This might be pseudoscience I got taught 20 years ago, but I have been under the impression that human evolution of breasts as a secondary sexual characteristic has to do with the shift to becoming bipedal. Like a lot of animals, early hominids would see the rear end of a fellow hominid and know this as a trigger for copulation. But when homo erectus started standing upright, butts weren't so universally erotic anymore. They had a whole back and head above them now. Breasts didn't need to be very large to fill their function, but an increase in size gave them a curvy appearance similar to a set of butt cheeks. And early humans were like, yeah, I think that's right, and selected for increased (or at least variable) breast size.