If we look around us, we may unintentionally group people simply based on skin color: black, white, yellow, etc. Eighteenth century people also defined race based on these observations. Sadly, we have used this definition until now.

Yes, I know there is less genetic variation between race than within race (to be scientific: 10% between race vs. 90% within race). However, from the sociological perspective, the underline genetic uniformity doesn’t matter. Skin color, facial appearance, body shape, and other physical characteristics are the only criteria people differentiate each other. The prejudice and stereotype against people of color is plain there.

Elaborating tedious genetics and numbers is of no use persuading people to abandon the race idea. Only after people know how human beings have developed different skin colors and other physical characteristics may they throw away stereotypic ideas and treat each other fairly. In this post, I will discuss the evolution of skin color.

The “out of Africa” hypothesis says that we were all from Africa. But were we all black a million years ago, similar to those who are still living there? How could Chinese end up yellow, while European look white and pale? If there were some prehistoric human beings trapped in deep ice, we would be able to answer these questions definitely. Unfortunately, we don’t have that luxury. As in most scientific disciplines, let’s start with some assumptions.

Both our cousins–chimpanzees and apes in Africa– have hairy bodies, and under the hair are light skins. If we feel comfortable to accept the common ancestor idea, we can assume that our ancestors were also covered by much hair and had light skin millions of year ago.

When first human-like animal (H. erectus, man who stands) walked out of jungle, the first thing happened to them must be hair-loosing. Directly exposed to the sun light, extra hair blocked the release of heat. However, without hair, skin got burned under the sun. The ultraviolet (UV) is also harmful to skin cells and causes skin cancer. It is known that melanin, the substance makes our skin dark, can protect cells from UV damage.

Well, things are not so simple. Although skin cancers do pose a threat to unprotected skin, it takes years to develop skin cancers. By the time of having skin cancers, most primitives might already complete his reproduction duty, or be killed by other diseases or during fights. The natural selection force from skin cancers seems not very strong.

Astute researchers noticed the importance of folic acid (folate) in our human body. Folic acid deficiency may cause neural tube birth defects. Folic acid is also an essential component in biochemical reactions in our body. Dark skin may serve as a protecting layer from destroying folic acid by UV.

Furthermore, we all know that exposure to sun light during winter can increase vitamin D in human body, thus preserving calcium for normal bone development. Therefore, people who live in cold places are likely to develop light skin so that enough UV can penetrate skin to initiate vitamin D synthesis, while people with dark skin reached a balance between the detrimental and beneficial effects of strong UV radiation in tropical areas.

So far so good. Modern human beings are descendents of Homo Sapiens (man of knowledge) in Africa, who were descendents of archaic Homo Erectus. H. Sapiens had already been hairless and had black skin for a million years. Nevertheless, those fretting earlier human beings started to move away from the middle of Africa around 150,000 year ago. They could go north which was a large grassland (that Sahara became desert was a recent development). Some kept moving on, crossed the Arabic peninsula, and reached cooler place such as Mesopotamia land. Some might psychotically kept moving on (or were probably defeated in tribe fighting) and reached Europe and south Asia. Because the UV is less in the northern world, some people might have better survival chance due to their mutated genes which could lead to light skin. In other words, skin color was reversed from black to light color.

Now here is the perplexing question: why modern East Asians (including Chinese) have yellow skin color while Europeans have white skin, given not much difference in environment between Asia and Europe? The above theories gave a generally explanation why people have dark or light color. It can’t explain why people have different light skin colors. As matter of fact, south-east Asian people who had similar amount of UV exposure still had lighter skin than those in tropical Africa. The residual between observed and predicted skin color index (based on UV exposure) was much larger among south East Asian people than those among African.

Furthermore, if early human beings move slowly and continuously, skin color should have a continuous spectrum. For example, black people live in the south of Africa do have lighter skin than tropical Africans like Sudanese do. Among American Indians who crossed Bering strait 11,000 year ago, Those who lived in tropical areas did have darker skin than northern American Indians did.

One possibility is the bottleneck phenomena. If for some reasons one subgroup who happened to have very light skin or yellow skin (due to non-random mating within its limited area) successfully conquered other places, the whole area may have light skin, even though some places may not be good for light skin. For example, northern Europeans might conquer the southern Europeans (who might have dark skin), thus creating a uniformly white European population. This type of migration might occur lately, say at the end of Neolithic period, or even later. For instance, the Aryan’s invasion to India practically eliminated all indigenous people (who had dark skin) and made all Indians look like the whites. Because later human beings could protect skin in other ways such as cloths, the evolution force of skin color didn’t matter much any more.

In Asia, it is possible that a group of east Asian people with somewhat yellow skin kept moving north and probably conquered neighbors too. Because yellow skin is just fine in either warm or cold weather (but not for tropical weather), they finally reached the farthest place. Most of them kept hanging around in the land of east Asia, while some crossed Bering Strait and kept marching in American land.

Some people thought that if the migration from Africa to other places started from the beginning of H. erectus, the later comers (e.g. H. Sapiens) might mix with old ones whose evolution was independent from that of Africa (multi-region hypothesis). The recent discovery of Homo floresiensis seemed to support this hypothesis. Nevertheless, most evidence until now suggested that all modern human beings were from the same African tribe.

Skin color is just one indicator of racial difference, the evolution of physical appearance is more complicated. I will discuss it in the next post and also give another explanations on skin color.