About 7 million years ago, when the human lineage progressively separated from our closest cousins, chimpanzees and bonobos, our ancestors, like other apes, had a lightly pigmented skin covered with body hair. Even if their skin was vulnerable to UV radiation, their hair protected them from the sun. About 2 million years ago, in Central Africa, our previously mostly vegetarian ancestors became effective hunters by specializing in long distance running. They could kill animals by pursuing them until they died of exhaustion during the hottest hours of the day. This specialization transformed the human body in two ways: humans lost most of their body hair and acquired the ability to sweat in large quantities in order to cool down the body during long runs. This is the remote origin of marathon runners.
One little precision is necessary here on the mechanism of natural selection. It is not possible to transmit to one’s children characteristics that have been acquired in one’s lifetime. Every child is born slightly different than his parents. In the previous example, a child can by chance be born with less body hair than his parents or with a greater ability to sweat. If the fact of being born with less body hair allows him to be a more efficient hunter (as he feels less hot when running), then he may have better chances to transmit this characteristic to his own children. Children born as hairy or more hairy than their parents probably had a harder time to hunt, feed themselves and have children, so their characteristics disappeared in the following generations.
Two million years ago, our ancestors thus became lightly pigmented and hairless in Central Africa. They did not stay lightly pigmented for long, because UV radiation has devastating health consequences on lightly pigmented skin. Among others things, UV radiation can destroy the vitamin B9 (also known as folate) present in the body. Since this vitamin is indispensable for cell division and pregnancy, our ancestors’ skin began to darken. Since scientists have understood the key role of this vitamin during pregnancy, it is given to every expectant mother. As in the case of body hair, some children were by chance born with a darker skin than their parents. They had better chances to have children than the others because they kept more of their vitamin B9, so their characteristic became dominant in the following generations.
This is how dark skin has served as a natural sunscreen for all the members of the genus homo in Central Africa for two million years. We will now see how the members of the genus homo who left Central Africa have acquired a lightly pigmented skin again.
In the last two million years, several members of the genus homo have left Central Africa at different times. I will speak here about Neanderthals (our remote cousins) and Homo Sapiens (our common species), in particular those who have settled in Southern Africa, Europe and Asia. Neanderthals are a dead branch of the genus homo and lived mostly in Europe and Western Asia. The separation between the Neanderthal lineage and the Sapiens lineage dates back to roughly 600.000 years in Africa. Neanderthals have thus far not been found by archaeologists after 30.000 BCE. Scientists have recently found out that they had mixed with the earliest homo sapiens in Eurasia and that they transmitted between 1 and 4% of their genes to present day non African people. The genetics of Neanderthals has shown that they had a lightly pigmented skin, and in some cases freckles, which some of us have inherited from them.
How did Neanderthals end up with a lightly pigmented skin? When the ancestors of Neanderthals left Central Africa on their way to Eurasia, they faced a less and less sunny climate. This soon created health problems, because exposure to UV radiation is the main source of vitamin D for humans. Vitamin D allows us to fix calcium on our bones as they grow. A lack of vitamin D can lead to a disease called rickets, in which leg bones are too fragile to support the weight of the upper body and become crooked. The dark skin of the ancestors of Neanderthals protected them from the sun but it also prevented the body from generating enough vitamin D because it blocked the penetration of UV radiation into the skin. Those who were randomly born with a lighter skin color than their parents have grown better in their new environment because they could generate enough vitamin D and have thus been able to transmit this characteristic to their children. Those born with a darker skin had a harder time to grow and have children so their characteristic disappeared in the following generations. Neanderthals’ skin color was a balance between the necessity to preserve vitamin B9 from being destroyed by UV radiation and the necessity to let enough UV in to generate vitamin D.
This scenario repeated itself a few hundred thousand years later for the Sapiens who have left Central Africa in the last 150.000 years. It is interesting to note that the same vitamin D problem has led to three different solutions among Homo Sapiens populations. The earliest Sapiens populations in Southern Africa, Europe and Asia have all acquired a lighter skin color, but in each case different genes are involved and different skin tones have appeared. The genes involved in the process are not at all the same as those of the Neanderthals. The San populations of Southwest Africa have a much lighter skin color than the populations of Central Africa, but their skin does not look like the skin of Asians or Europeans. This reminds us that in general, natural selection does not lead to uniformity but to diversity. From the common ancestor of all fish, natural selection has not formed one only species of super fish replacing all the others but instead a great variety of fish of all colors and sizes, each adapted to different ecological niches.
When did the skin of San, Europeans and Asians get lighter? We know that the ancestors of the San arrived in Southern Africa 150.000 years ago and that the ancestors of Europeans and Asians have settled most of Eurasia since roughly 50.000 years. However, they probably kept a dark skin for a long time. Genetic studies on European populations indicate for instance that the main genes now involved in the depigmentation of the skin have appeared less than 20.000 years ago, in the context of the last glacial maximum. The problem of vitamin D and the natural selection pressure must have been even more serious then because the very low temperatures in Northern Eurasia probably did not incite humans to stay outside for too long.
What shall we conclude from all this? Our ancestors were all black until a quite recent date in the history of the genus homo, which started more than two million years ago. Among Homo Sapiens as well as among Neanderthals before them, skin color is determined by the balance between two opposite necessities: the skin has to be dark enough to preserve vitamin B9 from UV radiation and to have good chances to have children; however, in a less sunny environment, the skin has to be lightly pigmented enough to allow vitamin D production during exposure to UV radiation. The result is an infinite variety of skin tone nuances in human populations depending on the level of UV radiation in the environment of their ancestors and the mixing of populations of different origins. Our skin color does not make us smarter or dumber, superior or inferior, it only allows us to regulate our vitamin intake according to the environment our ancestors lived in a few thousand years ago. For those of us who, voluntarily or involuntarily, do not live at the same latitudes at all than these ancestors, it is recommended to have a doctor regularly check our skin and our stocks of vitamin B9 and D in order to stay in good health.