Although the genes involved in the expression of the complex skeletal and dental traits we track through the hominin fossil record are still being identified, examples of selection-driven adaptation in living humans illuminate the recent and continuing evolution within our species, and exemplify the speed with which evolution can transform human populations.
The maintenance of a harmful allele like that for sickle cell trait shows how humans can adapt with their own biology to fight diseases. The recent evolution of lactose tolerance into adulthood in different populations shows how quickly dietary adaptations can evolve and also how readily convergent evolution occurs. Selection of varying amounts of skin pigmentation in humans is a striking example of how humans adapt to particular environments.
A gene with two or more alleles, each at an appreciable frequency, is a genetic polymorphism. In some African populations, the gene for the morphology of red blood cells is one such case. One allele (which we will call "A") codes for expression of normal red blood cells, but the sickle allele (which we will call "S") codes for sickle-shaped red blood cells. The S allele is caused by a point mutation changing only one amino acid. The two alleles are codominant, so neither masks the expression of the other. Individuals with two sickle alleles who are homozygous (SS) are affected with sickle cell anemia, which is a lethal disease where sickle-shaped red blood cells are essentially rendered handicapped and cannot perform their oxygen transport duties properly. Because of the negative effects of the S allele, one would expect it to exist in small frequencies, but it can reach frequencies of 20 percent or higher in populations in tropical regions of Africa, Southeast Asia, and Indonesia where malaria is prevalent.
Populations living in malarial infested regions have evolved a natural means of resistance to malaria using the S allele. Noncarriers (AA) have normal red blood cells but die more often from malaria than sickle allele carriers (AS) who are resistant to malaria. The malaria parasite (various species of the Plasmodium), which lives inside red blood cells, actually induces sickle-shaped red blood cells to rupture, so the parasite cannot eat, survive, and reproduce. The sickle allele carriers have some sickle-shaped cells but not enough to cause severe health problems or death. However, people with the homozygous genotype (SS) will most likely die from sickle cell anemia. Africans carrying the sickle allele who were taken to America in the slave trade no longer had an adaptive use for it in a land without malaria, so the successful adaptation in Africa is now a maladaptation in America. The sickle cell story is a powerful lesson that genes can be beneficial or harmful depending on the environment.
The ability to digest lactose into adulthood is a relatively new adaptation in humans that arose concomitantly with the advent of agriculture about 10,000 years ago.
Lactose is a component of milk and dairy products. Lactase is the enzyme that breaks down lactose during digestion so it can be absorbed by the gut. Normal or "wild type" humans have the ancestral condition, which is to stop producing lactase at about twelve years of age. They are lactose malabsorbers, or lactose intolerant, like other adult mammals.
However, some European and African groups that have long histories of pastoralism and animal domestication continue to produce lactase into adulthood and retain the ability to digest lactose. Early humans were strictly hunters and gatherers until about 10 Kya when some populations adapted new ways to obtain food: using domestication to keep the food source close. Selection pressures were strong on those people who were able to maximize these animal resources through digesting their dairy products throughout their lives. The ability to digest lactose as an adult must have been a huge nutritional advantage in these populations because selection favored it strongly. For example, nearly all Dutch and Swedish adults are lactose tolerant.
Genetic analyses by Sarah Tishkoff and others have shown that African and European groups converged on the same adaptation independently. Each group uses a different variant of a regulatory gene (i.e., a slightly different variant of an allele called an "SNP" for single nucleotide polymorphism) to control the gene for lactase production, which is called LCT. So far there are four different mutations that keep the lac-tase gene switched on. Each allele occurs in much higher frequency in populations that have long histories of dependency on domestic dairy animals: (1) Dutch and Swedes who are related to the ancient "Funnel Beaker" cattle-raising people of north-central Europe, (2) Nilo-Saharan-speaking groups in Kenya and Tanzania, (3) the Beja people of northeastern Sudan, and (4) Afro-Asiatic-speaking groups living in northern Kenya. The evolutionary tale of lactose tolerance is a powerful one because it highlights the influence of culture on biological evolution. Even more importantly, lactose digestion is a shining example of how quickly the biology of humans can adapt in order to survive better.
Because of the wide spectrum of variation, skin color is probably the most common and most conspicuous trait humans use to identify major phenotypic differences in others. However, Nina Jablonski and her colleagues have shown that there is a strong adaptive function behind human skin color variation.
Our closest relatives do not have skin color variation like ours. Chimpanzees have pale skin underneath their covering of black fur. Young chimpanzees have light faces, but with age and years of sun exposure adult chimpanzee faces develop dark spots or become dark all over. Based on parsimony, we assume our last common ancestor with chimpanzees was also pale-skinned with dark fur.
Regardless of how or when the drastic body hair reduction in humans occurred (probably with the emergence of the genus Homo at about 2 Mya), it is highly probable that darkly pigmented skin underwent very fast, very strong positive selection once hominins lost their furry coat. Pale naked skin is a great health risk under the equatorial African sun, so darkly pigmented skin would have been strongly favored.
Why is darkly pigmented skin better for people in the tropics? Melanin, the substance that gives color to skin, regulates the penetration of harmful ultraviolet radiation (UVR) from the sun. The more melanin in the skin, the more it can act as a natural sunblock as there is more solar radiation in the tropics than anywhere else in the world. The risks of sunburn and skin cancer are greatly reduced in people with dark pigmentation. One type of UVR, UVA, damages cells and DNA. It causes the breakdown of folate (a form of folic acid), retarding DNA replication and cell division in embryos that can then lead to their spontaneous natural abortion. This is a direct fitness affect of UVR on humans, since it negatively affects successful reproduction.
If dark skin is advantageous, why do some people have light skin? In regions further from the sunny tropics, darkly pigmented skin is actually maladaptive because it is too effective at blocking UVB (another type of UVR). UVB penetration of the skin is necessary for humans to metabolize vitamin D in their skin. At high latitudes away from the equator there is much less UVB present. Having less pigment in the skin allows more UVB to penetrate and permits the necessary metabolism of vitamin D (which is necessary for the body to absorb calcium, among other functions). In regions with significantly less UVB in the atmosphere, humans lost much of their skin pigmentation under the selective pressure to facilitate this process.
The consequences of vitamin D depletion include the onset of rickets, where bones do not form properly, often leaving the affected individual incapable of independent locomotion. A person's leg bones with rickets literally buckle under their weight. What's more, women who suffer severely from rickets have misshapen pelvic bones that render natural childbirth difficult to impossible. This is a direct link to fitness. In fact there are very few areas away from the tropics where people with darkly pigmented skin can live year-round and absorb enough UVB. Many people of African descent living in the United Kingdom, for example, must supplement their diets with vitamin D rich food like fish in order to live healthily there.
Molecular clock analysis of the MC1R gene associated with skin pigmentation points to a date of 1.7 Mya for the emergence of dark skin. It is quite possible that the people of Indonesia, Australia, and the South Pacific evolved dark skin separately, from an ancestral Asian stock that had reduced much of their pigmentation. And it is also possible that light skin evolved separately in Europe and in Asia. Geneticists will soon have the answers to these questions.
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