IANA anthropologist (or anything else that would let me answer this question). Please excuse any political incorrectness.
I assume there must be a reason that the asian/pacific rim country’s inhabitants have small, slanted eyes with an extra fold of skin (if I remember correctly). Perhaps an environmental pressure caused this evolutionary change. Two questions:
Is there an accepted reason in academic circles why this evolutionary adaption occurred?
Why does it appear that the animal life that lives in the region did not have the same need for this adaption? (or do they?)
It’s not really an extra fold of skin as much as the eyelid fold everyone else has has moved much further down. I’m Asian and I can’t do that thing where you draw eyes on your closed eyelids for laffs because they’re totally visible when my eyes are open. It’s anatomically equivalent to drawing on the area right above your eyelid, I suppose.
As for why animals don’t have it, I guess if the mutation doesn’t develop then it doesn’t develop. It’s not like the animals get to vote on what features would be really helpful. Also, it’s supposedly to protect against UV rays, so it’s plausible that the local animals might have some other mutation that does the same thing.
I have a degree in molecular Bio, but it’s only a B.S. and this is not my area, but I have come across a theory that makes sense. Just take it with a grain of salt.
It’s an adaptation to cold.
The epicanthic fold that gives the “slant” eyed effect covers the corners of the eye, and will protect the outside of the eye from freezing. Big eyes are exposed, but if the human can cover every part of the eye that’s not necessary, that’s an evolutionary advantage. It also explains why you find the fold in extreme Northern American populations.
On Edit: As FlyingRamenMonster points out, it’s not actually a fold, per se.
Just remember that genes build proteins. So there is no selection for a trait such as slant eyes, only for the protein(s) that case them (or for the regulatory genes that turn on another gene). So if a mutation results in an improvement in one function and not costly in others, then it can be selected.
In Steven Jay Gould’s “The Panda’s Thumb” he describes how an extra “thumb” on the forepaws (actually a modified wrist bone) is used by the panda to strip the edible leaves from a bamboo. The genes for this trait were presumably selected for this purpose. But they also resulted in an extra thumb on the hindpaws and they have no discernible function, but also very little cost. Although I have never had this confirmed, I have always assumed that the thumb of the essay’s (and book’s) title was the rear thumb, since the one in the forelimbs was just another evolutionary tale of no special note, but the rear thumbs have a story to tell of how evolution works.
Not every difference among groups has an evolutionary purpose.If my extended family was isolated, we might have developed into a “race” of humans with wide cheekbones who kept their hair.
Why can’t the simple answer be the correct one? Their society found beauty in a certain look. Over time those traits were selected for and became dominant. I’d be surprised if the epicanthic fold was such a survival trait that it fixed in the gene pool.
This doesn’t explain why it is also characteristic of Khoi-San populations, who, believe me, do not generally live in very cold regions. I’ve heard it might have been retained more as aan adaptation for harsh light conditions (sun off snow, sand)
To the OP’s question about the evolutionary change - The condition was already pre-existing in the human population before it left Africa, clearly, given the diversity of the Khoi-SAn genome vs other humans. It seems to be the baseline that other eyeforms develop from.
perhaps my theory going into this question (it must have an evolutionary function) was incorrect. I have heard of the idea of blocking out the cold, also, but wasn’t sure that was correct due to the wide distribution of populations with this particular trait.
Do any animals have this extra fold of skin around their eyes, or is it uniquely human?