Skin colour and the equator.

So, I'm trying to create a world where the only populated continent is mostly within the tropical zone (between 23°N and 40°S, more or less).

The predominant biomes in the world are Mountain Forest and Tropical Rainforest. There are a lot of rivers and lakes, and the different peoples of the world have never been too much into hunting (gathering, fishing and agriculture are big in that world).

The climate is also, mostly, tropical and because of"magic", there are a lot of sea breezes in that part of the world. In other words, it rains all the time.

Now, what I'm wondering is, would it be realistic if not every person in such a world was black?

I want to know if it's possible to also have lighter skinned people (thinking indigenous peoples from the Americas or from south-east Asia) if their ancestors didn't have the need to spend a lot of times hunting in the savannas.

I'm asking because I want to have a realistic reason for having both black and brown people in the world I'm writing. Because of reasons.

Between the rainforest canopy and monsoon season cloud cover, yes, I'd think it would be possible for there to be tan people.

Darker skin takes melanin, which takes energy to produce, so a species won't produce it if there hasn't been selection pressure for darker skin over the last few thousand generations. Such a species might live underground, or otherwise have protection from the equatorial sun.

Relative to other sorts of morphology evolution, changes in coloration can be rapid.

edited 18th Jul '15 6:07:43 PM by rodneyAnonymous

...Not all equator-dwelling people have OMG dark skin. Hawaii, the Philippines, and most of South Asia is a testament to the range of skintones in tropical areas.

The Philippines is actually a good example of skintone diversity despite what their bleach-pale actors might suggest, because there's an ethnic Chinese-Filipino population who aren't noticeably darker than most of their Chinese-from-China ancestors.

According to the books I've read, the skin colour on the indigenous people from south Asia and the Americas is a product of emigration from Africa.

That is, people went from living in Africa (dark) to northern Eurasia (light), to southern Asia and the Americas (darkish).

Since there is no such emigration in the world I'm building, I wanted to have a realistic explanation for the diversity that exists in the world.

Are you saying that gods (or precursors or whatever) just dropped humans at various points of the globe and left them to survive on their own?

Still, climate doesn't have AS MUCH of a hold on skin color as most people would think.

Nope.

In the world I'm creating, the ancestors of humanity (e.g. Australopithecus) originated in the rainforests of the continent within the tropical zone, and evolved from there.

However, they never spread to the rest of the world because the continents, unlike Afro-Eurasia, have never been connected. In this world there are not disappeared landmasses like Doggerland (which connected Great Britain to Europe) or Beringia (which connected Russia to Alaska). Also, the oceans between these continental masses are too vast and violent so there was no migration via island hopping like it supposedly happened with Australia.

The other continents were not colonized by humans because they were either too dangerous due to the local wildlife (e.g. dinosaurs), or they were frozen deserts (e.g. Antarctica).

Supposedly it's all about the effects of the sun and how it can cut the levels of Folic acid in half, which is bad. However, the same ultraviolet rays are necessary for the production of vitamin D, which helps the body absorb calcium (I think).

So, the different skin colours in our world came as a result of the exposure to the sun: in places where the sun is brightest (near the equator) people have darker skin to protect against the ultraviolet rays that affect folate, and prevents people from overdosing on Vitamin D, which can be toxic; and in places where the sun is weakest (closer to the poles), people developed lighter skin tones to absorb more ultraviolet rays to help them produce the vitamins they needed.

edited 19th Jul '15 2:19:41 PM by Lorsty

Recessive traits+isolated population+implications of breeding when reintroduced to wider group=what you are looking for?

edit:and recessive traits will still be expressed in later generations even if their holders are completely absorbed by a larger population with dominant traits.

edited 19th Jul '15 2:21:43 PM by IndirectActiveTransport

I have a question for my own setting that relates to skin color and the equator. If a world (or specifically a moon in my case) doesn't have a tilted axis , would the equatorial regions still receive more direct sun light? Or would the climate be the same everywhere (not counting geological features)?

It would still have different levels of insolation (the technical term of how much sunlight hits the ground. You'd still have the highest number of watts per square meter at the equator and the lowest at the poles. You just wouldn't have any seasonal variation in the amounts.

Sun isn't the only factor in skin pigmentation. Diet is equally important. For instance the Inuit are as far from the equator as you can get^* And wrapped up tight but still on the darker side. Since they get all the vitamin D they need their diet of fatty fish (which are less common near the equator).

Evolution can be very fast. Particularly if dark skin leads to rickets leads to brittle bones leads to death. That said, white Europeans might be Newer Than They Think. Some scientists reckon though humans migrated to Europe 40,000ya, there wasn't a strong change to lightened skin (within central and southern Europe at least) until within the last 8,000ya when a new group came down from the steppes of the near east.

Re: Oceans too vast and dangerous
The Polynesians laugh at you.

Re: Moon
Yes you would get more light at the equator and hence darker skinned people. Also, I think you'd get seasons based on when the moon was on the sun side of its planet (summer) and the far side (winter). Plus if the moon is tidally locked to its planet (one side stays facing it always^* Called a 1:1 harmonic, others are possible. Mercury is 3:2 with the Sun (three rotations per two orbits), Venus seems to be 5:1 with close approach to the Earth but that might be a coincidence) like most significant moons^* If it's big enough to make its spherical it's probably locked, then one side will only get light in summer, one only in winter.

Thanks guys.

@Luthen: It is tidally locked. I don't know what you mean with summer or winter side of the planet. Since it's tidally locked, a month equals a day. And to simplify things that's just about 24 hours in my case. That's hardly enough time for an occurence of seasons.

Actually, you might very well have a certain amount of seasonal variation depending on orbit. A highly eccentric orbit would provide for distinct seasons even on a tidally-locked planet...(of course, that'd also reduce the odds of it being tidally locked in the first place a little)

edited 20th Jul '15 7:11:04 AM by MattStriker

The "summers" would be shorter than the "winters" though. I don't want seasons however, so I'm good. All I wanted is differently skinned humans.