New light has been shed on why humans have differing skin colours.
Nina Jablonski, chair of anthropology at the California Academy of Sciences, and colleague George Chaplin, have analysed global measurements of ultraviolet light exposure.
In the Journal of Human Evolution, they conclude that the earliest members of the hominid lineage probably had a mostly unpigmented or lightly pigmented skin covered with dark black hair, similar to that of the modern chimpanzee.
This seems to have appeared early in the evolution of the genus Homo sapiens because of the higher activity and, therefore, heat levels associated with the modern limb proportions and bipedalism of our ancestors, compared with those of apes.
They suggest that in order to cool the body more efficiently, the skin became a vital organ in heat regulation, and achieved this through the evolution of an increased number of sweat glands, particularly on the face.
The brain is extremely heat sensitive, and its temperature closely follows blood temperature.
As the density of body hair decreased and sweat glands increased, the need to protect tissues beneath skin from the destructive effects of ultraviolet radiation also increased.
This protection was accomplished by darkening of skin.
The new research demonstrates that the previously observed relationship between skin pigmentation and latitude is traceable to the strong correlation between skin colour and UV radiation exposure.
It appears that this is because if you live in the tropics you need dark skin to protect the body's folate supplies from sunlight. The vitamin folate prevents 70 per cent of neural tube defects in humans but it can be readily degraded in the body by natural sunlight. Therefore, the solution to the evolutionary problem of maintaining adequate folate levels in areas of high UV radiation involved increasing the concentration of the natural sunscreen, melanin, in the skin.
So why, then, is not everyone dark skinned?
This is because we need vitamin D. In most humans, exposure to sunlight leads to formation of vitamin D in the skin. But dark skin becomes nonadaptive under conditions where the concentration of melanin is too high to permit sufficient amounts of vitamin D to be synthesised.
If the duration of UV exposure is not sufficient to produce vitamin D synthesis, a deficiency occurs, manifested in conditions such as rickets, osteomalacia, and osteoporosis.
So depigmentation of the skin was a necessary adaptation for humans attempting to inhabit regions outside of the tropics.
The research suggests skin colour has no bearing at all on how closely different races are related, but has more to do with the environment they are exposed to, in particular the amount of UV light.