RENEE MONTAGNE, host:
Now for a look at the science of skin color. Whatever your skin color, and even if you think your family has been that color for a long, long time, NPR Science Correspondent Robert Krulwich has some news for you.
ROBERT KRULWICH: According to Penn State Anthropologist Nina Jablonski, it now appears that most humans on our planet come from families that if you go back not that many generations…
Dr. NINA JABLONSKI (Penn State Anthropologist): Almost all of us were in a different place and we had different color.
KRULWICH: You may be as white as milk today or as black as deep chocolate, and yet, you're direct ancestors - and not because of intermarriage - as recently as 100 generations ago - and that's not far back…
Dr. JABLONSKI: No, it's not far. Really, it isn't far at all.
KRULWICH: Your direct ancestors could have had a very different color than you.
Dr. JABLONSKI: Our original estimates were that this occurred perhaps at a more stately pace, but now we think that that process of change may have had to occur much more quickly.
KRULWICH: And here's why. As everybody knows, says Nina…
Dr. JABLONSKI: Humans started in Africa.
KRULWICH: The part of Africa near the equator where it's intensely sunny with lots and lots of ultra violet light - which was a problem, because ultra violet light not only can cause sunburn, when you get too much of it…
Dr. JABLONSKI: Your connective tissues will break down and get very saggy and wrinkly, but perhaps even more importantly, the UV radiation will cause skin cancer.
KRULWICH: And skin cancer can sometimes kill you. On the other hand, if a human is plopped down, say in Norway, where the days can be very short and there's precious little ultra-violet light, then you discover there's a benefit to UV light.
Dr. JABLONSKI: And that is that it helps to initiate the process of producing Vitamin D in the skin. Vitamin D helps us absorb calcium and we need calcium to build a skeleton, to maintain a strong immune system, and for a host of other important functions in the body.
KRULWICH: So back when our earliest hominid ancestors were walking around central Africa, in order to survive, we had to figure out a way to screen ultraviolet light so we didn't die of cancer. But when humans then start to move away from the equator, then we had to get enough UV to build bones, stay healthy…
Dr. JABLONSKI: This is where evolution absolutely is at its peak.
KRULWICH: Because humans need skin that could adjust to changes in light, and so nature provided, says Nina.
Dr. JABLONSKI: An absolutely remarkable pigment called melanin.
KRULWICH: Melanin is produced by genes.
Dr. JABLONSKI: To balance the need to have sunscreen built in, versus the need to be able to still produce some Vitamin D in the skin, so it's a really cool molecule.
KRULWICH: And how does this molecule change skin color? Well, Darwin teaches that these things begin randomly. Somebody at some point had a baby and that baby just by chance has a little change in its DNA, which makes the baby's skin, let's say, a little lighter. So now, when that baby moves north to Europe, as a grown up, her light skin is an advantage because it helps her stay stronger, gives her more Vitamin D and so says Nina…
Dr. JABLONSKI: As they moved slowly northward, those generally that had the lighter pigmentation would have survived and reproduced preferentially.
KRULWICH: Meaning lighter skin people start to produce more babies than dark skin people.
Dr. JABLONSKI: That's really the bottom line.
KRULWICH: Producing more babies means you can survive as a group, and the further north you go, the better it is to have lighter skin.
Dr. JABLONSKI: Because by the time you get to lap land, boy there's precious little UV in the sunshine and there's precious little sunshine during the whole year.
KRULWICH: And that is why as you approach the Arctic, you do find more and more light skinned people.
Dr. JABLONSKI: And so, this in short, really creates the gradation of skin color that we see in modern humans today.
KRULWICH: And I didn't know this, but there are also instances of lighter groups turning darker over time.
Dr. JABLONSKI: The people now living in southern parts of India are extremely darkly pigmented.
KRULWICH: But once upon a time, those same people lived in central Asia. And up north, there, they were much lighter, until they moved south, where new evidence suggests…
Dr. JABLONSKI: They re-evolved dark pigment.
KRULWICH: And they're not the only ones. Aboriginal Australians may have gone from light to dark, Pacific Islanders, the same.
Dr. JABLONSKI: There's probably been a re-darkening of several different groups of humans. In other words…
KRULWICH: How would you know that? You don't have any photographs of their great-great-great-great-great grandpas and grandmas.
Dr. JABLONSKI: Oh but I have their genetic photograph.
KRULWICH: Meaning scientists have now studied the genes, the DNA, of populations all over the world.
Dr. JABLONSKI: What we can do in modern humans is isolate the genes that are responsible for pigmentation in most groups. And then, knowing the position of these genes in the genome, look at the pieces of DNA that we have isolated from some ancient ancestors and see how they compare.
KRULWICH: And by comparing DNA in ancient samples, they can very closely estimate when changes take place.
Dr. JABLONSKI: You can look backwards and see different generations of different colors. It's just phenomenal. But let's take that even farther.
KRULWICH: If you assume that each generation lasts 25 years, say, give or take…
Dr. JABLONSKI: Yes, yes.
KRULWICH: …it turns out that if a group of people keeps moving north or south to find food or to avoid enemies, to keep moving, that group needs to keep changing color at a remarkable pace.
Dr. JABLONSKI: We might be thinking let's say 50 to 100 generations…
KRULWICH: Wait, wait, wait, 50 to 100 generations, that's times 25, that's 2,500 years. You're saying that in 2,500 years, a lineage, a family group, can go from being black to being white or back the other way? I mean, that is lightening fast. That's like ancient Greece to now. I mean that's like a…
Dr. JABLONSKI: Yes, it's remarkable.
KRULWICH: But there is a caveat, she says.
Dr. JABLONSKI: We're talking about the rate of change during the earliest parts of the human evolution when we would not have had the nice togas or clothing that the man in Greece may have had.
KRULWICH: Because once people put on clothes, covering the skin, then the rate of change does slow down. So it's when we were naked and moving, that's when skin colors changed very quickly. But still, this means that this thing that we all notice when we think about race, skin color, it's not fixed at all. It is remarkably fluid.
Dr. JABLONSKI: We have now some fascinating genetic evidence of evolution acting on the skin, the antiphony of natural selection. We think that process of change may have had to occur much more quickly.
KRULWICH: Much, much, much more quickly.
I'm Robert Krulwich, NPR News.
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MONTAGNE: You can see a world's map that shows the range of skin color by continent at npr.org. This is MORNING EDITION from NPR News. I'm Renee Montagne.
LINDA WERTHEIMER, host:
And I'm Linda Wertheimer.