Why You Don’t Have Much Neanderthal DNA in Your Genome

Nov 13, 2016

By Stephen Yin

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.

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One comment on “Why You Don’t Have Much Neanderthal DNA in Your Genome”

  • 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
    “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.

    Here is a related but more negative view that Neanderthals and modern humans ever developed a “HYBRID SPECIES” through cross-breeding.

    But there are also big stretches of the AMH (Anatomically Modern humans) genome that contain no Neanderthal DNA at all. “To me, these ‘holes of Neanderthal sequence’ are the most interesting aspect,” Joshua Akey told me in an email. (Akey is an author of one of last week’s papers, which appeared in Science. )”[T]hey might provide a roadmap to positions in the genome that endow uniquely human traits.” Where they are absent, natural selection may have decided that the AMH versions were doing a better job and so eliminated the Neanderthal counterparts. Hence these “deserts” may help define the most important genetic differences between us and them.

    What the DNA “deserts” mean

    There are deserts on the X chromosome and in genes involved in the testes. Patterns like these in other species ranging from rabbits to fruit flies are regarded as signs that one species is about to split into two.

    Hybrid males descended from both branches tend to be infertile, like mules. That’s because males have only one X chromosome, and if it happens to be one that impairs their fertility, then they may not reproduce. Females have two X chromosomes, so even if one is impaired, if the other one is normal, it can rescue her ability to bear young.

    “So this suggests that the male hybrids might not have been fertile, whereas the females might have been fully fertile,” Svante Pääbo told Richard Harris of National Public Radio. Pääbo, the grand old man of ancient DNA based at the Max Planck Institute for Evolutionary Anthropology in Leipzig, was an author of the other paper, which appeared in Nature. We might have inherited most of our Neanderthal genes through hybrid females, he said.

    Fox News quoted Pääbo as saying Neanderthals must have been disappointed in their sons.

    Another author, David Reich of Harvard Medical School, told reporters that we and Neanderthals “were at the edge of biological compatibility.”

    “This underlines that modern humans and Neanderthals are indeed different species,” Fred Spoor told New Scientist. Spoor is also at the Leipzig Max Planck but was not a part of the Neanderthal research. Other scientists are more cautious about making so firm a declaration, but it’s clear that many lean toward that same conclusion, that Neanderthals were not Homo sapiens neanderthalensis but, rather, Homo neanderthalensis.

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