If modern humans came out of Africa 300,000 years ago, how did everyone who isn’t black get their skin colours?
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There’s a tradeoff between too much sunlight giving you sunburns and skin cancer, and too little sunlight resulting in vitamin D deficiency. The pigments in dark skin absorb much of the sunlight before it can do any of that, which is advantageous when you’re exposed to lots of sunlight, but problematic when you’re not.
As for how, there are many different genes that impact skin color, so there will naturally be variation in skin tone in any population. Of the people that left to higher latitudes, those with lighter skin had more babies. Repeat for thousands of generations.
There are a lot of genes that affect skin color, and as a result it’s something that can potentially change *very* fast.
Imagine that one dark-skinned African has a gene that says “build lots of pigment-producing cells” and another gene that says “don’t turn the pigment-producing cells on too frequently”. Meanwhile, a second dark-skinned African has “don’t build too many pigment-producing cells” but “run the pigment-producing cells almost constantly”. Their child could potentially inherit the combination of “don’t build too many pigment-producing cells” and “don’t turn the pigment-producing cells on too frequently”, and so be substantially lighter-skinned than either parent.
If there turns out to be a major survival advantage to being light-skinned (e.g., protection from vitamin D deficiency at high latitudes, or elite social status within the Proto-Indo-European caste system), such that light-skinned children live and dark-skinned children die, the family could end up completely light-skinned in just a generation or two.
That is, in fact, basically what happened. To the best of my knowledge, every individual gene that lightens skin color is found even in the darkest-skinned African tribes. The only thing Europeans did was combine those genes with one another. So there was no need to wait for new mutations to arise, which is the step that takes so long in normal evolution.
Because being black and lactose intolerant isn’t really helpful in a region with little sunlight and where milk is 50% of the diet, so evolution selected for straits for those regions.
The simple answer is mutation.
Some populations that left Africa mutated, and because of that mutation had more success in the new biomes they were inhabiting. They spread that mutation through their children until it became dominant in the areas they now inhabited.
More like 80,000.
Evolution doesn’t stop. People adapted to their environment. The indigenous skin tone of every area has a 1:1 correlation with [the average sunlight of an area](https://www.reddit.com/r/MapPorn/comments/44y90h/annual_sunshine_hours_map_of_the_world_2753_1400/). There’s a reason Sherpas are better in the mountains than you.
But now everyone sits indoors too much. Go get some more vitamin D one way or another.
Time…
300000 years means – if we count each generation as 15-20 years – 15000 – 20000 generations.
That means a lot of parents, lot of sex, a lot of oppurtunity for random mutations and adaptive changes to occur.
Keep in mind that, you encounter maybe 6-8 generations in your lifetime, 4 up when you born and maybe 4 down when you die.
Keep in mind that only 100-130 generation occured in total from year 0. and even now we can see how quick can skin color change from generation to generation, how mental capacity can change. etc….
That is evolution. and even 300000 years is not a very long time.
You can find africans with
Blue eyes
Blonde hair
Lighter skin tones
Some groups lost their melanin pigment due to sexual selection or environmental pressure.
Mutations and evolution.
Humans started out as black.
Black skin is a very useful thing to have if you are a primate who has just evolved to have no more fur.
Losing fur was a good thing for sweating but it was not so good for being in the sun all the time.
So human when they evolved had black skin.
When some of those humans migrated out of Africa to where there is less sunshine the black skin proved to be a problem.
Not enough sunshine was getting through to help make Vitamin D.
Not a big issue if you live in the Savannah where there is enough sunshine to go around but bad when you wander into the frozen north where the sun doesn’t shine as much.
At that point some freak mutants with pale skin had a big advantage and after enough time everyone who lived in that area was pale skinned.
People in Asia had s lightly different mutation for pale skin than people in Europe, but of course there was a lot of mixing and matching over the years.
Usually the close to the equator you were the darker your skin ended up, but evolution works a lot slower than human migration.
Chances are your current skin color is a mix on what was optimal for your various ancestors 10,000 years ago for wherever they lived at the time, not or where you live today.
You just described evolution. All lifeforms change to adapt to their environments. Darker skin is better for exposure to sunlight. Lighter skin is better to generate vitamin D from sunlight. There is less sunlight in the northern hemisphere due to where the land masses are.
Random mutation. One day a kid was born with light skin, they were able to take in sunlight and vitamin D more efficiently in their environment. In Africa we needed dark skin because it helpd stop solar radiation from giving us cancer, but in Europe it stopped us from getting enough vitamin D. So the people who had lighter skin were stronger and therefore more likely to survive long enough to have kids and pass on their genes. After a few thousand years, give or take, there weren’t anymore people in the population with that dark skin that the population originally had.
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