Researchers at the Center for Population Biology, University of California, Davis, published a research paper on 8 November 2016, that sheds new light on the hybridization between modern humans and Neanderthals. Entitled “The Strength of Selection against Neanderthal Introgression,” the study acknowledges the sexual mixing of human and Neanderthal populations but notes that “on average, selection appears to have removed Neanderthal alleles from the human population,” meaning that for some reason, much of the Neanderthal DNA from these combinations seems to have disappeared from modern human genomes. The question being addressed here is “Why?”
According the study, Neanderthals likely split from modern humans some 550,000 to 765,00 years ago. After migrating out of Africa to Europe, Eurasia, and East Asia (the study shows East Asians had initially higher levels of Neanderthal ancestry than others), modern humans re-encountered each other around 47,000-65,000 years ago and began interbreeding. Today, approximately 1.5-2.1% of the DNA of non-Africans was derived from Neanderthals. While it is impossible to determine what the precise Neanderthal population was in comparison with the human population, researchers determined that the predominant reason the percentage of surviving Neanderthal is so low was that modern humans greatly outnumbered Neanderthals. From the authors’ Summary:
Much of this Neanderthal DNA appears to be deleterious in humans, and natural selection is acting to remove it. One hypothesis is that the underlying alleles were not deleterious in Neanderthals, but rather represent genetic incompatibilities that became deleterious only once they were introduced to the human population. If so, reproductive barriers must have evolved rapidly between Neanderthals and humans after their split.
In non-science speak, it turns out that Neanderthals had large numbers of individually weak but perhaps collectively harmful genetic mutations. When Neanderthals interbred with modern humans, natural selection sought to remove them. One reason for this could be some form of genetic incompatibility—genes that were neutral in Neanderthals but harmful in humans. However, the scientists note, it takes millions of years for mammals to develop such a genetic incompatibility. There just wasn’t enough time. Instead, they think there is a simpler cause: when Neanderthals met and bred with humans, natural selection (avoiding sickly partners, dominant vs. recessive genes, or higher mortality rates) eliminated deleterious Neanderthal DNA from the resultant hybrid populations.
In general, harmful alleles are removed from populations over time, but there are a number of reasons why some dangerous alleles might persist. According to information published by the University of California at Berkley, there are circumstances when a single copy of a damaging allele might be advantageous. For example, the allele that causes sickle cell anemia is very dangerous if one receives two copies, but a single copy of the allele has shown to provide resistance to malaria. In one study performed by Oxford University, subjects were exposed to malaria for a drug trial. Only 1 in 15 subjects with sickle cell trait developed the disease, compared to 14 in 15 without the trait who developed malaria. So, for Africans in Africa, people with a single copy, the “sickle cell trait,” may outlive some without the trait. For African Americans, where malaria isn’t an issue, there are no advantages to the trait, and so one could expect it to disappear from the population more quickly than in Africa.
There are also other mutations that keep recurring, effectively preventing their being removed from the population. For others, time is the biggest factor. For instance, cystic fibrosis may have offered resistance to cholera, and so it persisted during the age of cholera. One might expect natural selection to be eliminating it now.
What does this have to do with our Neanderthal ancestors? Plenty. There were a half-million years from when the Neanderthal population split from humans to when they began to recombine. That was plenty of time for genetic changes to occur and plenty of time for any harmful mutations to be removed, provided their population was robust and growing. While researchers don’t currently know what the harmful alleles were, they speculate that Neanderthal populations were stable, and so the changes were rendered effectively neutral. Perhaps some mutations provided resistance to diseases or viral vectors that didn’t threaten modern humans. Or maybe genetic changes in Neanderthals lowered their resistance to diseases that humans brought to their populations. In the first case, there would be no genetic advantage to continuing the Neanderthal DNA. In the second, Neanderthals would die sooner, as would hybrids, until those with weaker genes were removed from the population.
Mother nature is ruthless.
One interesting tidbit for future research was the scientists’ having detected a sex bias in pairings between modern humans and Neanderthals. “[O]ur result would imply that matings between Neanderthal males and human females were about three times more common” than between Neanderthal females and human males. Now, while the researchers emphasize that these results are “very provisional” meaning it could be a false result, if true, it opens some interesting speculation.
One could imagine, for instance, hoards of brow-ridged, caveman horn-ball rapers ravaging the poor, innocent human females and leaving their sickly DNA to fester in their offspring. (There are certainly tons of examples of that behavior in so-called modern human history.) However, a simpler reason could be that due to millennia of interbreeding, maternal mortality during childbirth was high for Neanderthals. If Neanderthal mothers (and their babies) were likely to die at childbirth, then we’d see a non-randomly higher number of Neanderthal male – human female births than the opposite pairing, which we’ve seen.
All of this is fascinating, but inconclusive. However, it does lead to interesting theories, some of which match things we’ve seen in recorded history. The relative small size of the Neanderthal population implies that A) either a small number of souls left Africa one-half to three-quarters of a million years ago to become Neanderthals in the first place, or B) something in their biology prevented them from reproducing at a rate similar to humans’ birth rates. A small group of wayfarers, living in isolation as they were, are subject to genetic anomalies if, for no other reason, due to the lack of genetic diversity. Close contact and inbreeding lead to deleterious alleles, which in turn could restrict population growth. If the number of original Neanderthals was large, something in their environments—harsh climate, disease-bearing pests, etc.–caused their populations not to grow. Differences in alleles could be the result of the Neanderthals’ evolving to fight off individual biological threats at the cost of overall health.
But if that latter conclusion is true, wouldn’t modern humans have encountered the same genetic adversaries, and thus the same alleles? Not necessarily. The story of sickle cell trait tells us that what might be harmful in one place may not be in another. It could be that Neanderthals lived in places that were by definition more harsh than humans’ homes. However, a simpler, and Occam’s Razor would suggest, better theory is that Neanderthal populations didn’t grow and their genetic code became corrupted, simply due to a lack of genetic diversity.
Were Neanderthals the strong, robust beings we imagine them to be? Or were they, rather, genetically weakened populations of prehistoric hillbillies, waiting for modern humans to wash clean their genetic code? We await further research to find out.