Just because She's Topless

[Fireball]

Not so much that they signal fertility, which is a monthly thing, but rather, they indicate that the woman has better odds of surviving childbirth and being a contributing parent in the long term. As humans evolved, perinatal cranial capacity grew, and a fine line was drawn between optimal pelvic room and optimal brain size. A woman who could more easily give birth to a large-brained child was, I think more likely to give birth successfully, take part in parenting so that the child could have better odds of reaching maturity, and as a bonus, allow for children with larger brains.

Well, one aspect of this is that human females have masked fertility and are also 'receptive' at any time in the cycle. That is also quite unusual in primates. That means that signs of fertility are not directly associated with estrus (which technically doesn't happen in humans). Perhaps breasts and butts are used instead of baboon butts for signals in humans and are continual rather than cyclical to preserve hidden fertility.

Of course, that begs the question of why hidden fertility?

Italics mine. it is a god-given trait for the betterment of man.