Why do women have breasts?
Evolutionary theory has come up with nice explanations to many phenomena. But there are also a few exceptions. Female breasts are one. No female animals have enlarged breasts when they don't breastfeed. No animals except humans. There are a number of explanations to this, to be found on for example Wikipedia or in this article.
The extra-butt hypothesis (launched by zoologist Desmond Morris in his 1967 book The Naked Ape): Males were attracted to females' behinds. When our ancestors started to walk upright, fronts became more visible. Then females could appear more attractive with something buttock-like on their fronts. (Comment: Male sexuality is infamous for being superficial and stupid. But is it that stupid?)
Fat-hypothesis: Big breasts show that a female is well-fed. (Comment: But why not just place the fat in a more practical spot?)
Handicap hypothesis: female breasts are like the peacock's tail: Useful because it is useless. If a female can carry those ill-placed fat deposits through life, she must be truly outstanding. (Comment: Handicaps in sexual selection belong on the low-investment male side. To handicap oneself in order to brag only functions if one's main gift to one's offspring is superior genes. That has never been the case for human females.)
Age-indication hypothesis: A woman with big, round breasts shows that she is young and fertile. Too young, no breasts. Too old, empty and sagging breasts. (Comment: It doesn't take many pictures from primitive cultures to notice that women get pendulous breasts long before they stop being fertile. Many girls also never have round breasts from the beginning. For an overview of variation in teenage female anatomy, make an image search for Zulu Reed Dance.)
Fertility hypothesis. One study showed that women with bigger breasts and proportionally narrower waists had higher levels of a hormone linked to fertility. (Comment: Correlation is not causation. Besides, why would this unknown correlation between big breasts and fertility only work in humans and no other species?)
I think these explanations are all rather bad. Especially the theory that big breasts should signal fertility is downright counter-intuituve:
Breasts are the biggest, roundest and fullest during pregnancy and lactation. The first visual sign of pregnancy is not a notably larger belly. It is bigger, fuller and all around more silicon-style breasts. Ancestral men who were attracted to that type of woman ran a real risk of having sex with already pregnant women. Having sex with a woman in early pregnancy makes you a suspect of being the father of the child, if she didn't tell (or didn't know) that she was pregnant. In most cultures, also those with loose sexual norms, fathers are expected to provide for children and their mothers in one way or another. That way, having sex with a woman because early pregnancy gave her ballooning breasts seems like a very bad idea indeed.
Having sex with a lactating woman is also not reproductively great. Many women are not fertile while lactating, although their breasts are definitely big.
Despite this, males, at least Western males, hold a general preference for breasts that look like they belong to a pregnant or lactating woman. It is entirely possible that breast size is correlated with some fertility hormone. But every individual woman will have bigger breasts when infertile from pregnancy and less fertile from lactation. Breasts are as much of an anti-fertility cue as a fertility cue.
Mother I’d Like to Fuck
I would like to propose a totally different explanation: The MILF hypothesis. The purpose of breasts is not to correctly indicate the age or nutritional status of a female. It is to mislead males about the parity and nutritional status of a female.
To make this understandable we need to get back to the apes. Among the chimpanzees and the bonobos, males have a clear preference for somewhat older females. This seems to be because older females are better mothers. Being an ape-mother is difficult and first-time mothers often fail due to lack of experience. Males do little to help out: Since chimpanzees are promiscuous, they don't know very well which infant they fathered and they invest nothing or little in their children. Their chance of reproductive success is dependent on the female’s ability to rear their offspring. Therefore ape males have a strong preference for females who have already given birth and have the experience to raise an infant successfully. They commonly leave non-mothers to lower ranking males and let them father the first, experimental child. (Primatologist Franz the Waal present this information in his 1982 book Chimpanzee Politics)
My guess is that our ancestors had a mating system rather similar to today's chimpanzees. I'm not alone in guessing that. Several primatologists see the chimpanzee as a model for the common ancestor that diverged into humans and chimpanzees about 5 million years ago (for example Richard Wrangham in his 1996 book Demonic Males). In any case, the common ancestor should have looked rather much like a chimpanzee, because our earliest ancestors are very similar to chimpanzees. It wouldn't be completely strange if our ancestors shared the chimpanzee's preference for mothers then.
But are chimpanzee mothers breastfeeding when they mate for a younger sibling? This study suggests that the more well-fed mothers resume fertility while they still breast-feed rather intensely. In the study, a third of mothers had resumed fertility within 2 years postpartum. Another third resumed fertility between 2 and 4 years postpartum and a third resumed fertility later than 4 years postpartum. The resumption of fertility depended heavily on the mother's nutritional status. Mothers returned to fertility whenever they had available resources to do so successfully. In very good circumstances this could be less than a year after giving birth and in less advantageous circumstances it did not happen until their infants were completely weaned at the age of 4-5 years.
Male chimpanzees do not care much if the female is breastfeeding or not and hence the size of her breasts. Fertile female chimpanzees display a pronounced genital swelling, and that swelling is attractive enough for the chimpanzee male. Being a promiscuous animal the male chimpanzee wants almost every fertile female around.
Our pre-human ancestors took another path. Males started to invest in females and their children, probably little by little. This process is not complete to this day: Some females only manage to secure a limited amount of investment for limited times and some get none at all. When ancestral males started to invest in females, they sought to invest as little as possible so they had some resources left for other mating efforts. Therefore they actively sought out females who were as self-reliant as possible.
One clear marker of female self-reliance is if she is both breastfeeding and fertile. A breastfeeding and fertile mother basically says that she has high confidence she will be able to successfully raise another child. Even though human males invest in their offspring, human females invest a lot more, so the female's opinion is of tremendous importance for the male.
Seen from the ancestral human male’s perspective a female that was both fertile (some sort of visible estrus was probably still around in pre-human ancestors, some argue that modern human females still show their estrus, although very discreetly) and breastfeeding was an evolutionary jackpot. She was an excellent candidate to successfully raise your children: She could raise children with short interbirth intervals and she could do so without much help. And how did you know that the female was breastfeeding? By looking at her milk-filled breasts of course.
Alas, in the battle of the sexes good things seldom last, and if human males had developed a taste for large-breasted fertile females it was only a matter of time before non-breastfeeding females started developing large breasts too. Even females who had never given birth. If males favored females with large breasts, then females had to develop large breasts. Females fooled males into desiring a cheap bulb of fat instead of an expensive flow of milk. That cheap bulb of fat is still with us and males still desire it, although everyone has since long forgotten why.