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Meet Dr. Nina Jablonski, anthropologist at the California Academy of Sciences

Evolution of Human Skin Color It appears that our earliest modern human ancestors (Homo sapien sapiens), who lived 100 - 150,000

years ago in eastern Africa, had dark skin to protect them against the deleterious effects of ultraviolet

radiation. Many scientists used to believe that dark pigmentation evolved in Africa as a "sunscreen"

to protect against skin cancer. However, this could not be the only selective pressure since most

deaths from skin cancer occur only after reproductive age. According to the most recent theory,

different skin colors evolved to ensure reproductive success by regulating the production of two

critical vitamins.

Ultraviolet radiation (UV) catalyzes the synthesis of vitamin D, which is required for absorption of

calcium and development of the skeleton. Vitamin D deficiency can lead to rickets, a crippling bone

disease. But overexposure to UV radiation will break down vitamin B folate (folic acid), which is

necessary for fetal neural development and fertility. Anthropologist Nina Jablonski theorizes that

dark skin evolved near the equator. There, UV radiation penetration is high enough to stimulate

vitamin D production while the dark skin protects against the breakdown of folate. Light skin

evolved when early humans migrated to the high latitudes where UV radiation is much lower. The

amount of melanin gradually decreased to facilitate vitamin D synthesis under low UV conditions.

Today, as a result of recent migrations, many individuals do not live in the climate for which their

skin is adapted. Dark-skinned people in high latitudes can get their vitamin D from sources like fish,

while light-skinned people in the tropics can protect against folate breakdown by covering up with

clothing.

Science Interchange reporter Stacey Fowler recently interviewed Nina Jablonski, co-author (with

George Chaplin) of a paper entitled "The Evolution of Human Skin Coloration," published in the

July 2000 edition of the Journal of Human Evolution. Here are some highlights from the interview:

Dr. Jablonski, could you tell me about the recent research you conducted on the evolution of

skin pigmentation? Skin coloration is one of the most obvious ways in which humans vary from one to another. And so it

is of obvious interest to everybody because you look at one another and you say, "Oh, that person's a

different color than I am." What I've been interested in is what the evolutionary history of our skin

coloration is.

And what is some of that history? Well, skin is one of those things that isn't preserved in the fossil record. It's not like bones. And so,

reconstructing the history of skin, whether we're talking about its sweating abilities or its color, is

difficult and has to be done through indirect investigation. However, we've been able to shed some

interesting light on this phenomenon by looking at some of the physiological characteristics of skin.

For instance, skin-- especially dark-colored skin--is particularly good at screening out ultraviolet

radiation, and we consider it to be highly adaptive screening out ultraviolet radiation, and we

consider it to be highly adaptive. It turns out that ultraviolet radiation not only causes skin damage,

like wrinkling and things like that, but also it has much more sinister effects. It actually can cause the

breakdown of some crucial metabolites, or nutrients, in our blood capillaries such as the nutrient

folate, which turns out to be critical in normal development. So, if you get too much ultraviolet

radiation through your skin, the folate in your blood can actually be broken down by the radiation.

And this can have many deleterious effects. And so, having a natural sunscreen in your skin helps to

prevent that breakdown of folate.

On the other hand, if you are living in areas where ultraviolet radiation is particularly low, such as

areas near the Arctic or Antarctic circles, or actually as you move out of the tropics, you have another

problem to deal with. The skin is the place where Vitamin D is synthesized using ultraviolet rays to

catalyze the reaction. So you need some ultraviolet light to penetrate the skin in order to make

Vitamin D. Vitamin D turns out to be critical to your body because it provides the means whereby

you absorb calcium from your food in your digestive system. So if you don't have Vitamin D, you

can't absorb calcium from your food and you can't build strong bones.

Making the proper skin color turns out to be a balancing act between having enough natural

sunscreen to prevent a lot of damage to the contents of the blood system. On the other hand, you have

to let in enough ultraviolet light to still permit the formation of Vitamin D in your skin. So people

who live in conditions of lower ultraviolet light, away from the tropics and toward the poles, have to

have lighter skin than those people who live closer to the tropics or closer to the equator. Those

people really have to have darker skin to protect themselves from ultraviolet light.

Those of us who are sort of in the middle, like inhabitants of most of North America and most of

Eurasia, have to have skin that is capable of some level of tanning so that we can protect ourselves

from lots of ultraviolet radiation in the late spring and summer. But we can de-pigment ourselves as

ultraviolet light becomes less intense in the winter so we can take advantage of the ambient

ultraviolet radiation that does exist.

How did skin coloration evolve as our ancestors radiated out from Africa to inhabit other

continents?

