The DNA of all people around the world contains a record of how living populations are related to one another, and how far back those genetic relationships go. Understanding the spread of modern human populations relies on the identification of genetic markers, which are rare mutations to DNA that are passed on through generations. Different populations carry distinct markers. Once markers have been identified, they can be traced back in time to their origin – the most recent common ancestor of everyone who carries the marker. Following these markers through the generations reveals a genetic tree of many diverse branches, each of which may be followed back to where they all join – a common African root.
The mitochondria inside each cell are the power stations of the body; they generate the energy necessary for cellular organisms to live and function. Mitochondria have their own DNA, abbreviated mtDNA, distinct from the DNA inside the nucleus of each cell. mtDNA is the female equivalent of a surname: it passes down from mother to offspring in every generation, and the more female offspring a mother and her female descendants produce, the more common her mtDNA type will become. But surnames mutate across many generations, and so mtDNA types have changed over the millennia. A natural mutation modifying the mtDNA in the reproductive cells of one woman will from then on characterize her descendants. These two fundamentals – inheritance along the mother line and occasional mutation – allow geneticists to reconstruct ancient genetic prehistory from the variations in mtDNA types that occur today around the world.
Population genetics often use haplogroups, which are branches on the tree of early human migrations and genetic evolution. They are defined by genetic mutations or "markers" found in molecular testing of chromosomes and mtDNA. These markers link the members of a haplogroup back to the marker's first appearance in the group's most recent common ancestor. Haplogroups often have a geographic relation.
A synthesis of mtDNA studies concluded that an early exodus out of Africa, evidenced by the remains at Skhul and Qafzeh by 135,000 to 100,000 years ago, has not left any descendants in today’s Eurasian mtDNA pool. By contrast, the successful exodus of women carrying M and N mtDNA, ancestral to all non-African mtDNA today, at around 60,000 years ago may coincide with the unprecedented low sea-levels at that time, probably opening a route across the Red Sea to Yemen. Another study of a subset of the human mtDNA sequence yielded similar results, finding that the most recent common ancestor of all the Eurasian, American, Australian, Papua New Guinean, and African lineages dates to between 73,000 and 57,000 years ago, while the average age of convergence, or coalescence time, of the three basic non-African founding haplogroups M, N, and R is 45,000 years ago.
This information has enabled scientists to develop intriguing hypotheses about when dispersals took place to different regions of the world. These hypotheses can be tested with further studies of genetics and fossils.
Modern Human Diversity - Skin Color
Why do people from different parts of the world have different colored skin? Why do people from the tropics generally have darker skin color thanthose who live in colder climates? Variations in human skin color are adaptive traits that correlate closely with geography and the sun’s ultraviolet (UV) radiation.
As early humans moved into hot, open environments in search of food and water, one big challenge was keeping cool. The adaptation that was favored involved an increase in the number of sweat glands on the skin while at the same time reducing the amount of body hair. With less hair, perspiration could evaporate more easily and cool the body more efficiently. But this less-hairy skin was a problem because it was exposed to a very strong sun, especially in lands near the equator. Since strong sun exposure damages the body, the solution was to evolve skin that was permanently dark so as to protect against the sun’s more damaging rays.
Melanin, the skin's brown pigment, is a natural sunscreen that protects tropical peoples from the many harmful effects of ultraviolet (UV) rays. UV rays can, for example, strip away folic acid, a nutrient essential to the development of healthy fetuses. Yet when a certain amount of UV rays penetrates the skin, it helps the human body use vitamin D to absorb the calcium necessary for strong bones. This delicate balancing act explains why the peoples that migrated to colder geographic zones with less sunlight developed lighter skin color. As people moved to areas farther from the equator with lower UV levels, natural selection favored lighter skin which allowed UV rays to penetrate and produce essential vitamin D. The darker skin of peoples who lived closer to the equator was important in preventing folate deficiency. Measures of skin reflectance, a way to quantify skin color by measuring the amount of light it reflects, in people around the world support this idea. While UV rays can cause skin cancer, because skin cancer usually affects people after they have had children, it likely had little effect on the evolution of skin color because evolution favors changes that improve reproductive success.
There is also a third factor which affects skin color: coastal peoples who eat diets rich in seafood enjoy this alternate source of vitamin D. That means that some Arctic peoples, such as native peoples of Alaska and Canada, can afford to remain dark-skinned even in low UV areas. In the summer they get high levels of UV rays reflected from the surface of snow and ice, and their dark skin protects them from this reflected light.
Modern Human Diversity - Genetics
People today look remarkably diverse on the outside. But how much of this diversity is genetically encoded? How deep are these differences between human groups? First, compared with many other mammalian species, humans are genetically far less diverse – a counterintuitive finding, given our large population and worldwide distribution. For example, the subspecies of the chimpanzee that lives just in central Africa, Pan troglodytes troglodytes, has higher levels of diversity than do humans globally, and the genetic differentiation between the western (P. t. verus) and central (P. t. troglodytes) subspecies of chimpanzees is much greater than that between human populations.
Early studies of human diversity showed that most genetic diversity was found between individuals rather than between populations or continents and that variation in human diversity is best described by geographic gradients, or clines. A wide-ranging study published in 2004 found that 87.6% percent of the total modern human genetic diversity is accounted for by the differences between individuals, and only 9.2% between continents. In general, 5%–15% of genetic variation occurs between large groups living on different continents, with the remaining majority of the variation occurring within such groups (Lewontin 1972; Jorde et al. 2000a; Hinds et al. 2005). These results show that when individuals are sampled from around the globe, the pattern seen is not a matter of discrete clusters – but rather gradients in genetic variation (gradual geographic variations in allele frequencies) that extend over the entire world. Therefore, there is no reason to assume that major genetic discontinuities exist between peoples on different continents or "races." The authors of the 2004 study say that they ‘see no reason to assume that "races" represent any units of relevance for understanding human genetic history. An exception may be genes where different selection regimes have acted in different geographical regions. However, even in those cases, the genetic discontinuities seen are generally not "racial" or continental in nature but depend on historical and cultural factors that are more local in nature’ (Serre and Pääbo 2004: 1683-1684).
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