to aesthetic differences.

“As for those genes that were unsuccessful, one would have to look for some of the genes coding for the morphological characters in which we and Neanderthals differ, like the large brow ridges, the shape of the frontal sinus and the shape of the mandible,” he explains.

“A hybrid between a Neanderthal and modern human would likely have disharmonious facial features and these might well be quickly filtered out because they made chewing or breathing in cold weather more difficult or even because they appeared less sexy and would get fewer mates.”

But Groves expresses hesitation that Neanderthal populations were really as small as Juric and colleagues suggest.

“There are fewer Neanderthal sites in Europe than there are Homo sapiens sites, but does that mean that their population was that small?” he says. “I’m really not sure.”