This is a good example of the influence of the environment on natural selection. A trait that is better in one environment could be worse in another. An evolutionary anthropologist named Nina Jablonski helped solve this mystery in the 1990s.

Human hair, eye, and skin color all come from a pigment called melanin. The more of this substance you have, the darker you can be. Some genes tell you to make more or less of it, other genes tell it where in your body to go.

People who live in areas where there is a lot of sun are in greater danger of getting skin cancer and cataracts (a cloudiness in the eye). Also, too much sun causes people to use up their folic acid (a type of vitamin B). Folic acid is needed for an embryo's nervous system to form correctly, so pregnant women who get too much sun are more likely to have babies with brains and spinal cords that are not formed correctly. So people with lighter skin and hair in sunny climates probably did not live as long as people with darker skin and hair, and left fewer children.

People who live in areas where there is not a lot of sun have a different problem. People need sunlight in order for their cells to make vitamin D. People need vitamin D in order to absorb calcium. You probably know that we need calcium for our bones and teeth, but it is also vital for lots of other things, including making our muscles contract.

So people with darker skin in northern climates (such as northern Europe) probably did not live as long as people with lighter skin and hair, and left fewer children.

The eye color question may not be quite as clear. There may be advantages to having different eye colors in different climates, but it could also be that some of the same genes for having lighter or darker skin lead to lighter or darker eyes.

How do you think global climate change, vitamin supplements, or movement of people influence what will happen with human skin and eye color in the future?

The Scandinavian people from way up north, where there is not much sun have evolved to having light skin to let in what little sunlight there is. However, now so many people move around, so that in Europe there are not so many people with blond hair and blue eyes anymore! The Roman people settled in southern Europe thousands of years ago; now you have Middle Eastern people settling there to escape the horrible wars in their own countries. And Indian people settled in Europe, too.

It has to do with pigments and sunlight, and accidents of genetics. In parts of the world that don't get much sunshine, it is healthier to be able to absorb more sunshine through the skin, including ultraviolet light (UV). A small amount of UV can help our bodies generate Vitamin D, which is an important vitamin for our bones and immune systems.

But UV also causes cancer. To reduce this, our skin also contains pigments, which absorb some of the UV (but don't make Vitamin D). We all have some pigments, which make our skin brown, black, red, or pink. Northern Europeans have the least amount of pigments because they need the least protection from UV, and because they historically needed the most UV. The genes that produce pigments for skin are similar or close to the genes that produce pigments for hair or eyes. So if you have light skin, you're more likely to have light eyes and light hair--and to get skin cancer.

Good question, but it probably has to do with the amount of sunlight that Europeans are exposed to. This is true in northern Europe than in southern Europe, by the way. Blue eyes and blond hair allow more sunlight to come in than brown eyes or dark hair, which is a good thing if there isn't much sun to begin with (Europe is pretty far north), and a bad thing if there is a lot of sun (like in the tropics). This is also why most Europeans have light-colored skin, and,by contrast, why most Africans (Africa is tropical)have dark skin.

One thing I do know is why some people have light colored skin and some have dark colored skin. As it happens the Sun produces large amounts of ultra-violet (UV) radiation. Too much of it and you get skin cancer, too little and you are unable to synthesize vitamin D, needed to prevent rickets (a softening of the bones in children potentially leading to fractures and deformity). In other words, you need the right amount of UV radiation.

Melanin is a substance in your skin cells that has the capacity to absorb UV radiation. Melanin is what determines the color of your skin. Large amounts and your skin has a dark color, small amounts and your skin has a light color. If you live close to the equator you will receive a larger amount of UV radiation from the Sun than if you live far from the equator. This means that you need larger amounts of melanin if you live near the equator than if you live far from it in order to protect you from excessive UV radiation.

People living near the equator and having great amounts of melanin have a greater chance of surviving than people with small amounts of melanin also living near the equator. On the other hand, people living far from the equator and having large amounts of melanin will not be able to absorb enough UV radiation to synthesize vitamin D, therefore at those latitudes people with small amounts of melanin have the better chance of survival. Thus through evolution, most people living close to the equator have dark colored skin and most people living far from the equator have light colored skin. At least that used to be the case several thousand years ago, when groups of migrating people would take literally thousands of years to move from one region of the earth to another.

