Darwinists are eager to get a grip on the large and curvy breasts of women, which blossom lusciously in defiance of their expectations, since female apes are flat-chested.
So those scientists who are still backward enough to be fans of Darwinism have proposed that big breasts evolved when humans began to walk erect. They then supposedly began to prefer to engage in sex face-to-face, rather than doggie-style, as apes do.
Thus males who had long been turned on by the sight of a butt, often became horny instead at the sight of curvaceous tits: a frontal pair of which, at least in the feverish imaginations of some Darwinist biologists, resembles a butt. So runs this Darwinist tale of running after tail.
See my link "Can Darwinists explain why women have big tits??" In which a Darwinist-believing biologist fantasizes about this "great theory" that "ass-men" evolved into "tit-men."
Darwin-fans speculate that women with larger breats had their choice of the best male partners. Thus natural selection took hold of their tits and made them bigger and bigger: the Theory of the Expanding Titties!
Thus do believing Darwinists strive to grapple with women's breasts.
But unfortunately for this piece of typically wild Darwinist fantasy, the asses of female apes do not resemble a pair of boobs. There are many parts of the world in which face-to-face sexual intercourse hasn't been very popular. There are also regions in which men have preferred smaller breats. And worse, if the Darwinist "Big Boobs Theory" were correct, men would have an inborn preference for large breasts in their sexual partners. So gay men would chase those men who have some hints of soft and curvy tits (some men do have them.)
But there's no sign of that. Glance at any gay male magazine and it becomes clear that hard, flat, muscular chests are greatly preferred in that set. Not a hint of soft, curvaceous breasts anywhere.
So the Darwinist Big Boobs Theory falls flatter than pendulous tits in a broken bra when faced with the facts.
Like Darwinist theory in general, it belongs to the realm of woolly, speculative daydreams: not to the sphere of genuinely scientific realities which have been demonstrated by experiments.
Then why do men often (not always) like women to have rather large breasts? Think for yourselves for a minute, all ye who cry for True Faith in Darwinism, and it won't be hard to see why.
Larger breasts are one of the prominent secondary sexual characteristics which distinguish men from women. So straight men, who lust after women, associate bigger breasts with a desired partner: and thus with satisfying sexual action. Thus it's no mystery at all why straight men are often sexually excited by the sight of large boobs.
And why, then, do women have large breasts in the first place?
The only genuinely scientific answer is, "Science doesn't know the answer. So your guess is as good as mine, dude. Or gal. Don't be a BIG DARWINIST BOOB."