system
Given the following post, respond as if you're commenting on Reddit:
human
If differences in human phenotypes such as skin colour, hair colour, etc. are due to clinal adaptation, what clines lead to the development of East-Asian slanted eyes?

Since there is good evidence to suggest we radiated from Africa, I presume that primitive humans did not have slanted eyes, but I guess a possibility is that primitive humans did in fact have slanted eyes, and the change to non-slanted eyes is actually an adaptation. In either case, why/how did this happen?

EDIT: Also, I'm running on the assumption that eye shape is a product of clinal adaptation, when it may very well not be. However, I would expect that it is based on the fact that we have many forms of slanted and non-slanted eyes as you move East-West, and this smooth variation is expected with clines.
gpt
The explantation for epicanthus is that Central Asian steppes, the area where proto-Mongoloids origated, were very dusty with common dust storms and the epicanthus helped them to protect their vision. But it's also possible that it was a random mutation and an example of the founder effect.

Because epicanthus doesn't occur only in Mongoloid-descended populations, but also in some tribes in Africa, like the Sudanese Dinkas and some Bushmen. But the Malagasy people on Madagascar have it from Mongoloids, since the first settlers there were Austronesians who originally came from Taiwan.