no you don't understand, this argument has only stood in the past because it was pointed out none of the other mammals had boobs because they didn't walk upright, butt the thing is the other mammals don't have booty either even if they should by being on all 4 according to you
the truth is that mammals are not primordially assfags, but rather legsfags: the way an animal move is the first and probably best indicator of health of an animal and mammals, by being the animals that the most value migrations, see enormous importance in proper walking.
when humans switched from a quadrupedal to a bipedal erection they had to change standards of measurement for the quality of a walk, want to know what they saw? the longer the stride the more the hip sway
in order to accentuate this women put on fat on their cheeks
it's arguable whether boobs came before ass but I think it's inevitable to think they may have developed independently from one another