The history of our own species, Homo sapiens, in terms of skin is a fascinating history. If we look at

our earliest Homo sapiens ancestors (about 100 to 150 thousand years ago in eastern Africa), we can

reconstruct that those ancestors would have had dark skin to protect themselves from the deleterious

effects of ultraviolet light. But those populations began to move out of the tropics and colonize areas

that were much less intense in terms of ultraviolet light. As they first moved into the Circum

Mediterranean, Western Asia, then onward into Eastern Asia, Europe, Southeast Asia, Australia and

so forth, these populations would have to undergo some depigmentation in order for them to be able

to synthesize enough Vitamin D in their skin.

Imagine, for instance, the populations that went from East Africa and slowly made their way into

central Asia or northern Asia. These populations would have had to undergo quite extensive

depigmentation in order to maintain enough Vitamin D synthesis potential in their skin. But imagine

some of these populations that were eventually on their way into Southern India, or what is now Sri

Lanka. Those populations that also originated, ultimately, in eastern Africa would have undergone

some depigmentation as they moved out of the most intense UV of the tropics, and then they would

have undergone repigmentation as they moved down, back into the intense ultraviolet regimes of

southern India and Sri Lanka.

This same pattern of intense pigmentation to start out with, followed by a period of depigmentation

perhaps 10, 20, or 30 thousand years long, followed again by another period of repigmentation, I

think has been followed by many different populations as they have gone from one part of the world

to another. It's not a deterministic process; it's simply an adaptive process as these populations have

changed from one area with one particular ultraviolet light regime to another.

Are we seeing any evidence that skin pigmentation is changing in response to current

environmental factors? One of the most interesting changes that we are seeing today, of course, is that people are moving

from one part of the world to another. You have lots of very light-skinned European people who are

moving into areas where there's a lot of ultraviolet light -- either to the southern United States or

people moving from England to northern Australia, for example. And so we're seeing people who are

inherently well-adapted to low levels of ultraviolet light moving into areas where there's a lot of

ultraviolet light, causing them to suffer tremendously from ultraviolet light damage to their skin.

On the other hand, we have an interesting phenomenon with people who are moving from where

ultraviolet light is very intense, such as Africa and India, into regions where it's less intense, such as

the United States or the UK. For instance, these days there are a lot of people from the subcontinent

of India, including Pakistan, moving into the UK and the United States where there are much lower

levels of ultraviolet light than they're used to. It turns out that these people are particularly

susceptible to Vitamin D deficiencies of various kinds.

Although we don't see human skin changing in response to environmental changes because our time

frame is too short to see any evolutionary change, what we are seeing are the dramatic

effects of human migrations as people move from areas of the world that they are well-adapted to

areas of the world where they are not well-adapted in terms of ultraviolet radiation.

If, for instance, an Indian family moved to the UK and lived there for several generations, at

what point would their descendants begin to adapt to the climate? It's hard to say how long this adaptation would take because these days adaptation in any human

characteristic is very much mediated by our cultural behavior. Humans do a lot of stuff : They wear

clothes, they take shelter, they take vitamin supplements, they do all these things to change the nature

of their interface with their physical environment. So it's now almost impossible to predict how long

it might take for a human population to adapt to a different ultraviolet light regime because we do so

much meddling.

A final comment? I think one of the most important findings of our research is that skin color is a highly adaptive

feature of the human body. It has changed over thousands of years to reflect environmental

conditions. That is a wonderful thing in itself because it means that, basically, the skin is a highly

flexible organ. We know this already from other types of physiological studies, but in terms of

evolutionary biology it is also very flexible. It can change depending on the environmental

conditions, which means that skin color itself is really of no value when we look at evolutionary

relationships per se among different human populations. You can have individuals from different

populations that share a similar bone structure, for instance, but have a completely different skin

color. The two are unrelated. And so we can't use skin color for determining relationships between

human groups.

The map above shows the potential for synthesis of vitamin D in human skin, as computed from

annual average UV radiation at the Earth's surface (UVMED). The highest annual values for

UVMED are shown in light violet, with incrementally lower values shown in dark violet, then in

light to dark shades of blue, orange, green and gray. White denotes areas for which no UVMED

data exist (Mercator projection). In the tropics, the zone of adequate UV radiation throughout the

year is delimited by bold black lines. Light stippling indicates Zone 2, in which there is

not sufficient UV radiation during at least one month of the year to produce previtamin D3 in human

skin. Zone 3, in which there is not sufficient UV radiation for

previtamin D3 synthesis on average for the whole year, is indicated by heavy stippling. In short this

means that within the tropics, people can meet their vitamin D needs through casual sun exposure. As

you go farther north or south, this becomes an increasing problem. In the area we refer to as Zone 3,

this is an acute problem for human populations. Successful habitation of that zone has required

evolution of greatly depigmented skin and inclusion in the diet of lots of vitamin D-rich foods (like

fish and marine mammals).