No one is *really* sure. It’s like a lot of evolution… we aren’t a result of someone sitting down to design every single feature intentionally, for specific reasons.
Plenty of who we are have good reasons of course! And most of those reasons are pretty obvious. The opposable thumb is SUPER useful, so you can see why that evolved.
Breasts are one of those grey areas. The main theory is that it’s just visually sexually appealing. Either on their own, the way birds slowly get more elaborate feathers that serve no purpose other than attracting a mate, or to somehow replace various visual cues that are now hidden because we walk upright, and certain other areas are less prominent.
As you say – just about all mammals have breasts in some form, but humans are the only ones that keep them around other times beyond just the time needed to feed children.
Who knows for sure though? Maybe it’s some fluky side effect we don’t even know about! Maybe breasts were even *more* prone to breast cancer before they were permanent. Perhaps the growth and fading of breasts over many cycles of having kids made cancer much more common, and a fluky mutation to keep breasts around at all times actually resulted in a much lower incidence of cancer!
(I don’t think that’s *really* the case of course, but just pointing out that there is certainly the possibility that permanent breasts are almost a side effect of something else we aren’t aware of.)
There was a documentary I saw forever ago that suggested a rather dark theory explaining breasts and the lack of clear signals for when a woman is fertile. Infanticide. Breasts that are feeding a baby are large, and signal that the woman has given birth. If the male sees this and wants a mate who can for-sure give him offspring, he looks for the woman with round breasts and kills her infant. After a time, she will be ready to get pregnant again.
The lack of clear fertility signals, in this scenario, helps conceal paternity so the male will not kill the infant, on the chance that it’s his own.
It’s a waste of energy if you’re gonna need to regrow it every few months if you’re pumping out baby after baby, which happened not too long ago. Evolution takes time and usually happens because something was more favorable.
In the old days, a woman with working titties 24/7/365 would feed her young more efficiently.
Today, big tiddies are nice, so while a small titty woman has a 99.9% chance to get a guy willing to creampie them, a big titty may have a 99.91% chance. Why would they evolve away in that case?
Especially since the tiddy is specifically needed to raise the next gen.
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