Why the Neanderthals Lost the Race

Based on both physical and DNA evidence, anthropologists classify humans as members of the family of great apes, which includes orangutans, gorillas, and chimpanzees, as well as various human species. The branch to which we modern humans belong includes at least six species belonging to the genus Homo whose fossil remains have been found in East Africa over the past 60+ years. There was once a tendency to think of the latter as ancestors of modern humans, as if they were stages up a single branch of the tree, rising from primitive to advanced over hundreds of thousands of years, with Homo sapiens at the very tip, i.e., the “highest” form of human. On the contrary, the human family tree is both more complicated and more interesting. These various other members of the genus Homo are now recognized as separate twigs off the human branch of the great ape lineage. They are not so much our ancestors as our cousins. That branching continued to occur for millennia alongside the twig that we now recognize as modern humans (Homo sapiens). Some of those other humans made it to Europe before we (Homo sapiens) did.

The migratory path of the several human groups took them north out of Africa, then through the Middle East, ultimately colonizing Europe and Western and Southern Asia. Evidence for the European in-migration of Homo sapiens is clear and points to a time about 40,000 to 50,000 years before the present. But that’s not early enough to qualify as “first.” The discovery of settlements by other species of Homo throughout Europe, extending as far east as Western Siberia (e.g., the Denisovan people, >50,000 yrs ago), indicates that they predated the arrival of Homo sapiens by thousands of years.

The best studied of these earlier migrants out of Africa are the Neanderthals, whose remains and cultural artifacts are found throughout Europe, and who clearly precede the arrival of Homo sapiens. Anthropologists and paleontologists have puzzled over how it was that Homo sapiens, as late arrivers, came to displace the Neanderthals. Many theories have been proposed, ranging from superior weapons and technology to superior intelligence, and most recently to the use of what Scientific American termed “the ultimate weapon,” cooperation.

For the most part, these explanations have not been completely satisfying. Nevertheless, one factor does seem certain: Homo sapiens effectively “swamped” the Neanderthals. There were simply many more of us than of them. But that alone does not explain the apparent, complete disappearance of the Neanderthals. DNA evidence indicates that there was some limited interbreeding between resident Neanderthals and immigrant Homo sapiens. So, in one sense, some of the Neanderthal genome has survived. Interestingly, the presence of Neanderthal DNA in the modern human genome, which amounts to something like 2–3 % of the total genome, is largely confined to modern Europeans and Asians, indicating that the interbreeding occurred after the arrival of Homo sp. in the Middle East and/or Europe.

Still, why did the Neanderthals disappear from Europe? Recently, Leonard Greenfield, a physical anthropologist/paleontologist at Temple University, has set forth a persuasive case for a critical role of vitamin D, both in shaping the evolution of modern humans, and in explaining the disappearance of the Neanderthals.
It is generally agreed that the ancestral home of all of the various Homo species was East Africa, a location that would have provided abundant vitamin D in the form of sunlight. (Solar UV-B radiation converts a precursor compound into vitamin D.) Contemporary individuals from East African tribes exhibit a vitamin D status, derived mainly from cutaneous synthesis, which is equivalent to what would be produced in a Caucasian by a purely oral intake of 5,000 to 8,000 IU/day. However, it is also known that solar input of vitamin D inexorably diminishes as individuals move north out of equatorial latitudes.Thus north-migrating peoples coming out of East Africa pretty much all faced some degree of vitamin D deficiency.

That fact is generally considered to be the main explanation for the rapid loss of skin pigmentation among the migrating tribes of Homo sp. The heritable mutation that lead to the shift to pale skin thereby enhanced cutaneous synthesis of vitamin D and thus partially offset the diminished solar UV-B irradiance at higher latitudes. Individuals without that change in skin pigmentation would have been even more seriously vitamin D deficient than the others, and their pelvic bone structures could have been so distorted by D-deficiency rickets that delivery of babies from below would have been difficult or outright impossible, leading ultimately to extinction of those tribes and families that failed to develop pale skin.

But that simply means that all migrants coming out of Africa would have had marginal to deficient vitamin D status. The farther the northward migrants got from their place of origin, the worse their vitamin D status. But that tells us nothing about why Neanderthals, particularly, lost the race to survive in Europe. The only sources of vitamin D available to European Homo sp. would have been what little sun exposure might have been available and a diet rich in seafood & marine mammals. The high latitude of most of Europe and its extensive and persistent cloud cover mean that most individuals would have gotten little vitamin D by the solar route, which leaves only food. Greenfield points out that only the Homo sapiens immigrants had developed cultural practices that included fishing and/or eating the meat and fat of marine mammals. As a result, the Homo sapiens “immigrants” would have had been better able to achieve and maintain a healthy vitamin D status than the Neanderthal “natives”.
But general health, alone, is probably not a satisfactory explanation for what appears to have been the fairly rapid extinction of the Neanderthals. There’s more to the story. Adequate vitamin D status is absolutely essential for an organism to mount an adequate immune response, particularly in the face of foreign antigens, to which the “natives” would have had no prior exposure. (There are many contemporary examples of populations being “wiped out” by infectious diseases with which they had had no experience, brought to them, even if unwittingly, by “discoverers” or colonizers.)

Thus it appears likely that native, Neanderthal populations, would have declined both in numbers and in dominance simply because, unprotected by adequate vitamin D and hence with compromised immune competence, they succumbed to diseases brought to them by the invading Homo sapiens, whose vitamin D status was better and who, in addition, had inherited some degree of resistance to the diseases concerned. Also, as just noted, the invaders had dietary practices that, in comparison to Neanderthals, better suited them to live and thrive in a vitamin D-deprived environment (i.e., fish eating). Presumably, had the resident Neanderthals been able to achieve a more adequate vitamin D status they would have been better equipped to deal with the diseases brought to them by the invading Homo sapiens migrant bands.

There is a moral to this story, namely that nutrition is important after all, not just for the health of individuals, but for the survival of whole populations. But there is yet another insight to be gained. We are able to discern the association between poor population-level survival and low vitamin D status in the Neanderthals, but only from our great distance in time. Individual Neanderthals with inadequate immune competence would have been prone to become sick or to die, but up close one could not have been certain that it was the vitamin D status that was responsible, even if we had been there. Nor would every individual with low vitamin D status have succumbed. There is great deal of variability in sensitivity to, and need for, vitamin D from person to person. It’s just that, considering the population as a whole, the risk of a Neanderthal individual’s developing one of those unfamiliar diseases would be elevated, and, as a group, Neanderthals would thus be less competitive in a Darwinian sense. This was the reason Greenfield puts forth and it seems the most satisfactory of extant explanations for the fact that the Homo sapiens population grew and prospered, while the Neanderthal population, already fewer in numbers, shrank.

Further reading:
Greenfield, L.O. Vitamin D Deficiency in Modern Humans and Neanderthals. (2015). OutskirtsPress, Denver, CO

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