How could natural selection account for skin colour?

Question

My husband is studying anthropology, and I am currently editing his assignment regarding the evolution of the human brain and it's relationship to the development of culture.

He wrote: "Queensland has the highest skin caner rate in the world. This is due to those currently living in Queensland not having evolved here. If natural selection was left to select those that had darker skin then over time the people of Queensland would progressively get darker."

It just occurred to me that this can't be true. Why?

At what age do white people get skin cancer as a result of sun damage? Usually after the age of 30. Since we reach puberty at around age 12, and many cultures in the past have begun reproducing at this age, most adults over thirty would have already had many children by the time they came down with skin cancer.

Those adults who died from skin cancer would have reproduced long before this malady killed them. Hence, how could natural selection account for skin colour?

Answer 1

Hi! I think you just got a li´l confused. Skin cancer is due mainly to the hole in the ozone layer caused by human beings in the last  40 years, let´s say. That´s like a fraction of a second to the cosmic time. Evolution takes hundreds and hundreds of years to happen. Probably 70 years from now, our grandchildren´s kids will be born only if their parents´ skins will be able to afford UV at least until they can give birth to a new breed. And that probably means that they´re are gonna have a different skin color than you do now.....

Answer 2

You wrote: "If natural selection was left to select those that had darker skin then over time the people of Queensland would progressively get darker."

This is not necessarily true.  However, if a population with darker skin were to move towards the poles, they would over generations become lighter, because a chronic vitamin D deficiency would lend a reproductive advantage to anyone with lighter skin, regardless of age.

Remember that dark hair, dark skin and brown eyes is the default setting for humanity.

Most other physical attribute are just a matter of what different populations find, or found, physically attractive.

Answer 3

Those are good questions you raise, but I can think of three problems with your thinking.

Female puberty occurs at 12, these days, but even 100 years ago, it didn't occur until at least 16 or 17. A lot of nutritional factors probably play into that.

You also say that TODAY, skin cancer usually starts at 30. I don't know if that is true, but consider that today, we spend most of our time indoors and often wear clothing over most of our skin. When humans were first evolving, we wore no cloths and lived outdoors so our exposure to sun was much greater. It is possible that light skinned people living a few hundred thousand years ago would have gotten skin cancer at a much younger age.

A third factor is that even if people die at age 30-40, that gives them less opportunity to raise all of their children to the age when they themselves can have children nor can they help in raising their grandchildren. We must have grandchildren or our genes die out in our children.

All three of these factors would contribute to people with light skin having a smaller window of time to reproduce and pass on their genes. Inevitably, people with darker skin would predominate in the population.

Answer 4

You forget that the children are at a disadvantage because their parents may not be there to help them grow up due to cancer.

Answer 5

From NewScientist

10 March 2007

NewScientist.com news service

Jessica Marshall

"So why did different populations evolve different skin tones? The leading theory, proposed by Nina Jablonski, also at Penn State, is that our colour reflects a balance between conflicting needs. Not only can sunlight damage our skin, it also breaks down folic acid (also known as folate), an essential B vitamin. On the other hand, we need ultraviolet light to make vitamin D.

Jablonski and colleagues have shown that skin colour around the planet correlates more closely with winter UV levels than with summer levels (New Scientist, 12 October 2002, p 34). This suggests that our skin colour has evolved to optimise folic acid and vitamin D levels during winter, with tanning allowing us to adapt to higher UV levels in summer."

Answer 6

You're basing your skin cancer rates on indoor cats. Naked, all day everyday in the sun would produce a different early skin cancer rate as well as heat stroke, sun burns etc....  Add in a small population of ancestors & there it goes.

Answer 7

Melanin would build up in successive generations if they were continuously exposed to high levels of UV radiation.  The argument not put forth here is in simple sexual selection.  Not all traits are inherited or continued because of natural selection.  Humans pick disadvantageous traits a good deal of the time because we find them appealing.  Tall, thin women are not prime candidates for child birthing.  The birth canal is narrowed and the child has to have its still soft skull forced between constricted pubic bones.  Today they are often C-sectioned to prevent complications, not exactly natural.  So as for skin cancer, tough I disagree with the age of death from skin cancer (I would put it closer to 50) by age 30 many people will show spots from cancerous cells around age 30 and these are not exactly attractive.  You are less likely to pick up a lover if they have discolored spots covering their bodies.  Throughout all societies symmetry is always considered attractive and random blotches covering your exposed skin is not exactly symetrical.

Answer 8

Good thinking Momof4!

Answer 9

First, I would suggest studying the National Geographics Genography Project.

