Homo sapiens evolved in Africa as a hairless ape. That creature spent LOTS of time outside, with a prodigious amount of skin exposure in a nice sunny environment. Today homo sapiens is largely an indoor mammal, and typically covers up a good deal of skin when venturing outside. Even when in a relaxed stance, say on a beach vacation, gobs of sunscreen is applied. This all adds up to much less Vitamin D synthesis happening in the skin cells.