Humans evolved ability to tan 'to cope with changes in amount of sunlight'
Humans developed the ability to tan to cope with changes in the amount of UV radiation their bodies received during the year, scientists believe.
Researchers from Penn State University in the US studied the way the sun illuminates different parts of the Earth.
They found that the amount of ultraviolet B light - which produces vitamin D in human skin - was extremely variable further north due to atmospheric scattering of the light and absorption by oxygen.
The researchers believe that humans began to tan as a way of combatting the huge variations in the amount of ultraviolet light our skins were receiving throughout the year.
The reason people tan is so the skin could cope with fluctuations in the amount of sunlight their skin was receiving, a new report suggests
'The variation of ultraviolet radiation, especially in the middle and high latitudes is great,' said Nina Jablonski, professor of anthropology at Penn State.
'Tanning has evolved multiple times around the world as a mechanism to partly protect humans from harmful effects of ultraviolet radiation.'
Ultraviolet B radiation produces vitamin D in human skin. Ultraviolet radiation can, however, destroy folate.
Folate is important for the rapid growth of cells, especially during pregnancy where its deficiency can cause birth defects.
'We actually demonstrate that in those middle latitudes where highly fluctuating levels of ultraviolet radiation occur throughout the year, tanning has evolved multiple times as a mechanism to partly protect humans from harmful effect of the sun,' said Jablonski.
The tanning process evolved for humans who by and large were naked all the time. As the ultraviolet B radiation began to increase in the early spring, the skin would begin to gradually darken.
As the sun became stronger, the tan became deeper. During the winter, as ultraviolet B waned, so did the tan, allowing Vitamin D production and protecting folate.
'What we now recognise is that some of the medical problems seen in darkly pigmented people may be linked at some level to vitamin D deficiency,' said Jablonski.
'Things like certain types of cancer in darkly pigmented people and in people who use a lot of sunscreen or always stay inside could be partly related to vitamin D deficiency.'
Scientists have understood for years that evolutionary selection of skin pigmentation was caused by the sun.
As our human ancestors gradually lost their pelts so they could cool through sweating, their naked skin was directly exposed to sunlight.
In the tropics, where humans evolved and where ultraviolet radiation is high throughout the year, natural selection created dark skin to protect against the sun.
'Past arguments about the selective value of dark pigmentation focused on the protective effects of melanin against sunburn, skin cancer, and overproduction of vitamin D.
'These factors can no longer be considered significant selective pressures,' the report says.
This means that because sunburn and most skin cancers do not alter an individual's ability to have children, they are not selection factors.
The human body also has a mechanism to prevent overproduction of vitamin D.
The researchers concluded that dark skin pigmentation in the tropics protects people from folate destruction by ultraviolet B, but, because levels of ultraviolet B are high year round, the skin can still allow enough in to manufacture vitamin D.
As humans moved out of Africa, they moved into the subtropics and eventually inhabited areas up to the Arctic Circle where ultraviolet radiation is very weak.
North or south of 46 degrees latitude, which includes all of Canada, Russia, Scandinavia, Western Europe and Mongolia, there is insufficient ultraviolet B through most of the year to produce vitamin D.
Populations in these areas evolved to have little skin pigmentation.
In the latitudes between 23 and 46 degrees, an area that encompasses North Africa, South America, the Mediterranean and most of China, ultraviolet B radiation is much more variable.
Heavily pigmented skin in the winter would block the development of vitamin D, and lightly pigmented skin during the summer would allow destruction of folate.
The report appeared in a recent issue of the Proceedings of the National Academy of Sciences.