> The entire reason for skin color variations is a genetic optimization for UV absorption at specific latitudes vs sunburn risks.

This seems obvious but was not confirmed by genetic evidence. The rate of adaptation turns out to be much higher than can be explained by skin cancer.

The real cause appears to be fertility. UV radiation breaks down folate (vitamin B9) in the bloodstream, and folate is critical for DNA synthesis and repair. Folate deficiency causes serious problems in pregnancy, neural tube defects like spina bifida, and may impair sperm production. So darker skin in high-UV equatorial regions likely evolved partly to protect reproductive capacity.

In the other direction, lower melanin production helps with vitamin D synthesis in lower sunlight environments. Vitamin D requires UV-B radiation to be synthesized in the skin, and melanin inhibits this. Vitamin D is also linked to fertility. It's involved in sex hormone production and has been associated with successful implantation and pregnancy outcomes.

If you're curious, check out Nina Jablonski and George Chaplin's work. Their hypothesis is that skin color evolution as fundamentally about reproductive fitness: dark enough to protect folate, light enough to synthesize vitamin D. Both nutrients affect fertility, fetal development, and offspring survival. They have an immediate primary impact on fertility and success, while skin cancer even in the most extreme environment/phenotype mismatch, has an onset after reproductive age.