Comment by defen | Hacker News Reader

Comment by defen | Hacker News Reader

Milk does not naturally contain very much vitamin D, so that's a silly reason to expect lactase persistence to have evolved. A much simpler explanation is that it massively increases the calories available to pastoralist populations.

SLC24A5 is an interesting gene - I don't think Vitamin D is the full story. It's an SNP that is alone responsible for 25-40% of skin tone difference between Europeans and West Africans, and is virtually fixed in European populations (99%). However it's also under selection in places like the Ethiopian highlands, where sun exposure is presumably not an issue. Furthermore, if it were just about Vitamin D, you would expect that just about any loss-of-function allele would do. Compare to mutations that confer malaria resistance - there's a whole wikipedia list: https://en.wikipedia.org/wiki/Human_genetic_resistance_to_ma... - why do we not see the same for skin tone variation in Europe? Why is this single point mutation omnipresent, if it's theoretically only lightening the skin so that Vitamin D synthesis is easier?

Milk does not naturally contain very much vitamin D, so that's a silly reason to expect lactase persistence to have evolved.

The original study[1] doesn't suggest a connection between lactase persistence and vitamin D. It does discuss those topics in close proximity, though, which I suspect misled the author of the (popularizing) Science article.

[1] https://www.biorxiv.org/content/early/2015/10/10/016477

> vitamin D

That is the driving force for lighter skin (unless your diet is mostly fish, such as the Inuit). Skin gets darker again when migration towards sunnier climates happens: while UV light is essential for the synthesis of D3, it also destroys folic acid / vitamin B9 [1], which is essential when the neural tube of a fetus is being formed.

And since this mutation is so very visible, maybe here sexual selection plays a role, just like it is suspected with blond hair?

1: https://www.sciencedaily.com/releases/2014/03/140321095240.h...

I'm not saying vitamin D isn't involved; I'm just wondering whether it's the whole story. Is this single nucleotide change in SLC24A5 really the only way to decrease melanin production without having other harmful side effects? Naively I would expect lots of ways to have a loss-of-function mutation; why is this one so common? Sexual selection could be a factor, but that is not politically correct to say.

> Is this single nucleotide change in SLC24A5 really the only way to decrease melanin production without having other harmful side effects? Naively I would expect lots of ways to have a loss-of-function mutation

In Asia, light skin evolved separately and is using different genes.

https://en.wikipedia.org/wiki/Human_skin_color#East_Asia

>>However it's also under selection in places like the Ethiopian highlands, where sun exposure is presumably not an issue.

The Ethiopian highlands have been historically productive in agriculture. Agricultural diets contain less Vitamin D, increasing the selective pressure for lighter skin tones.

The highlands are also cooler than other regions of Africa at similar latitudes, resulting in more clothing being required on average. More clothing necessitates more UVB absorption per unit of skin exposed, which would further select for lighten skin tones.

What's the story with East Asians? They have light skin as well, right? Is it the same mutation?

Completely different set of mutations. So whatever advantage is conferred by SLC24A5 mutation is probably matched by something else in East Asians (possibly EDAR)

No it's different, and thought to have evolved independently. See e.g. http://journals.plos.org/plosgenetics/article?id=10.1371/jou...

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