When I was in school those many years ago, I was taught that the fair skinned people were from the northern climes and that only the northern Eurpeans had red or blond hair.  Well, since starting to search for my ancestors on the web, I have found that blondes and red-haired people also lived in the Middle East.  I have found that blacks have migrated to cold climes 10,000 years ago, and they are still black; that white people migrated to Africa's coast and 10,000 years later, they are still white.

This "natural selection" business is all part of Darwinism, which is, at most, a theory.  There is no proof anywhere that one species evolved into another; there is no proof of natural selection.  Consider: if "survival of the fittest" is a scientific fact, then people who are fat, lazy, weak, and prone to be on welfare are the fittest, because they reproduce the fastest!

You mentioned the evolution of the human brain and its relationship to the development of culture.  Find a copy of "The Neanderthal's Necklace" and read it.  The Neanderthals were huge, standing more than 6 feet tall, very well adapted to the cold of the last ice age, had huge cranial capacities, were stong...but are now extinct.

Of everything I have ever read, the two greatest factors in culture have been the female of the species (wanting protection, comfort, companionship, etc.) and agressiveness (the cultures support war-like postures).

As to cancer rates, I would tend to believe it has more to do with diet; again, a real-life observation: the Orientals used to have such difference in the appearance of the fold of the inner eye.  Darwinists claimed it was to protect them from the extreme cold of the Orient.  (They obviously never wintered in Nebraska or Montana.)  Anyhew, nowadays this feature is fading.  Why?  Diet.

Perhaps Queensland people spend more time in the sun; perhaps there is something in their diet.  They have recently reported on CBS-TV that diet is a major factor in skin cancer, as life observations have already pointed out.

Again, as to "natural selection", look at the people you know, and find out why they picked the mates they have!

Answer 10

I find the fact that, among the answerers who think they understand evolution, not a one among the first 5 answers understands!  This is apalling!  

A naturally inherited dark skin (a possible heriditary advantage in a very sunny place) has NOTHING TO DO WITH SUNTANS except it may LOOK THE SAME!  Lots of people having suntans does NOT PRODUCE tan newborns!  

This false belief was discredited over 100 years ago!  

All dark human skin is due to melanin in the skin.  This can be produced from sun exposure OR it can be inherited, OR it can be a sum of the two.  

You are right that skin cancer arising after having children would have no relationship to whether those children might, themselves, be susceptible to skin cancer.  Skin cancer, as you suspect, might be a consequence of someone having too much sun exposure, after they have reddened, blistered, and/or tanned.  But whatever color the adult's skin ends up, skin cancer just means the coloration (the acquired or the inherited or the two added up) was just NOT ENOUGH to protect the sensitive now-diseased layers of the skin.  WIth different people, the amount of sun required to cause disease may be more or less, and it will probably depend somewhat upon skin color.  It will also depend upon patterns of exposure, because different parts of the solar spectrum may produce malanin coloration than produce cancer, and those spectrum variations vary with the time of day and the type (thickness) of the clouds.  Two people of apparently equal darkness may have different levels of likelihood for developing skin cancer.  

Because of the truth of your observation, that reproduction may precede the first signs of cancer, a genetic line of people who are born pale and darken as they age is likely to have cancer unaffected by the adults being dark.  The cancer can occur before sufficient darkening occurs in any generation.  However, skin cancer can cause a person to die before they might have had SOME children -- those will not be born and they WOULD HAVE BEEN sun-sensitive and probably skin cancer-susceptible.  All of THEIR offspring are likely to be fewer in number than those from the born-dark.

A family (a bloodline) where darkness is clearly inherited to a degree that provides significant solar protection is LESS likely to show skin cancer occurrences in young people and people not getting a lot of sun.  

How does a population change from being mostly pale to mostly dark at birth?  It happens by the VERY GRADUAL process of diseases like skin cancer "killing off" SOME PEOPLE who are inadequately protected by their skin color.  

In any given population,.the tan people may have tan skin for any of the reasons I gave previously:  inherited, acquired, or the sum of the two.  Just a very few of the tanned-by-the-sun people will have just slightly fewer offspring for the reasons connected to death by skin cancer.  The tan-by-inheritance people will not suffer this SLIGHT reduction in offspring.  

Over hundreds of generations (like the centuries the aboriginals have lived in Australia), more of the born-tan people are born than born-pale.  The aboriginals have so few born-pale babies, that you can say the aboriginal people now have a survival advantage.   In fact, it is the individual born-dark aboriginals that have that advantage.  Those that might be born-pale simply become fewer and fewer (not by dying, but by not being born) with each new generation.  

I hope this clears things up.

Answer 11

There is a difference between natural selection and artifical selection. If the people determined that people of darker skin were more attractive for what ever reason, and the people over generations became darker, then that's artifical selection.

