2016-11-09 17:34:19
Trilobites: Why You Don’t Have Much Neanderthal DNA in Your Genome

17:34, November 09 218 0

Neanderthals and modern humans diverged from a common ancestor about half a million years ago. Living in colder climes in Eurasia, Neanderthals evolved barrel chests, large skulls and strong hands. In Africa, modern humans acquired shorter faces, a prominent chin and slender limbs. Then, roughly 50,000 years ago, the two species encountered one another and interbred, as modern humans spread out of Africa.

The legacy of this interbreeding has been the subject of much scientific inquiry in the past few years. Today, up to 4 percent of the genes of non-Africans are Neanderthal in origin.. These may have influenced a diverse range of traits, including keratin production, disease risk and the propensity to sneeze after eating dark chocolate. Where did all the other Neanderthal DNA go? Why did a Neanderthal-human hybrid not prevail?

Two recent studies converge on an explanation. They suggest the answer comes down to different population sizes between Neanderthals and modern humans, and this principle of population genetics: In small populations, natural selection is less effective.

“Neanderthals have this small population over hundreds of thousands of years, presumably because they’re living in very rough conditions,” said Graham Coop, a genetics professor at the University of California, Davis, and an author of one of the studies, published Tuesday in PLOS Genetics.

As a result, Neanderthals were more inbred than modern humans and accumulated more mutations that have a slightly adverse effect, such as increasing one’s risk of disease, but do not prevent one from reproducing (and thus, passing such mutations along).

“After Neanderthals started mating with humans, natural selection in the larger human population started purging” those mutations, said Ivan Juric, a geneticist at 23andme who studied with Dr. Coop and was a co-author of the study.

In 2014, a group led by David Reich, a genetics professor at Harvard, found that Neanderthal DNA tended to be located far away from important genes in the human genome. This provided one of the first pieces of evidence that natural selection was working against Neanderthal DNA.

At the time, Dr. Reich’s group attributed some of the finding to possible infertility in Neanderthal-human hybrids. “But we also showed that infertility couldn’t explain most of the pattern,” Dr. Reich said.

In their study, Dr. Juric and Dr. Coop found that differences in population size could explain it. They analyzed the current-day frequency of Neanderthal ancestry along the human genome. Fitting a mathematical model of natural selection onto it, they found that a pattern of weak natural selection because of population size differences between Neanderthals and humans could account for the distance between Neanderthal DNA and genes in the human genome today.

Their results confirmed those reported in another paper published in April, which approached the puzzle from the opposite direction. In that study, the authors started with historical estimates of Neanderthal and human population sizes. Based on those sizes and the probability that new mutations that arise are harmful, they computed the load of genetic mutations for each population, and found that Neanderthals most likely had a greater prevalence of slightly disadvantageous genes.

“Our simulations showed that early hybrids would have been much less fit than pure humans,” said Kelley Harris, a postdoctoral fellow at Stanford and one of the authors of that paper. Over time, this would have decreased the amount of Neanderthal DNA around modern human genes.

Together, these papers tell a convincing story, Dr. Reich said. “The reduction of Neanderthal ancestry near genes that we’ve observed is a strange and remarkable phenomenon,” he said. “These two papers have provided a plausible and exciting explanation.”