Why did Asians from colder climates develop monolids but (most) Europeans from colder climates didn't?

[u/glitchy_thefnaffan]

From my understanding, monolids were a feature developed to see better in the cold and harsh climate. Populations from Mongolia, China, etc, have monolids because the climate has almost always been historically cold, but Europeans from places like Russia and Finland often have double eyelids or hooded eyes even though the weather conditions are similar.

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[u/regular_modern_girl]

First off, there actually are some pretty old populations (I mention them being “old” populations to note that they aren’t necessarily descendants of more recent migrations from further east in Eurasia) in the far north of Europe who have epicanthal folds on their eyelids (“monolids”) similar to eastern Asians and indigenous North American groups with some regularity, including the indigenous Sámi groups of Scandinavia and the closely-related Karelians of Russia, certain other ethnic groups in the Arctic of European Russia, and apparently they also can occur in some insular Celtic-descended populations in the British Isles, Iceland, and elsewhere, although they are today rare in these populations due to a lot of admixture with populations who don’t have this trait. The Icelandic singer-songwriter Björk Guðmundsdóttir (who probably doesn’t need any introduction for most, but I figured I’d link her wiki in case she isn’t that known these days) notably has fairly prominent “monolids” if you look at pics of her without makeup, and as far as I am aware is of entirely Icelandic descent, but her naturally dark hair and epicanthal folds lead to believe at least part of her family might be primarily descended from some of the captive Irish people (mostly women) who were brought to Iceland as thralls (basically slaves) of the early Norse settlers in the early middle ages, as again these traits are historically present in Irish populations, but have become less and less common over time. In any event, this is an example of a northern European from a very isolated population having these sort of eyelids.

It’s not actually an entirely settled matter that epicanthal folds were selected entirely due to life in cold, snowy climates by the way, that’s one theory, but there’s evidence I’ll mention momentarily that seems somewhat hard to square with it, but even if this trait is originally selected for due to environmental factors, that doesn’t mean it inevitably must occur in every population that lives in such an environment, the mutation ultimately is something that presumably popped up randomly, and would’ve just so happened to be selected in certain environments, so this trait being more rare in some far northern Eurasian populations than others could simply be due to the mutation having only shown up in certain geographic areas.

However, I actually don’t think epicanthal folds are something geographically bound entirely to northeast Asian populations originally, as even besides the fact that they appear to have been present, or historically present, in some European populations for a long time, they’re also present in certain populations in Subsaharan Africa, populations which are in some cases thought to be some of the earliest-branching lineages of Homo sapiens sapiens, or at least who are descended in large part from such lineages; these populations are found in southern Africa, and are historically known to ethnologists and linguists as “Khoisan” peoples (from Khoekhoe and San, the names of two of the more prominent of these ethnic groups), although we now know their languages, despite some superficial similarities (like making extremely heavy use of click sounds as consonants, which are only really found in natural languages in Subsaharan Africa, but these languages use them especially frequently and in a high diversity, along with most information being conveyed through separate words rather than inflection, and the use of vowel tones) are less likely to actually constitute a single distinct language family, but instead to actually be several small language families and isolates that may not be especially closely-related.

How genetically-related all of these ethnicities actually are is also somewhat disputed last I saw, but they at least do tend to share some similar distinctive physical traits, such as somewhat shorter than average (but not “pygmy”) stature, relatively lighter skin for Subsaharan Africa (due to being at a higher latitude further south for millennia), extremely densely curled “peppercorn” hair texture, high cheekbones, and (most relevant to this topic) epicanthal folds or “monolids” on their eyes. They are distinctive-looking enough that historically raciologist pseudoscience sometimes grouped them as a distinct “race” from other Africans. Considering most of these groups live in subtropical desert, semi-arid, or (especially far south) mediterranean climates where snow is rare or basically non-existent, I can’t imagine protection from snow glare was what caused selection for epicanthal folds in these populations, if anything it might just have popped up as a mutation and then become prevalent due to relative isolation (many of these groups live in desert areas that weren’t suitable for traditional agriculture, so most other ethnic groups such as the Bantu-speaking cattle-pastoralist cultures of the Zulu and Xhosa coming from up north had relatively less territorial overlap with them) and therefore limited outside genetic input, or maybe due to sexual selection (people tending to prefer partners who look like them, in this case).

There’s even historically been a theory that all Homo sapiens once had eyes like this, and they are something that was specifically lost in many populations, rather than being gained by certain ones, but I don’t know if this has been borne out by evidence.

Besides the case of “Khoisan” peoples mentioned above, epicanthal folds clearly occur prevalently in many populations throughout the tropics, where protection from snow-blindness can’t have been a selective pressure, including Southeast Asian groups, Austronesians (seafaring speakers of Austronesian languages who are found in many, many places ranging from Indonesia, the Philippines, Polynesia, Micronesia, Taiwan, and even Madagascar and New Zealand, being thought to have possibly have originated on the South Chinese island of Hainan, or at least that’s where their language family is thought to have started), the “Naga” tribes of India, Andaman Islanders, and indigenous groups from throughout the tropical Americas. Founder’s effect (sharing a relatively small group of founding ancestors) can be invoked as an explanation in several of these cases (indigenous Americans, many of the Austronesian groups, and Andamanese), with these groups simply having come from ancestors who had “monolids” (and therefore could’ve been from colder areas), but not quite with all of them, and in general the extreme prevalence of this trait in certain tropical areas that have likely seen a decent amount of migration in and out still suggests that the trait cropped up in more than one population independently.