But the only way for it to be natural selection would be if the lighter people died off before they reached the reproductive age. You are correct.

Answer 12

That of course assumes that whatever we evolved from had light skin, and not dark skin. Could it not be that our ancestors were all dark skinned, and that light skinned people are the ones who mutated to suit (or perhaps mutated despite) their environment - not dark skinned peoples.

Though you certainly raise a puzzler, that's for sure! I wonder if this has ever been discussed by scientists. You should email some learned institution and ask them!

Answer 13

your premise is way off, the only way you can adapt to the extreme sun is sun tan lotion.  and that doesn't guarantee that you wont develop skin cancer.  The only way you can adapt to these conditions is have an offspring by someone with darker skin and hope that they are born with their color.

Answer 14

Queensland has a high skin cancer rate because of Australian culture. The white Australians think it's a great idea to lie in the sun on the beach all day and go around with your shirt off in the sun. Most Europeans would disagree, and those of them that do like to sunbathe can only experience a Queensland-style climate by going abroad, which lasts for a few weeks at most. Aussies on the other hand can sunbathe all year round - and also suffer equivalent damage all year round.

I think your husband will ultimately be proved right as the paler Aussies will either suffer visible skin damage before their reproduction age (usually their 20s) or simply move to a more sheltered country. The darker ones would breed amongst themselves, or even with Aborigines - the darkest people of all.

Answer 15

It can't because it isn't true!  God created us, not single celled organisms.

Answer 16

I agree with Blue_Bee, she hit the nail on the head.

Answer 17

children entering puberty would find mates of darker skin more attractive, since they will know from the elders that darker lives longer.

Answer 18

People with lighter skin allow more sunlight to enter their bodies.  Therefore, if the people had darker skin, less sunlight would get in.

Answer 19

Each race has various risk for particular diseases...

you gave one example. Another example is that sarcoidosis is primarily found in black people(and various other examples, based on different groups, gender, and so on).

Natural selection is not based on a large group of people, but more of an individual issue...(It's a micro issue, as opposed to a macro issue).

Also, natural selection is not based on one issue(one illness), but the person 'as a whole'.

Good luck on the paper.

Answer 20

You don't have to get cancer to get darker skin. Its called a tan. As a population developed, they would produce more pigment (melanin) as they spent more time in the sun. Hence each generation would produce more and more pigment over generations. What the h**l does Cancer have to do with evolution? It is a result of one having NOT evolved in a certain environment, not the other way around.

Answer 21

I have no idea if people can evolve from light to dark

Early humans were dark skinned, and as some moved into temperate regions, their skin got lighter.  This was probably because with more pigment we could not absorb enough vitamin D.

There was no need for people in regions with more sun to evolve this way, because they got more sun hence more D.

There is no mechanism I know to pass down pigment from parent to child.  Just because you got tanned every year of your life and ended up far darker than you were born, does not mean you are going to magically have darker skinned children.  You pass on your genes and whatever habits you can while the child is in the womb.  Tans fade, while pigmentation is genetic.

One thing you might want to look into would be the earliest onset on skin cancer - if only a small percentage die before they are done with children, it would have a cumulative effect.

Also, consider when skin cancer might occur if people used no sun screen - he could alter his argument to include that if it helped.

Answer 22

You are quite correct, the primary factor that dictates the reproductive success of melanin levels has very little to do with skin cancer.  Skin colour is a factor of melanin concentration and has shown regional variations in the past to counteract the intensity of the sun, or lack there of. Prehistorically melanin was selected for based on the reproductive results between how much melanin each individual had and how that effected our bodies chemistry with respect to health and reproduction. Our differences in skin tones, therefore, was because of evolution. Ultimately the driving force of evolution is reproductive success and while there is a variance in every population the overall tone will drift to what is best suited for each environment over generations. As is seen across the globe, there is the general rule that people who live in equatorial regions are darkest and this blends until artic areas are reached where people are the lightest. Like I said, the reason why this was prehistorically was because of reproductive sucess. For instance at equatorial areas the harsh uv rays from the sun breakdown certain vitamins and chemicals that are active in the operations in the human body. One of these is vitamin B, but the most important chemical that is broken down by sunlight as far as reproduction is concerned is folic acid. Folic acid is crucial to rapid cell development so if an extremely light skinned individuals are habituating in tropical regions the males will experience a reduction in their sperm count, and women would experience more miscarriages because Folic acid is extremely important for foetal development. Conversly, dark people habituating in northern hemisphere localities will have a reduced absorption of vitamin A & D which is also important for foetal development and for general health which would cause a reproductive discrepancy that would cause this darker toned characteristic to have been selected against.

