Health

Dark-skinned H. erectus had tuberculosis?

Gene ExpressionBy Razib KhanDec 7, 2007 7:14 AM

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Most ancient case of tuberculosis found in 500,000-year-old human; points to modern health issues:

Although most scientists believe tuberculosis emerged only several thousand years ago, new research from The University of Texas at Austin reveals the most ancient evidence of the disease has been found in a 500,000-year-old human fossil from Turkey. ... The research team identified two shared characteristics in the communities: a path of migration from low, tropical latitudes to northern temperate regions and darker skin color. People with dark skin produce less vitamin D because the skin pigment melanin blocks ultraviolet light. And, when they live in areas with lower ultraviolet radiation such as Europe, their immune systems can be compromised.It is likely that Homo erectus had dark skin because it evolved in the tropics, Kappelman explained. After the species moved north, it had to adapt to more seasonal climates. The researchers hypothesize the young male's body produced less vitamin D and this deficiency weakened his immune system, opening the door to tuberculosis.

The point about dark-skinned people and weaker immune systems due to Vitamin D deficiency is well taken. I've blogged Vitamin D deficiency before. I find it interesting because I am curious about its role in the evolution of human skin color. It seems that many Eurasian populations have become much lighter skinned within the last 10,000, and I've wondered why. For example, imagine that infectious diseases became a bigger selective pressure as population densities increased due to agriculture. Additionally, with agriculture the range of nutrient deficiencies, including of Vitamin D, would also loom larger with a less varied diet. Perhaps this was a selective environment which shifted these populations toward rapid depigmentation as an adaptive response? But, I do have to offer that the hypothesis that Eurasian Homo erectus suffered Vitamin D deficiency because of dark-skin is rather unlikely. Populations can lose their pigmentation rather quickly, over a few thousand years (see the selection coefficients around SLC24A5). These are loss of function mutations, and it is often quite easy to break things as opposed to gaining function. I read once in my youth that Neandertals might have had dark-skin because of bone morphology which resembled rickets. We now have data which suggests that Neandertals were depigmented. This makes sense, Neandertals and their precursors resided in northern Eurasia for hundreds of thousands of years. Evolution works. Unless a hominid population are very recent immigrants to a region it seems unlikely to posit that because their ancestors had dark-skin they must also have had dark skin. After all, the ancestors of Swedes only emerged from Africa in the last 50,000 years, but they no longer have dark skin (though it turns out that most of the selection for lack of pigmentation might have occurred within the last 15,000 years, at least on three loci, SLC45A2, SLC24A5 and OCA2). Note: The Dmanisi fossils are well over 1 million years old, so I see no reason to assume that this individual was a descendant of recent immigrants are a priori grounds. Update: John Hawks is skeptical of the skin color connection too.

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