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

Science Interchange reporter Stacey Fowler recently interviewed Nina Jablonski, co-author (with George Chaplin) of a paper entitled "The Evolution of Human Skin Coloration," which will be 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).

To hear Quirks and Quarks, go to

http://www.radio.cbc.ca/programs/quirks/archives/00-01/logs0001.htm

The anthropologist Nina Jablonski explains.  Go to February 10, 2001.

Here is another way to access it:

http://www.radio.cbc.ca/programs/quirks/realaud/01-02-10-quirksandquarks.ra

SUMMARY

1.     dark-colored skin--is particularly good at screening out ultraviolet radiation

2.     ultraviolet radiation not only causes skin damage

3.     can cause the breakdown of folate.  So, having a natural sunscreen in your skin helps to prevent that breakdown of folate.

4.     But, the skin is the place where Vitamin D is synthesized using ultraviolet rays to catalyze the reaction.

5.     you need some ultraviolet light to penetrate the skin in order to make Vitamin D

6.     So vitamin D allows you to 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.

7.     It is a balancing act between having enough natural sunscreen to prevent a lot of damage to the contents of the blood system,  and having to let in enough ultraviolet light to still permit the formation of Vitamin D in your skin

8.     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.

9.     Early African populations that left Africa had to undergo some depigmentation in order for them to be able to synthesize enough Vitamin D in their skin.

10.   The populations that moved from central Asia down into deep India would have undergone repigmentation as they moved back into the intense ultraviolet regions of southern India and Sri Lanka.

11.   Now we have lots of very light-skinned European people who are moving into areas where there's a lot of ultraviolet light, causing them to suffer tremendously from ultraviolet light damage to their skin.

12.   People are  also 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.  It turns out that these people are particularly susceptible to Vitamin D deficiencies of various kinds.

13.   We don't see human skin changing in response to environmental changes because our time frame is too short to see any evolutionary change.

14.   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.

15.   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 north, successful habitation has required evolution of greatly depigmented skin and inclusion in the diet of lots of vitamin D-rich foods (like fish and marine mammals).

Skin Colour 2

I talked about skin colour, and how if your hide is loaded with lots of a chemical called melanin, making your skin is dark in colour. Sunlight stimulates the production of more melanin, thus making your skin darker. But exactly how did we end up with the situation that the people with the lighter skin bunker down near the poles, and the people with darker skin hang out near the Equator?

Well it's all to do with vitamins. So claims a paper published in the Journal of Human Evolution, by Nina Jablonski, who's an anthropologist, and her husband George Chaplin, who's a Geographic Information Systems Specialist.

Now their theory makes a few assumptions.

Let's assume that our earliest ancestors had a light skin, like our closest relatives the chimpanzees. They evolved some 2 to 4.5 million years ago, in the rain forests of Africa. They were competing for food with a whole bunch of other animals in the rain forests. But out on the open savannas, there were not many competitors.

Now remember that when the Theory of Evolution talks about Survival of the Fittest, it doesn't mean the critters with biggest muscles - it means the ones with the most children. But too much sunlight can actually interfere with baby-making.

It's all because of a very important chemical called folate, part of the B-complex group of vitamins. Just one hour of bright sunlight can drop the folate level in a white-skinned person by 50%.

Folate is very important in making babies. If the mother doesn't have enough folate, the baby can be born with a Neural Tube Defect such as spina bifida. In the USA alone, about 2,500 babies are born every year with Neural Tube Defects, and about half of them are because the mother didn't have enough folate. There are even three cases where pregnant women spent time in tanning studios or solariums, and then gave birth to babies with Neural Tube Defects. Low folate can also cause other abnormalities in the foetus in the palate, lip, aorta, kidney, skeleton and gastrointestinal system.

But folate is also important for baby-making by the man. Low folate levels can stop the production of sperm. At one stage, the search for the male contraceptive led to drugs that would block folate - but they were too effective, and blocked folate everywhere in the body.

So, out on the sun-drenched African savannas, there was natural selection for people who had darker skin, which would stop the sunlight from destroying the folate.

But what about people who live far from the Equator where the sun is very weak?

It turns out that these people actually need a very light skin, that will allow as much sunlight as possible to penetrate. In this case, it's to make another vitamin.

Back in the 1960s, a biochemist, W. Farnsworth Loomis, suggested that some people have a light skin colour so their body can make vitamin D. Vitamin D is necessary for proper metabolism of calcium.

Now like everything in the human body, vitamins are complicated, and there are many different vitamin Ds. This biochemical pathway starts off in the skin with a chemical related to cholesterol. Sunlight turns this into vitamin D3. If you don't get enough vitamin D3 by sun exposure, you can get it by swallowing a fish extract, such as cod-liver oil. D3 gets converted into another form of vitamin D in the liver, which in turn gets changed into another vitamin D in the kidneys.

People with a darkish skin need between two and six times as much ultraviolet light, as compared to light-coloured people, to make the same amount of vitamin D3.

Children who don't get enough vitamin D get rickets, which makes their leg bones weak and curved. Rickets in adults is called osteomalacia. In fact, in Australia we are seeing a resurgence of osteomalacia in Muslim women who wear the chador - a loose, usually black, robe that covers the body from head to toe, and most of the face. Medical problems related to low vitamin D have also been seen in dark-skinned Indians migrating to the cloud-shrouded United Kingdom.

If you don't soak some sunlight on your skin, and if you don't take any fish oil, you get a vitamin D deficiency.

So people who live near the Poles need a light skin, and maybe a fishing industry, to get their Vitamin D. Luckily vitamin D is fat soluble, so you can store enough to prevent a deficiency for about three or four months, which would get you through the worst of a deep Arctic winter.

So there you have it - dark skin near the Equator to stop folate from being destroyed, and light skin near the Poles, to make vitamin D.

Now a theory is nice, but it's just a pile of words unless it's backed up by experiments. Luckily, NASA launched the Total Ozone Mapping Spectrometer satellite in 1978. It measured ultraviolet light all over the surface of the planet. Our friendly anthropologists, Jablonski and Chaplin took these measurements, and compared them with skin colour in more than 50 countries. The link was obvious - the stronger the ultraviolet light, the darker the skin.

So just remember, that in our world of fly-by trans-continental mass migrations, from a vitamin point-of-view, the best place for you under the Sun, is where you're from...