It’s generally known that dark colours absorb the light and also the heat. And light colours reflect the light and heat.
So why is it that the hot countries have dark skinned races, and the cold countries have light skinned races? Surely that’s backwards to logic and science, yet it’s remarkably consistent.
Evolution, you got some ‘splainin’ to do.
The Master speaks. See the second line, fourth paragraph.
Light color also transmits light more. If the choice is between absorbing all the light at the skin and transmitting some of it further into the body, it’s better to absorb it all at the skin.
It happens that I’m doing a lot of reading on a related subject for a Staff Report. Most likely the purpose of melanin is to prevent UV damage (skin cancer and sunburn). Light-skinned individuals living in the tropics (including albinos with dark-skinned parents) have very high rates of skin cancer. Also, light skin sunburns much more readily and sunburnt skin can’t sweat as efficiently, potentially leading to hyperthermia.
There are several competing theories that used to be more popular than they are now. One has to do with vitamin D synthesis, which occurs in the skin when it is exposed to UV radiation. The idea here is that a light-skinned person will overproduce vitamin D in the tropics (leading to a potentially dangerous condition known as hypervitaminosis-D) and a dark-skinned person will underproduce vitamin D outside the tropics (leading to rickets in children and osteomalacia in adults).
This first part is based on a false assumption that more UV always means more vitamin D. In fact, the body stops making vitamin D when a certain amount is present. In other words, it is impossible to get hypervitaminosis-D from sunlight alone (but it is possible to get it from ingesting too much vitamin D from foods rich in it, such as fish liver).
The second part is based on the false assumption that dark-skinned people can’t synthesize enough vitamin D outside of the tropics. Before the Industrial Revolution and urbanization, it was not difficult for even the darkest-skinned people at high latitudes to get enough UV in summer to make a year’s worth of vitamin D, at least as far north as the Arctic Circle (and probably farther north). Rickets and osteomalacia were quite rare until the Industrial Revolution when people started spending more time indoors in cities with high levels of UV-blocking pollution. Data are inconsistent, but it is probably true that dark-skinned people are more prone to rickets than light-skinned people in the same environment. However, rickets has always been rare in agricultural communities, no matter the skin tone and no matter the latitude.
“Darker” colors also radiate heat more effectively than “lighter” colors. Which will keep you cooler will depend on how much time you spend in the shade versus direct sunlight.
I put “darker” and “lighter” in quotes, because it’s not actually the “color” that we see in visible light which is most important, but the “color” in whatever wavelength you’re radiating most at: Infrared, for body temperature. Infrared “darkness” isn’t necessarily the same thing as visible “darkness”, but they’re related.
Yeah, the “black skin radiates better” hypothesis is another one debunked in the book I just read (Biological Perspectives on Human Pigmentation by Ashley Robins). I was a bit surprised to learn
Robins cites the 1963 article “Thermography of the human body” by R. B. Barnes in volume 140 of Science.
Wow! Thanks for the answer, that makes more sense - I hadn’t even considered UV. Excellent!