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So if the places on the equator of my world has light skinned people of the deserts while the places a bit south of the equator can have dark skinned folks of tropical rainforests, is that explainable by other methods (certain genes, wind/materials/rocks (reflected rays))?

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    $\begingroup$ Is light-skinned absence of pigment or they have an actual light pigment? E.g. some light-colored pigments like titanium and zinc oxides are used in sunscreen to increase protection $\endgroup$ Sep 16 at 5:40
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    $\begingroup$ It could be anything, absence of melanin or presence of titanium/zinc oxides. Or even something else. $\endgroup$
    – Abhay123
    Sep 16 at 10:42
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    $\begingroup$ Your question makes sense. Your question title make sense. but the two ask diametrically opposite things! $\endgroup$
    – PcMan
    Sep 16 at 12:40
  • $\begingroup$ People adapted to shed their pigmentation in order to boost vitamin D production. Solve the vitamin D issue and people would keep their melanin also at high latitudes. $\endgroup$
    – FluidCode
    Sep 16 at 15:30
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    $\begingroup$ I mean, people move around. There are plenty of dark skinned people living in the colder latitudes and light skinned people living near the equator on Earth right now... $\endgroup$ Sep 16 at 17:13
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Dark skin is an evolutionary adaptation to the amount of solar radiation a population has to deal with, the dark pigment melanin absorbs UV that could otherwise damage the DNA of skin cells. As such UV pigmentation response varies with insolation in sedentary populations where those populations have adapted to local conditions over many generations.

So because there is a genetic basis for the degree of skin pigmentation of particular population groups you can look at founder effect and genetic drift to justify the pigmentation of different groups as a matter of, relatively, recent historical population migrations.

There is some basis for this in our own world (at least in the historical theories I was taught, DNA studies have changed our understanding of many migrations drastically in recent years and I haven't always kept up):

  • Migrations coming back out of Western Europe and the Eurasian Steppe into North Africa and Mesopotamia are thought to be the reason that groups like the Bedouin are significantly paler of skin than tribes to their south to spite the stronger desert sun.

  • Such migrations are also thought to be behind the fact that the Marsh Arabs occasional express phenotypes indicating the presence of recessive genotypes for blonde hair and blue eyes in the local genepool.

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    $\begingroup$ Could do with breaking up into discrete bite size portions / bullet points for clarity in getting specific points & elements across / ease of reading & comprehension, otherwise a good answer [+] $\endgroup$
    – Pelinore
    Sep 16 at 11:15
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    $\begingroup$ In addition to having strong evolutionary advantages for high melanin levels in high solar radiation areas, there are strong evolutionary disadvantages against high melanin levels in low solar radiation areas. High melanin levels increase the amount of solar radiation necessary for the same level of vitamin D production. Low vitamin D levels have significant negative effects on the immune system and a variety of other negatives, including negative outcomes in pregnancy. The net effect being that adaptation will be relatively quick upon population migration, but still many generations. $\endgroup$
    – Makyen
    Sep 17 at 2:47
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    $\begingroup$ @Pelinore Sorry I default to "wall of text" is that easier to follow? $\endgroup$
    – Ash
    Sep 17 at 8:46
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    $\begingroup$ @Ash Yep that's great, I suffer from the same wot default mode & I'm often less successful at correcting for it (nor do I always remember to) than that go up there :) I'd upvote again if I could. $\endgroup$
    – Pelinore
    Sep 17 at 10:36
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    $\begingroup$ @Tristan We have existing populations that are fixed for both extremes and a number of variants that create other unique UV response levels in their particular populations as well. Also the OP doesn't specify human nor a similar evolutionary history so I've tried not to make any more assumptions than I had to. $\endgroup$
    – Ash
    Sep 17 at 10:54
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Complementing Ash's answer, you could also have a few other mechanisms, depending on how you set your world

Incorporation of pigments from food

Your humanoids incorporate different pigments to their skin in the same way flamingos or salmon.

So your equator residents eat zinc or titanium dioxide to incorporate those pigments to their skin and provide them with protection against the sun. Your polar counterparts (north and south) might incorporate other types of pigments to their diet to help them be more effective in absorbing certain wavelengths that they need for their metabolism (for example, the same way we synthetize vitamin D) and are not that abundant closer to the poles

Their tanning is white

As mentioned in the other answer, melanin is an adaptive response to UV exposure to protect the cells from damage. Perhaps your humanoids evolved a different response to sun radiation (not necessarily UV) that makes them produce white pigment in response to radiation exposure. So your humanoids could be darker skinned normally and then become lighter with exposure to radiation from their sun

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    $\begingroup$ They could be orange, carrots are a holy food, you're required by holy scripture to eat at least 4 pound of raw carrot a day to prove your devotion :) $\endgroup$
    – Pelinore
    Sep 16 at 11:30
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    $\begingroup$ @Pelinore HERESY!!! The orange tribe will burn in hell, everyone knows that grapes are the sacred food and purples will prevail! $\endgroup$ Sep 16 at 11:33
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    $\begingroup$ Hadn't heard there was a pigment in grapes that gets transferred to the skin the way keratin does though? . but regardless of that the holy orange skins will prevail over the weak & foolish grape munchers! all that acid will give them stomach ulcers & make them vomit, we'll just have to wait for them to fall over, known medical conditions resulting from excess consumption of the infernal grape are our friends :) $\endgroup$
    – Pelinore
    Sep 16 at 11:42
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    $\begingroup$ @Pelinore I think you mean beta-carotene (that's the orange pigment in carrots that gets transferred in flamingos and salmon) tho. And of course grape is the holy food due to its superior healing powers! $\endgroup$ Sep 16 at 12:19
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    $\begingroup$ "carotene " Dang! damn my godawful spelling, spellchecker & my inattention to detail! yes, that one. $\endgroup$
    – Pelinore
    Sep 16 at 12:23
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Gloger's rule – humidity is correlated with darker coloration.

This principle is also vividly demonstrated among human populations.[Ember et al. (2002)] Populations that evolved in sunnier environments closer to the equator tend to be darker-pigmented than populations originating farther from the equator. There are exceptions, however; among the most well known are the Tibetans and Inuit, who have darker skin than might be expected from their native latitudes. In the first case, this is apparently an adaptation to the extremely high UV radiation on the Tibetan Plateau, whereas in the second case, the necessity to absorb UV radiation is alleviated by the Inuit's diet naturally rich in vitamin D.

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Can humanoids be of dark skin despite living somewhat lower than the equator and vice versa?

Yes, right now there are lots of light skinned people living in tropical climates and latitudes, and dark skinned people living in high latitudes with cold weather.

In fact there's historical evidence for this going back thousands of years from across europe.

So if the places on the equator of my world has light skinned people of the deserts while the places a bit south of the equator can have dark skinned folks of tropical rainforests, is that explainable by other methods (certain genes, wind/materials/rocks (reflected rays))?

Just move the people. Say they migrated enmasse to a new land long ago, it'll give more plot devices to work with. You don't need to invoke genetics/chemistry/weather etc, and it's more believable.

The process of altering skin colour via evolutionary pressure from sunlight/UV will thousands of years of isolation from others, maybe longer.

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