Now, earlier I mentioned "in prehistoric times", and that was on purpose. Our knowledge and technology has already deafeated these prehistoric influences on skin colour from having an effect on reproductive sucess in modern times. Light toned people living in tropical areas can simply take vitamin B/folic acid supplements as well as use sunscreen to block the intensity of uv in breaking down the chemicals/vitamins that are important for reproduction and dark toned people living in northern environments don't even particularily need to focus on taking supplements because even in the winter, when their vitamin A & D deficiency should be the worst, the contemporary supermarket is full of imported fresh produce and milk with these vitamins added, and this will allow an ample offset to any loss of vitamin A & D that may be experienced.

So in modern Day Queensland this selective pressure will no longer be an issue as I'm sure that most women there are put on folic acid as soon as they conceive, and those who are experiencing reproductive difficulties will be put on it earlier.  If all human populations mixed then we would just be a widely varied mix of colour everywhere around the globe that would be primarily brown in tone and there would be no reason for colour to be selected for in either direction so long as darker toned people continued to eat their produce and lighter toned people remembered to take their folic acid and wear sunscreen.  The later, sunscreen, may even reduce some of these skin cancer rates, but as far as reproduction is concerned it is it's protection that it affords to the bodies vitamin balance that counts!

Hope this all helps!

PS, for those that said that parental investment would offer their children a greater "success"... this "success" would be consistent in terms of financial and social, but this will not neccessarily translate into reproductive success.  In fact in modern populations it doesn't as "successful" people show the lowest reproductive rates as they tend to start focussing on retaining the "wealth" and "success" of their offspring by restricting heirs, where as less priviledged individuals tend to have more children so as to try and hedge their bet that some of their offspring can find success without their financial inputs.

Answer 23

Overall, it sounds acceptable, although I do have a few critiques for both you and your husband, which follow:

First, he's assuming a negative correlation between skin cancer and increased skin pigment concentration. I'm not sure that such a relationship is that well defined. I could be wrong on this point; he had just better be able to back it up with a study or two.

Second, one needs to consider the length of time that humans have inhabited Queensland. Evolution seldom acts quickly, and according to the movers and shakers of evolutionary theory, evolution does not happen quickly by our standards. This takes hundreds generations, at the absolute minimum, and that is referring to something that is always lethal in the homozygote. Something like pigmentation would take even longer.

The third problem that I have with it is that his verbiage and tone implies that evolution has stopped. Natural selection has yet to take a break. I'd also apply my first critique with your argument as well. While it is logically sound, you must establish that most skin cancers do not strike until after reproductive age, and even more importantly, that skin cancer susceptibility is heritable. As far as I know, skin cancer has truly limited heritability. It occurs as a result of mutations caused by exposure to the sun, and may or may not be passed on to offspring. You are also assuming that the mutation that produced the cancer occurred early and did not express until late in life. I fear that this is a flawed assumption.

I'm no expert on skin cancer, it's timeline for action or its heredity, but you need to address the questions that I have outlined in order to make the claims that you both do. As an aside, what you describe is a perfect example of the evolution of senescence, a truly fascinating topic in my opinion, and one that you seem to be versed in. Whatever the case may be, kudos for thinking critically! Those conclusions would have escaped most people, so rock on for catching what most wouldn't!

Answer 24

selective advantages don't have to be very large. the effect is cumulative over many generations. it's like compound interest. also you neglect other factors such as sun damage itself, which can occasionally be deadly (especially without modern technologies like sunscreen).

Answer 25

Quote

"He wrote: "Queensland has the highest skin caner rate in the world. This is due to those currently living in Queensland not having evolved here. If natural selection was left to select those that had darker skin then over time the people of Queensland would progressively get darker."  "

Reply

I agree with his statement given enough time and assuming there weren't any factors working against this direction of evolution.

Quote

At what age do white people get skin cancer as a result of sun damage? Usually after the age of 30. Since we reach puberty at around age 12, and many cultures in the past have begun reproducing at this age, most adults over thirty would have already had many children by the time they came down with skin cancer.

Reply

A miniscule advantage will be incorporated over a large number of generations.  

"Natural" selection would imply a more primitive existence.  We would have a very hard time competing with the Aboriginies since we would obviously have higher cancer rates.  The thinning in the ozone in the south, the winter summer, and the Sun worshipping Australians all account for higher skin cancer rates.  Skin cancer can manifest itself in younger people as well.  There is an advantage to having older people around to help teach younger ones so I wouldn't necessarily dismiss that benefit to society for people beyond the age of giving birth.

Answer 26

Natural selection is not the only factor it is a big part of it but not the only one, Adaptation is also accruing.