Human Skin Color - Evolution of Skin Color

Evolution of Skin Color

It is theorized that about 1.5 million years ago, the earth endured a megadrought that drove hominids from lush rainforests into arid, open landscapes. This, coupled with the loss of dense body hair, caused early human skin to endure excess UV-B radiation and xeric stress. Rogers et al. (2004) performed an examination of the variation in MC1R nucleotide sequences for people of different ancestry and compared the sequences of chimpanzees and humans from various regions of the Earth. Rogers concluded that roughly five million years ago, at the time of the evolutionary separation of chimpanzees and humans, the common ancestors of all humans had light skin that was covered by dark hair. Additionally, our closest extant relative, the chimpanzee, has light skin covered by thick body hair. Human hair gradually disappeared to allow better heat dissipation through sweating and the skin tone grew darker to increase the epidermal permeability barrier and protect from folate depletion due to the increased exposure to sunlight. By 1.2 million years ago, around the time of homo ergaster and homo erectus, the ancestors of all people living today had exactly the same receptor protein as modern Africans. Evolutionary pressure meant that any gene variations that resulted in lighter skin were less likely to survive under the intense African sun, and human skin remained dark for the next 1.1 million years.

About 70,000–100,000 years ago some modern humans began to migrate away from the tropics to the north where they were exposed to less intense sunlight, possibly in part due to the need for greater use of clothing to protect against the colder climate. Under these conditions there was less photodestruction of folate and so the evolutionary pressure stopping lighter-skinned gene variants from surviving was reduced. In addition, lighter skin is able to generate more vitamin D (cholecalciferol) than darker skin so it would have represented a health benefit in reduced sunlight if there were limited sources of vitamin D. Hence the leading hypothesis for the evolution of human skin color proposes that:

1. From ~1.2 million years ago to less than 100,000 years ago, the ancestors of all people alive were dark-skinned Africans.
2. As populations began to migrate, the evolutionary constraint keeping skin dark decreased proportionally to the distance north a population migrated, resulting in a range of skin tones within northern populations.
3. At some point, northern populations experienced positive selection for lighter skin due to the increased production of vitamin D from sunlight and the genes for darker skin disappeared from these populations.

The genetic mutations leading to light skin, though different among East Asians and Europeans, suggest the two groups experienced a similar selective pressure due to settlement in northern latitudes.

There is a long-standing hypothesis that the selection for lighter skin due to higher vitamin D absorption occurred soon after the Out of Africa migration some time before 40,000 years ago. A number of researchers disagree with this and suggest that the northern latitudes permitted enough synthesis of vitamin D combined with food sources from hunting to keep populations healthy, and only when agriculture was adopted was there a need for lighter skin to maximize the synthesis of vitamin D. The theory suggests that the reduction of game meat, fish, and some plants from the diet resulted in skin turning light many thousands of years after settlement in Europe and Asia. This theory is supported by a study into the SLC24A5 gene which found that the allelle associated with light skin in Europe may have originated as recently as 6,000–10,000 years ago which is in line with the earliest evidence of farming.

One of the most recently proposed drivers of the evolution of skin pigmentation in humans is based on research that shows a superior barrier function in darkly pigmented skin. Most protective functions of the skin, including the permeability barrier and the antimicrobial barrier, reside in the stratum corneum (SC) and the researchers surmise that the SC has undergone the most genetic change since the loss of human body hair. Natural selection would have favored mutations that protect this essential barrier; one such protective adaptation is the pigmentation of interfollicular epidermis, because it improves barrier function as compared to non-pigmented skin. In lush rainforests, however, where UV-B radiation and xeric stress were not in excess, light pigmentation would not have been nearly as detrimental. This explains the side-by-side residence of lightly pigmented and darkly pigmented peoples..