Wikipedia:Reference desk/Archives/Science/2019 January 13

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January 13

Human skin colour evolution

I dimly remember reading childish texts (a long time ago) that explained that black skin colour evolved to protect humans in hot climates.

However, if humans all came from Africa, presumably, we were originally all black and some [of what became] races eventually evolved lighter coloured skin. Except, I can't think of a survival mechanism that would make that work.

I'm well aware of the potential here for racist nonsense and trolling so references only please to where I might be able to read about this in reliable sources.

Thanks --Dweller (talk) Become old fashioned! 17:52, 13 January 2019 (UTC)

Probably doesn't answer your immediate question, but is on-topic: [1] Bazza (talk) 18:01, 13 January 2019 (UTC)

Vitamin D synthesis seems to have been a factor - see [2] and [3]. PaleCloudedWhite (talk) 18:19, 13 January 2019 (UTC)

I see we have articles for both Nina Jablonski and David Reich (geneticist). Martinevans123 (talk) 10:55, 14 January 2019 (UTC)

Note though this quote from the article "The paper doesn’t specify why these genes might have been under such strong selection. But the likely explanation for the pigmentation genes is to maximize vitamin D synthesis, said paleoanthropologist Nina Jablonski of Pennsylvania State University (Penn State), University Park, as she looked at the poster’s results at the meeting."

This raises the following issue. Vitamin D is used to make calcitriol which is involved in gene regulation. Evolutionary pressure to get the skin color changed in order to get more vitamin D in order to affect gene regulation, sounds strange as any suboptimal gene regulation can be modified directly. So, if the demand for more vitamin D synthesis did play a role, then that points to an advantage to have as strong seasonal vitamin D induced gene regulation variation where in winter time you would want to have low vitamin D levels and in the summer a higher level. Count Iblis (talk) 19:51, 13 January 2019 (UTC)

Being deficient in vitamin D potentially results in several deleterious effects on human health. PaleCloudedWhite (talk) 20:13, 13 January 2019 (UTC)

Yes, but this begs the question of why calcitriol is used for gene regulation in the first place. Count Iblis (talk) 20:55, 13 January 2019 (UTC)

As always with evolution, it's a mistake to look at things from an engineers perspective. Perhaps changes in gene regulationthe genes involved in biosynthesis would have had the same effect without a change in skin colour and it would have been as effective for things to happen that way, but that doesn't mean it should have happened that way. Evolution doesn't care how you got there, just that you did get there, if there was no evolutionary advantage to either pathway. And this isn't even a good engineers perspective since you can't just randomly decide it's suboptimal, I see no analysis of the changes required for either pathway to succeed, and the risks and costs of changes in these areas has been presented so you can actually say one pathway is simpler/easier. (I'm not sure increasing biosynthesis rates is as simple as you seem to think. It's not like sun exposure is an arbitrary regulator. Instead it's a physical requirement for biosynthesis.) Besides, if we're looking at some reason for it to have happened via skin colour, rather then making up some reason for which AFAIK we have no evidence exists (an advantage to seasonal vitamin D), we should consider the possibility of sexual selection something it's generally accepted does occur, and which is believed to be one possible reason for the evolution of different hair colours and eye colours [4]. (In fact that mentions extreme skin depigmentation.) Besides of which, if we believe that there is an advantage to seasonal vitamin D regulation, we should consider why (AFAIK), there's no evidence it has evolved in some fashion in places where it's not possible to rely on significant seasonal sun exposure differences. Nil Einne (talk) 05:36, 14 January 2019 (UTC)

BTW, in the same vein you can't assume an alternative pathway for vitamin D biosynthesis should have arose that isn't so dependent on the sun. Although it sounds like there's no simple alternative pathway anyway Vitamin D#Biosynthesis, although not surprisingly the pathway we do use arose very long ago so it's probably a moot point. As to why we don't just produce it in abundance so that no one is ever deficient, well despite what the vitamin megadose people like to think, too much of something important can be harmful. And even before we get to the level of kidney damage etc Vitamin D#Effect of excess, it's a reasonable hypothesis that excessive amounts would be more harmful than beneficial. And that's before we even consider the costs of producing so much vitamin D. P.S. Clarified a few things in my original post when writing this. Nil Einne (talk) 10:52, 14 January 2019 (UTC)

Reading a bit more I realise I may have misunderstood something Count Iblis was trying to say, but most of my comments still stand. You can't assume genetic changes in another area could have happened which would have had the same effect and that there are simpler and so should have happened because that's simply not how evolution works. Especially when you haven't even demonstrated that the changes are more likely.

Notably, changes could easily have negative effects especially when there is also selective pressure in the same direction anyway. (E.g. changing the regulation system so that it's less depend on calcitriol could be a negative when there's also selective pressure towards increasing vitamin D production by lightening skin ton anyway.) To be fair, since it's likely you need a lot of changes you would probably expect some co-evolution of both until a reasonable balance is achieved provided either are reasonable changes. But no evidence has been presented this didn't happen anyway. (Note it's even possible that there were changes in regulation but because of the other advantages there were also changes in skin tone and eventually the changes in regulation went the other way.)

The fact that calcitriol is involved in regulation doesn't mean it's an arbitrary regulator. (Again, I don't know if the evidence is out there but Count Iblis has not presented it.) The article calcitriol and calcitriol receptor make me think that it could easily be an important physical component, in other words, you can't have the transcription factor without calcitriol or some alternative. This doesn't mean some other pathway wouldn't have worked, but like with the biosynthesis issue if for whatever reason it did evolve in this way, it's not going to magically change. And there could be so many reasons (likely more than one) why it evolved in this way, it doesn't have to be better than any alternative since again, we're talking about evolution not a super-engineer. It's not impossible that seasonal differences could be a factor, but then again, there's a very good chance this evolved where seasonal differences in sun exposure were minimal to whatever, perhaps single cell organism it first evolved in.

