Introduction

Nina Jablonski, now chairman of the anthropology department at the California Academy of Sciences, and her husband, George Chaplin, a geographic information systems specialist, formulated the first comprehensive theory of skin colour as an evolutionary adaptation.
Their findings, published in their article “Human skin pigmentation as an adaptation to UV radiation” in the Journal of Human Evolution, show a strong, somewhat predictable correlation between skin colour and the strength of sunlight across the globe. But they also show a deeper, more surprising process at work: Skin colour is largely a matter of vitamins.

Earliest Ancestors

With the current genetic and paleontological data, scientists agree that human ancestors once had bodies covered with dense hair with fair skin. An examination of the variation in MC1R nucleotide sequences for people of different ancestry that compared the sequences of chimpanzees and humans from various regions of the earth 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 just like chimpanzees, our closest biological relatives.
It is theorized that Between about 4.5 million and 1.5 million years ago, a change in earth’s climate drove early humans to move into hot, open environments in search of food and water not only having to cope with more exposure to the sun but they also had to work harder to gather food. So our ancestors had to develop a better cooling system to dissipate body heat more efficiently upon physical activity. The adaptation that was favoured involved an increase in the number of sweat glands on the skin. Enhancement of thermal sweating was a key innovation in human evolution because it allowed maintenance of homeostasis during sustained physical activity in hot environments by dissipating heat through evaporation. Early humans probably had few sweat glands, like chimpanzees, and those were mainly located on the palms of their hands and the bottoms of their feet. Occasionally, however, individuals were born with more glands than usual. The more they could sweat, the longer they could forage before the heat forced them back into the shade. The more they could forage, the better their chances of having healthy offspring and of passing on their sweat glands to future generations. A million years of natural selection later, each human has about 2 million sweat glands spread across his or her body.
Over time human hair disappeared to allow better heat dissipation through perspiration and cool the body more efficiently. But that was a problem because hairless skin is particularly vulnerable to damage from sunlight when exposed to a very strong sun, especially in lands near the equator. The once-protected skin became exposed to a multitude of environmental elements, including abrasion, infection, various chemicals and weather conditions.

UV Radiation

Under those conditions natural selection would have favoured mutations that improve barrier function in human skin; one such protective adaptation is the pigmentation of interfollicular epidermis. Melanin, especially eumelanin, is an excellent natural sunscreen. Dark skin, with its abundant, large, dispersed melanosomes rich in eumelanin, absorbs more UV light in its dead outer layers than light skin does, thus preventing UV light from penetrating to the living layers beneath.
It was previously suggested that the protective effects of melanin against DNA damage, which could lead to skin cancer, made dark pigmentation beneficial in some situations But while UV rays can cause skin cancer and sunburn they both have negligible effects on a person’s reproductive success, as most survive to reproductive age, so this is unlikely to be the reason for the evolution skin colour.
Until the 1980s, researchers could only estimate how much ultraviolet radiation reaches Earth's surface. But in 1978, NASA launched the Total Ozone Mapping Spectrometer. Jablonski and Chaplin took the spectrometer's global ultraviolet measurements and compared them with published data on skin colour in indigenous populations from more than 50 countries. To their delight, there was an unmistakable correlation: Variations in human skin colour are adaptive traits that correlate closely with geography and the sun’s ultraviolet radiation. The weaker the ultraviolet light, the fairer the skin.
A recent review reports that skin reflectance is lowest (i.e. melanin is highest) at the equator, then gradually increases, about 8% per 10° of latitude in the northern hemisphere and about 4% per 10° of latitude in the southern hemisphere. This pattern is inversely correlated with levels of UV irradiation, which are greater in the southern than in the northern Hemisphere.

Jablonski’s Theory of Human skin pigmentation as an adaptation to UV radiation

The above findings led Jablonski to develop the leading hypothesis for the evolution of human skin colour. It proposes that 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 since folate deficiency could potentially affect both male and female fertility and can interfere with normal development and is correlated with neural-tube defects such as spina bifida and anencephaly, in which infants are born without a full brain or spinal cord. It can also lead to anemia, neurological or psychiatric problems, cardiovascular disease, and increased risk of certain cancers.  While folate comes from diet, UVA exposure harms levels of folate’s main serum blood form, 5-methylhydrofolate. An hour of intense sunlight is enough to cut folate levels in half in people with light skin. Darker pigmentation is able to absorb and scatter UVA and in result protect folate in blood vessels from being destroyed by UV radiation. Therefore under high UVR conditions in Africa, it was an evolutionary advantage to have dark skin to maintain folate sufficiently.
However, UV light also has an important benefit: it helps in your body’s synthesis of vitamin D (cholecalciferol). Vitamin D is important for calcium absorption and so a deficiency in vitamin D can lead to bone abnormalities, such as rickets. It can also lead to viral and bacterial infections and contribute to auto immune diseases such as multiple sclerosis, type 1 diabetes, and rheumatoid arthritis. In environments where UV light is weak, natural selection would tend to favour lighter skin, which increase the body’s ability to capture UV light and maximizes the ability of the body to produce vitamin D in the conditions of reduced sunlight.
This delicate balancing act between Folate and Vitamin D explains the variation in skin colour in different geographic areas. It also explains why humans living in intermediate climates with higher seasonal variation of UV exposure developed tanning as a means of maintaining both adequate levels of vitamin D during the winter but also adequate levels of folate during the summer.

Migration

Paleontological data, cultural comparisons among living humans and genetic analysis helped sketch out the migration path of humans out of Africa. It has been suggested that within the past 100,000 years, some of the descendants of those ancient Africans gradually migrated around the globe, eventually becoming the “indigenous” peoples of Asia, Europe, Australia, and the Americas. This “Out of Africa” or “Recent African Origin” hypothesis has found strong support from paleontologists, anthropologists, and geneticists and is now widely accepted.
The prevailing view is that people first spread from Africa to the Middle East. Some of the Middle Eastern humans seem to have spread along the southern Asian coastline, learned something of sea craft, and travelled to the Polynesian islands and then to Australia by about 50,000 years ago. Other people spread into Central Asia by about 45,000 to 40,000 years ago. Various subgroups of the Central Asian population spread and settled the four corners of the Asian continent; some of these Asians crossed over a land bridge into North America on the order of 20,000 years ago. Central Asia is also where the ancestors of modern Europeans originated; a group of Central Asians spread into Europe about 30,000 years ago, eventually replacing the Neanderthals who had lived there before them.
There remains some debate over whether or not the recent African emigrants interbred with hominids (e.g., Neanderthals) who had settled Eurasia after a more ancient African origin. However, it is clear that we all share a relatively recent common ancestor.