So basically, the evidence isn’t totally clear on epicanthal folds for sure having been selected for primarily by the environmental factor of being in a snowy area, perhaps that was a factor in them becoming prevalent in some populations, but it also seems highly likely they independently appeared in areas without snow as well. It also appears that East Asia is not the only region where the mutation emerged independently, even if now most populations that have this trait most prominently are found primarily in East (and Central) Asia and the Americas. I’m not sure if the exact gene variant (assuming it is just one, it might be several like we see in some other phenotypical traits like red hair) behind the trait has been identified, and if it has whether or not a range of people from different geographically-distant populations who all have epicanthal folds have been studied to determine if it’s all the same gene variant (I assume, based on distribution, it has to be more than one).

[u/silicondream → u/regular_modern_girl]

Considering most of these groups live in subtropical desert, semi-arid, or (especially far south) mediterranean climates where snow is rare or basically non-existent, I can’t imagine protection from snow glare was what caused selection for epicanthal folds in these populations, 

What about protection from glare in general, though? Ultraviolet incidence at ground level is highest in arid and oceanic (no forest, no shade) tropical and subtropical regions. There's a popular and reasonably well-supported hypothesis that languages indigenous to such regions are less likely to have separate words for green and blue, because older people in those societies have yellowed lenses from UV damage.

There are a couple of "grue" languages in the Arctic as well, and that could be thanks to the snow glare factor. I wonder if anyone has included the effect of year-round snow reflection in their UV incidence estimates, and examined how well such estimates predict the development of "grue" languages and of epicanthal folds?

[u/Odd-Suggestion4569 → u/regular_modern_girl]

TIL that Björk is over 60

[u/sci3nc3r00lz → u/Odd-Suggestion4569]

I just had this realization last week after going down a random Sugarcubes rabbit hole. I thought, That can't be right... but yup.

[u/BadCatNoNoNoNo → u/Odd-Suggestion4569]

And Björk is a grandmother.

[u/SpecialPomegranate → u/regular_modern_girl]

I have to note that the Sami and the Karelians (in Finland and on the Russian side) are in fact descended from migrations from east asia (although Finns/Karelians are 95% european and Sami are 80+% european by now). The monolids/epicanthic folds are a relic of this ancestral trait, not indigenous to Europe. Björk almost certainly has admixture from northern Scandinavia, again ultimately deriving her eye shape from Northeast Asia.

[u/Acegonia → u/regular_modern_girl]

This guy anthroplogises!

[u/Amelaista]

Mutations dont develop because of a pressure. Mutations are random, and are selected for because of a pressure.
So to answer your question, the mutation likely never developed in other locations. Or was never able to spread.

[u/gujjar_kiamotors → u/Amelaista]

Evolution is so accidental, no design😁 great precise answer 👍

[u/Beginning_March_9717 → u/gujjar_kiamotors]

mutations are accidental, selection is directional. evolution is sexy

[u/EmperorBarbarossa → u/Amelaista]

san people?

[u/Lockespindel]

Here in the Nordic countries you see epicanthic folds quite frequently, especially in Finns, who tbf have a small Siberian ancestral component.

I've only realized it after travelling to Southern Europe, where I reacted to how almost no one has those features

[u/Thousandgoudianfinch]

Because the steppe has a lot of open sky. Think Mongolia. This means there is a lot more light than in protected forested areas of Europe. The lidded eyes protect against sun damage from living in the exposed steppe country.

[u/Mysterious_Fix_7489 → u/Thousandgoudianfinch]

And unlike today 90% of Europe would have been covered in forest 5000+ years ago.

[u/7LeagueBoots → u/Mysterious_Fix_7489]

I’ve always said Europeans are forest people trying to turn themselves into grassland people.

[u/Strict_Depth8978]

Modern Scandinavians came from Germany, the original inhabitants of Scandinavia have monolids, like inuits and sami people. The same applies to Russians.

[u/grapescherries]

I read it wasn’t developed because of cold, but because of sand and dust blowing in people’s eyes, when ancient people migrated across deserts.

[u/dashsolo]

From school I remember this is mostly considered a “family resemblance” and not adaptive.

[u/Sensitive_Tap_7365]

not everything is an adaptation, a lot is just chance - presumably Genghis Khan had monolids

[u/diffidentblockhead]

Heavy wind and dust. Look up Loess Plateau

[u/Mister_Way]

If there were significant interbreeding between populations, then and only then would you expect the same traits to spread. There's no guarantee that convergent evolution would occur.

[u/Tradition96 → u/Mister_Way]

For some traits, convergent evolution seems almost guaranteed.

[u/Endward26]

The question may be even more complicated. The archaeological findings of so-called “microblades” imply a migration route from the south, at least as far as I know.

[u/Unit266366666]

I think it’s worth noting that the most people do not readily and rapidly see the epicanthic fold per se and instead perceive a much more complicated set of features of the eyes and face. It runs in my family and if not for it combining with our brow structure to lead to pseudoblepharoptosis and related conditions in many aging members of the family I’m not even sure the family would be aware of it much besides us noticing a less specific similarity in the eyes. Speaking with the typical layperson they don’t notice it and can’t readily pick out which of us have it or don’t.