When teaching about the evolution of humankind, students often ask me about skin colour. Some people feel awkward about asking questions about skin colour, but we shouldn’t: “understanding” and asking questions are at the heart of anthropology.
So, students ask, “What happened after we came down from the trees and began to walk on two legs rather than four? Did the intense sun in Africa cause our skin to get darker?” Another student will join in and add: “I’ve heard that our ancestors were dark first and dark skin colour protected them from cancer.” These are great questions, and by examining the evolution of skin, colour anthropologists have illustrated that the so-called differences that have been thought to set us apart are actually what makes us unique as a species. “Look around the room,” I say to my students, “the faces and colours that you see are faces of success.”
FROM HAIRY TO SWEATY & DARK, THEN LIGHT
Developing an ability to walk on two legs gave our early ancestors an adaptative advantage over other species; and while we are on the topic, we didn’t decide to come down from the trees. Instead, our ancestors faced new challenges related to environmental change: as the Pliocene saw warmer global temperatures, trees began to disappear from certain areas of Africa. Our ancestors found themselves in a changing environment where managing body temperature became essential to their survival as a species.
To be sure, most mammals do not cope well with heat: they are covered in hair and are poor sweaters (think about how poor of a long distance runner a dog is in the heat). In fact, most animals cannot endure excessive physical exertion in hot environments without overheating.
In contrast to other animals that need to rest during the hottest periods of the day, hair loss and an increase in the number of sweat glands allowed ancestral humans to cope with the extreme heat of Africa. This took place around 5 million years ago when worldwide temperatures were on average 5 degrees higher than they are now. “With less hair, perspiration could evaporate more easily and cool the body more efficiently. But this less-hairy skin was a problem because it was exposed to an unyielding sun, especially in lands near the equator,” explain anthropologists at the Smithsonian (2017).
“Why are ultraviolet rays a problem?” you ask. This is another great question, but not for the reasons you might think. Most people believe that the most significant problem that our ancestors faced is related to developing skin cancer when it is actually related to preserving folic acid.
This is another great question, but not for the reasons you might think. Most people believe that the most significant problem that our ancestors faced is related to developing skin cancer when it is actually related to preserving folic acid.
“Whaaat?!” you exclaim.
I know! It ‘s hard to imagine that avoiding skin cancer isn’t the reason why skin colour darkened, but it makes sense. Skin cancer is something that develops during the course of one’s lifespan, and for something to increase a species’ fitness (from an evolutionary perspective), it must impact their reproductive success.
With respect to evolution, then, any features that give a species an adaptive advantage only emerge over successive generations. In other words, the adaptation in question must help ensure that an individual lives long enough to have babies. The offspring will benefit from the new genes mom or dad passed on because they will live long enough to have babies while individuals without the new feature do not. As noted, considering that skin cancer typically emerges later in life – after the individual has had babies – dark skin wouldn’t give them a reproductive advantage.
These initial changes in skin colour are thought to have taken place around 1.5 million years ago among a group of hominids (ancestors of humans) we have named Homo erectus.
In contrast to the explanation that may of us are familiar with (skin colour and cancer), the real answer is found when examining the impact UV radiation has upon an unborn fetus vis-a-vis the mother. Put simply, as anyone who plans to have or has had a child knows, doctors typically recommend increasing one’s intake of folic acid – during pregnancy – because it is essential to the development of a fetus. UV radiation, however, strips one’s body of folic acid and therefore is destructive to a developing fetus. Dark skin, then, ensured the success of our species. This doesn’t mean, however, that there weren’t other challenges our ancestors had to face, that is, from a biological perspective.
As we know, our ancestors eventually migrated out of Africa. These people had dark skin. In fact, as pointed out by Gibbons (2015, para. 5),
modern humans who came out of Africa to originally settle Europe about 40,000 years ago are presumed to have had dark skin, which is advantageous in sunny latitudes. And the new data confirm that about 8500 years ago, early hunter-gatherers in Spain, Luxembourg, and Hungary also had darker skin: They lacked versions of two genes—SLC24A5 and SLC45A2—that lead to depigmentation and, therefore, pale skin in Europeans today.
“If people had dark skin as late as 8500 years ago in Europe, why did light skin colour emerge?” you ask.
Well, the answer revolves around absorbing vitamin D. Recent research has identified that “one amino acid in one gene plays a significant role in determining why people of European descent have lighter skin than people of Africa.” While many people had dark skin, people who migrated into areas of Northern Europe were faced with another UV radiation issue: in areas with low UV radiation, people with dark skin will not absorb enough vitamin D through sunlight and develop Ricketts (a debilitating disease that causes malformed limbs).
The evidence for these changes in skin colour come from an archaeological site in Sweden called Motala. This archaeological site analyzed the genes of seven individuals who lived 7700 years ago. As explained by Gibbons (2015, np), these people were found to have both “light skin gene variants, SLC24A5 and SLC45A2. They also had a third gene, HERC2/OCA2, which causes blue eyes and may also contribute to light skin and blond hair.” As people migrated and interbred, light skin colour became more frequent.
Dass, A. (nd). Panatone chart of every human skin tone colour. Humanae.
Gibbons, A. (2015). How Europeans evolved white skin. Science, AAAS. http://www.sciencemag.org/news/2015/04/how-europeans-evolved-white-skin