Human Skin Color Variation

Skin tone variation among humans. Photo courtesy of National Geographic/Sarah Leen

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 the 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.