Recently, researchers in genetics have concluded that if an isolated group of people were to remain in only one region of the earth, after about 25000 years their descendants will acquire a skin coloring best suited for the latitude of that region.

This is a good question and an active area of research for scientists studying human evolution.

I should first point out that blonde hair and blue eyes are prevalent only in certain parts of Europe - northern France, northern Germany and Scandinavia (Denmark, Norway, Sweden, etc.), primarily. These are regions where "Nordic" peoples dominate, but in other regions of Europe like the Mediterranean and Balkan areas, darker hair, eyes, and complexions are more common.

The best evidence we have suggests that these traits became common in Nordic populations during the last Ice Age because of "sexual selection."

Sexual selection is a type of evolution in which one sex has an arbitrary preference for a particular trait in the other sex. For example, there is a small fish in Trinidad called the swordtail. Male swordtails have one extra-long fin that doesn't really do anything. But female swordtails prefer to mate with males with a long fin, for no apparent reason. If you trim a male's fin, females refuse to mate with him, and if you artificially lengthen his fin, females go crazy for him. This is a trait that has evolved just because females have this unexplained preference for males with long fins. This is called sexual selection because one sex is driving the evolution of a trait in the other sex.

It seems like the same thing might have happened in northern Europe in the Ice Age. Around this time, humans were hunter-gatherers, so most men were off hunting for (and being killed by) wooly mammoths and such. So many men died doing this that there were fewer men than women in the population, so men got to be pretty choosy about the women they would mate with. For whatever reason, men in these populations developed a preference for blonde hair and blue eyes. So the genes for blonde hair/blue eyes might have been rare in the population at first, but because women with those traits were found to be more attractive, they could find mates more easily and had more children. Pretty quickly (on an evolutionary timescale) blonde hair and blue eyes became very common in those populations.

People with blonde hair and blue eyes might have popped up every now and then in human populations outside of northern Europe, too, but because the ration of men/women was more even in those other populations, sexual selection was less intense, so those traits never became common. (Although a similar sort of thing seems to have happened with red hair and green eyes in ancient Irish populations).

In your question you didn't ask about skin color, but of course having fair skin also tends to go along with blonde hair and blue eyes. Unlike hair and eye color, it seems that light skin color may have had a real advantage for early northern Europeans. Vitamin D is an essential vitamin that humans can produce only by exposing their skin to UV radiation from the sun. If you live in a very sunny place this is no problem, but humans in very high latitudes (like northern Europe) see very little sunlight for most of the year. One way to avoid a vitamin D deficiency in that sort of environment is to have lighter skin, allowing more UV radiation to penetrate and create vitamin D.

So anyway, there once was a real advantage to having lighter skin if you lived in northern Europe, and that's why people from that region are so pale now. Of course today we can get plenty of vitamin D from dairy products that have been enriched with that vitamin, and having fair skin is actually a detriment if you live down in the tropics, because you'll get sunburned all the time.

Hope this answered your question.

COMMENT FROM A SCIENTIST:

This site claims that lighter eye, hair, and skin color evolved by natural selection, controlled by sun exposure in the area in which people lived.I don't know when this answer was posted, but an alternative theory has been proposed based on sexual selection rather than natural selection. This USA Today article cites Jared Diamond: "dark peoples of equatorial West Africa and the New Guinea mountains get no more UV radiation than the light-skinned folk in Switzerland, if you take cloud cover into account." The article also cites Charles Taylor of UCLA, who finds Diamond's proposal logical, but only because he apparently accepts the argument that UV radiation isn't the likely cause, and there doesn't seem to be any other reasonable theory at this point.

Peter Frost ( here ) argues for sexual selection as well, and plots variation in skin/hair/eye color against areas of the world where the environment caused an imbalance in the sexes--not enough men, or women, to go around. Any recessive trait that would make a potential mate stand out could give that mate a selection advantage, and allow him/her to pass on the recessive genes. He has maps to show the distribution.

Light colors do appear randomly occasionally--apparently, blonde hair has shown up in some Australian Aborigines and in other unexpected places.

I just thought you might want to bring your site up to date to let readers know there are competing theories, and the case is by no means settled yet.

Thanks for providing this great service to the public!