Comic Strips

https://en.m.wikipedia.org/wiki/Cultural_history_of_the_buttocks

Butts are the original boobs.

The baboons have been oddly quiet since you asked this

Something that blows my mind is realising how much of our understanding of sex and attraction is socially constructed. For example, there are plenty of documented cultures where women's breasts don't have the erotic connotation we attach to them. The thing that really threw me off was learning about some people who don't kiss as a show of affection — I found this a surreal concept, because in terms of romantic interactions with a partner, I'm fairly meh about sex, but I'm a big fan of kissing/making out; There's a sense in which I obviously know that preference towards kissing is likely not an evolved trait, but more sociocultural, but it feels so intuitive that something so visceral isn't necessarily an innate trait.

Anyway, this is a long way of saying "did we evolve to like butts, though?". Evolutionary biology, the field that would consider questions like these, is unavoidably pretty heavy on the speculation side — given that humans have evolved to be such social creatures, we can't really separate out the sociocultural aspects of development from the genetic side, and that makes asking evolutionary questions on large timescales to be a tricky endeavour.

Edit: This isn't to say that asking these questions is pointless to do. I appreciated your question precisely because it's the kind of thing that cooks my brain (and I enjoy that)

Does walking upright make the butts more or less visible? They're slightly higher but less prominently displayed.

I dunno, but some other brother might deny