Physics Evolution Contradiction

Physics Evolution Contradiction

In basic physics we learn that dark colors absorb electromagnetic
energy and light or white colors reflect it.

So it would seem that evolution would be kind and thus select darker
human skin colors in Northern locations where it is very cold
AND conversely select lighter or white skin color in hot tropical areas
where heat is oppressive.

But darker skin is found in hot locations like the Middle East, Africa,
Thailand, India, Philippines, Mexico and Latin America while lighter skin
color is found in frigid locations like Russia, Northern China, Canada,
Northern Europe and Scandanavia

What is the explanation for this apparent contradiction ?

Or my assumptions incorrect ?

 

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Vitamin D synthesis. Plus a fair amount of sexual selection.
There is no contradiction.

 

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Short answer: Rickets.

Longer answer:
Sunlight on skin produces vitamin D.
Lighter skin (less melanin) produces more vitamin D for the same sunlight exposure.
A deficiency in vitamin D leads to insufficient absorption of calcium from the diet, which leads to rickets.

 

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Nope. This is a biological fact which is centered around the chemical, melanin. Melanin forms a protective layer in the skin, which prevents electromagnetic radiation from penetrating through your skin, and causing, sometimes cancerous, mutations in your DNA. The darkness of the skin is just a byproduct of increased volumes of melanin. That is what a tan is. It is the body producing more melanin to protect your cells and DNA, which then causes skin to darken.

Genetically, you may naturally have higher or lower levels of this chemical. In evolutionary terms, if you live nearer the equator and have higher levels of melanin, you are less likely to get some forms of skin cancer, and so live longer to propagate your genes. Though the reason for darker skin in hot countries is a combination of this evolutionary factor and the body naturally producing more to compensate for increased levels of sunlight.

Hope I've helped.

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Edit: Bugger. I spent ages formulating that answer, as this is not really my field of expertise, and the pete summarised it in 4 lines.

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No, you've added an at least as important part of the story.
I didn't even think of the role of melanin in protection from UV damage.

 

Until around 10,000 years ago, humans lived in an ice age. We stopped running around naked and used clothing, so there was no point in the body producing a pigment. Standing naked on a glacier, trying to heat up by the sun on your black skin is a ridiculous proposition, especially when furs come for free with your meals.

 

Once you get north or south of the 40th parallel, there's not enough sunlight (and too much cloud cover) to produce enough Vitamin D, and rickets is the result. Plus, modern Humans were moving into Europe during the cold, foggy, cloudy Ice Age, which would have made the problem worse. A tribe of hunters cleans out an area and moves on, with the pressure of other hunters behind them. In an average lifetime, say 40 years, a tribe might cover ten degrees of latitude, 620 miles--say 15 miles a year. After a while, they cross the Tagus River in Spain, or reach the area of Toranto in Italy, or the plains of southern Albania, or the Dardanelles in Turkey.

As they move north from then on, with every generation the darker-skinned babies are weeded out because they can't absorb enough Vitamin D; as they develop rickets, they can't walk properly, can't hunt, and can't contribute much to the welfare of the tribe. They don't find partners, and don't pass on their genes. The babies who are born with lighter skin don't suffer from the same problem; no rickets--they can walk, they can hunt, they can migrate with the tribe when it moves to new hunting grounds. They find partners, and they produce families. Lighter skin is Nature's way of dealing with the vagaries of northern Europe's often appalling weather. The tradeoff is that we suffer from higher rates of skin cancer, which couldn't be helped.

 

Bergmann's Rule explains how animals deal with body temperature in relation to latitude. If surface are is increased relative to volume, it is easier to lose heat, while larger volumes to surface area loses heat. Compare the huge people of Northern Europe to the very tall and thin people of Subsaharan Africa.

 

Consider also the short, squat-built, ice-age neanderthals compared to the taller and slighter build of the modern humans.

 

In addition to vitamin D synthesis, which many people have already correctly pointed out, there's also the minor fact that sunlight has a lot of UV radiation, which is bad for you. If you get a lot of direct sunlight, you will need darker skin to help block the UV.

 

Getting back to physics. Remember too that darker bodies emit EM better than light bodies.

What we have here is an interesting thought you propose, but the situation has other issues to consider as well.

 

You are a little off on your dating. The Younger Dryas was a time of major glacial activity. The rapid melting that accompanied its end led to a 100 metre rise in sea level. I don't think you can call that a minor ice age. That was 11,800 years ago - quite close to the 10,000 figure used by spidergoat.

Modern humans arrived in Europe 35,000 ot 40,000 years ago. The genera consensus is that Neanderthals were gone by 28,000 years ago, though some recent research suggests a few may have held on for three or four thousand years more. So your displacement date is also faulty.

 

If nature required that melanin made us lighter, while still blocking uv it would have occured; therefore I believe that the darker the complexions at the equator have every thing to do with color as nature intended, not by a coincindental effect of melanin which was inplanted to block uv

 

What do you mean "as nature intended"?

 

Excuse me!- as nature "selected"

 

How would color be selected? Camouflage? Sexual preference of females?

 

Yup.
Melanin is important if you don't want skin cancer.

 

Because the color blocks harmful UV radiation.

Oh wait nevermind.