Out of Africa

December 8, 2004 9:02 PM

EvolutionFilter: Regarding the out-of-africa theory and skin color. [mi]

If we all came from Africa, why is it that such a large percentage of the world's population is light-skinned? I know that there are other places besides Africa where there are dark-skinned people, but why aren't we all still black? Is there some evolutionary advantage to being light-skinned?

posted by BuddhaInABucket to Society & Culture (16 answers total)

It's a trade-off that occurs in places with less sunlight. If you don't need protection from UV rays you can get some vitamin D.

http://en.wikipedia.org/wiki/Melanin.
posted by Firas at 9:12 PM on December 8, 2004

So the evolutionary advantage is location-based. If you're going towards the equator with light skin, you're disadvantaged.
posted by Firas at 9:15 PM on December 8, 2004

Pigmentation effectively acts as a sunscreen... dark skin prevents sunburn and skin cancer. When some of our ancestors migrated to cooler climates, it became beneficial to soak up as much sunlight as possible in order to absorb vitamin D.

See here for more information (I've just skimmed it and can't vouch for all the info)

On preview: I knew I'd be beat.
posted by painquale at 9:16 PM on December 8, 2004

so, how did dark-skinned people get vitamin-d before enriched milk?
posted by BuddhaInABucket at 9:30 PM on December 8, 2004

As you move north, importantly, there is less sunlight, and in partiular, UV light that is responsible for Vitamin D.

Based on this, Buddha, people at the equator may be less efficient at obtaining vitamin D, but because there is more sunlight there, it balances out. Dark skinned people living at locations with less sunlight, however, have problems with vitamin D.

By the way, it is now believed by some that rather than protection from UV rays, dark skin may be an adaptation to loss of folate. As you move away from the equator to areas with less sunlight, loss of folate becomes less of a problem.
posted by Jimbob at 9:37 PM on December 8, 2004

so, how did dark-skinned people get vitamin-d before enriched milk?

From the meat you eat, especially fish. Hence why populations north of the Arctic Circle aren't very pale-skinned, they traditionally eat so much fish and blubber (and Vitamin D is fat soluble) that they didn't need to soak in the sun.

I suggest this article, the maps of skin tone are interesting, and it talks about the recent changes that could have occured in skin colour (pre- vs. post-agricultural Europe, etc).
posted by nelleish at 9:48 PM on December 8, 2004

wow, that's really fascinating, and now my question is completely answered. Thanks you guys!
posted by BuddhaInABucket at 10:52 PM on December 8, 2004

I can't remember the name of the book, but I remember reading that perhaps skin color, hair color, etc. isn't necessarily the result of evolutionary advantage, but actually resulted from what various groups found physically appealing (and thus propagated).

It's entirely possible (ok, likely) that I misunderstood what I read. Is this anywhere near a mainstream theory?
posted by Doug at 12:41 AM on December 9, 2004

One more thing however....

I don't think there is any reason to believe humans (Africans that is) were dark skinned prior to migrating to the north and loosing their dark skin. Skin color doesn't show up in fossils and I am not aware of any proof one way or another to suggest that humans (Africans...) were not light skinned at one point and evolved darker skin.

When humans (or proto-humans...) had 100x as much body hair as they do now, the color of their skin wouldn't matter too much. The color of their hair however would matter a great deal. Without getting much sunlight, like animals at the bottom of the ocean, I would imagine the skin would be lighter due to the lack of a need to create pigment for the skin.

I know this sounds like an argument someone in the KKK would make, but really, I can think of no reason to suggest either option (humans originally being light skinned, or humans originally being dark skinned) is known or widely trusted in the scientific community.
posted by pwb503 at 12:47 AM on December 9, 2004

It is interesting to identify the advantages various adaptions might have, but it is impossible to verify the "reason" why a particular trait spread. It could have been natural selection for the reason you identify, natural selection for a reason you have not identified (including sexual selection), or pure random chance.
posted by grouse at 2:20 AM on December 9, 2004

perhaps skin color, hair color, etc. isn't necessarily the result of evolutionary advantage, but actually resulted from what various groups found physically appealing (and thus propagated).

If its physically appealing then it is an evolutionary advantage. The question is why they developed to find those things physically appealing.
posted by biffa at 4:07 AM on December 9, 2004

pwb503- that is an interesting question, and the thing is that it isn't thought that we were very dark to begin with. From the article I linked above:

"...the default human complexion is apparently the light brown of the Khoisan and Ethiopian peoples. It is easy to see one’s own complexion as normal and others as needing explanation, but this is an illusion... In fact, both the deep darkness of the Bantu people and the extraordinary near-albino lightness of the Scandinavians seem to be relatively recent and selectively driven adaptations. One is a paleness adaptation. The other is a darkness adaptation."

grouse- Sexual selection IS natural selection, as biffa states. I'd think healthy babies are pretty vital to the success of the species. If in an ancestral setting those who bear healthy babies (free of neural tube defects and/or rickets) were dark or light, respectively, then it's in your best genetic interest to develop attracted-ness to that coloring. There's plenty of evidence for this in the animal kingdom: those traits able to produce the most viable offspring become dominant. You mate with who you think will help you produce the most viable offspring, likely to live to sexual maturity, because everybody wants their genes to be passed on. Hence, certian tones become typical of certain climactic and dietary conditions, because they were the traits that give the best advantage for you and your offspring in that climate.
posted by nelleish at 8:10 AM on December 9, 2004

pwb503: even without nelleish's link, i'd have positied that the dark skin was ancestral: all the great apes have dark skin. chimps, gorillas, orangs. as only one member of this group (us) appears to include light-skinned versions, the most parsimonious explanation is that dark skin is ancestral and light skin is a derived characteristic unique to specific groups of humans.
posted by caution live frogs at 11:51 AM on December 9, 2004

nelleish: What part of "natural selection for a reason you have not identified (including sexual selection)" implies that sexual selection is not natural selection?
posted by grouse at 2:42 PM on December 9, 2004

oooh, heh. Read that strangely. Sorry there grouse, I can be an idiot.

I think I reacted as I did, because your first sentence ("It is interesting to identify the advantages various adaptions might have, but it is impossible to verify the "reason" why a particular trait spread.") didn't make sense to me at first. I was thinking: Wouldn't the adaptations be the exact reason a trait spreads? But now I realize you were talking about vectors for evolution, not evolution itself. In the end, it does all come down to who has the most babies, but there are reasons why one individual reproduces more than another.

I think that made sense.
posted by nelleish at 4:37 PM on December 9, 2004

nelleish: Say you have a particular trait that you think confers a selective advantage. You can do a controlled experiment to verify that advantage.

Now design an experiment that can falsify your hypothesis that this trait was propagated because it was more adaptive for the reason you specified than another alternative trait or for a reason you did not specify at a particular time in history. You have to rule out both (1) genetic drift, and (2) alternative explanations for why this particular trait is adaptive to the one you posit. This is made all the more difficult by the fact that you can only infer what the environment was like at the time. Geologically fairly accurately, but as far as the kinds of other organisms around, not so much.

You would be hard-pressed to create this experiment.

Not all changes are adaptive and an inherent assumption in asking these "what is the evolutionary reason" questions is that they are, which is simply begging the question¹. In science "why" is a question we don't ask about things we can't measure.
posted by grouse at 2:07 AM on December 10, 2004