Ultimately of course, it's often not possible to be certain why something complex, especially something that happened a long time ago, evolved precisely the way it did.

Nil Einne (talk) 12:33, 15 January 2019 (UTC)

One has to consider the hundreds of millions of years that complex life has existed. During that time any problems with calcitriol having to be made from vitamin D which then requires enough UV radiation to reach the skin, would have had to be dealt with. This strongly suggests that it isn't actually a problem, rather a solution to a problem. So, low vitamin D levels causing gene regulation being modulated in a different way is not problem in the natural context, rather it is the intended optimal regulation. E.g. during very long periods of drought, animals may burrow underground for many months, vitamin D levels will go down and gene regulation is done differently. I'm then inclined to assume that this modulation in gene regulation isn't going to add to the problems of the animals, rather that it is going to help them survive. Or animals that have to survive in harsh winter conditions, their vitamin D levels will be lower too. I'm then inclined to assume that this lower vitamin D level helps such animals to make it through winter rather than making life for them even more difficult.

Vitamin D is also stored in body fat; animals in winter time who still have large fat reserves will have higher vitamin D levels than animals with smaller fat reserves. This suggests that low vitamin D is used by the body as a signal that energy supplies on the long term are not secure. To optimize survival probability then requires modulating processes such that less energy is used. This fits in well with a lot of the recent research results e.g. on how vitamin D levels modulate the immune system. Count Iblis (talk) 18:22, 17 January 2019 (UTC)

[Edit conflict] An excellent recent review is: Jablonski, N.G., Chaplin, G. 2017. The colours of humanity: the evolution of pigmentation in the human lineage. Philosophical Transactions of the Royal Society B 372:20160349. https://doi.org/10.1098/rstb.2016.0349 The article concludes that the advantage of dark skin is in reducing the photodegradation of folate; the advantage of light skin is an increased production of vitamin D, which is catalysed by UV light. The optimal balance between these opposing factors depends on ambient light intensity and thus on latitude. Jmchutchinson (talk) 18:29, 13 January 2019 (UTC)

I see. So a darker-skinned population that moved north would suffer increasingly from vitamin D deficiency and mutation of lighter skin would have made it more likely that their offspring survived to reproduce before the vitamin D deficiency killed them (or I suppose rendered them down the social order so they wouldn't get a mate)? --Dweller (talk) Become old fashioned! 22:07, 13 January 2019 (UTC)

You're right to think of black skin as ancestral. Note that dark skin coloration depends on a complex biology of melanocytes and melanosomes, with pseudopodia extended to regulate skin tanning on a week to week basis. The intricacy of these mechanisms speaks to the importance of protecting the skin, especially in ancestral African conditions. Apparently albino Africans not merely develop skin cancer, but develop it early enough for it to be a major cause of death. [5] Our article talks about a selective force to darken skin color 1.2 million years ago, but dark skin in primates has been around much longer than that. So the need to pigment skin is longstanding and has evolutionary mechanisms and distribution that indicate a deep evolutionary history, but there are selective forces at play to turn the regulation of the process up and down at much shorter intervals. It appears that light skin in Europeans and East Asians represents convergent evolution, presumably based on the inescapable realities of latitude. [6] Wnt (talk) 02:22, 14 January 2019 (UTC)

Since we now know that humans mixed with Neanderthals, why would this mixing not have led to us simply using the Neanderthal genes for pale skin? Count Iblis (talk) 04:24, 14 January 2019 (UTC)

Did Neanderthals have pale skin? --Jayron32 13:21, 14 January 2019 (UTC)

It would seem so. From Neanderthal genetics#Interbreeding with modern humans:

"It is suggested that 20 percent of Neanderthal DNA survived in modern humans, notably expressed in the skin, hair and diseases of modern people.[14] Modern human genes involved in making keratin—the protein found in skin, hair, and nails—have specially high levels of Neanderthal DNA. For example, around 66% of East Asians contain the Neanderthal skin gene,[clarification needed] while 70% of Europeans possess the Neanderthal gene which affects skin colour. POU2F3 is found in around 66 percent of East Asians, while the Neanderthal version of BNC2, which affects skin color, among other traits, is found in 70 percent of Europeans."

{The poster formerly known as 87.81.230.195} 90.217.251.247 (talk) 19:23, 14 January 2019 (UTC)

That is related to Count Iblis' question but I think it provides minimal evidence towards Jayron32. The fact that many of those with paler skins have genes from Neanderthals which affect skin colour doesn't mean Neanderthal's had pale skin, it doesn't even mean the effect of these genes was paler skin. But my impression is it's generally believed they did, see e.g. Neanderthal. "A 2007 genetic study suggested some Neanderthals may have had red hair and blond hair, along with a light skin tone.[52]" Nil Einne (talk) 09:24, 15 January 2019 (UTC)

Yeah, my point was not to say that they didn't, it was to ask for evidence that they did. It's tautological to say that our evidence that Europeans got their pale skin from Neanderthal DNA is that Europeans have pale skin, and also have more Neanderthal DNA. That's a post hoc fallacy problem. Again, Neanderthal's may (or may not) have had pale skin, but you can't just drop a statement like that as though it were obvious and not have any clear evidence to back it up. It's not like saying "the sky is blue". I can't look at a Neanderthal today and see... All I wanted to know is "what is the evidence that they had pale skin" Nil Einne's last statement is the sort of thing I was looking for. That's a good start. --Jayron32 14:08, 15 January 2019 (UTC)