the evolutionary mystery of human breasts

There’s just something mysterious about breasts. No, not how entire business empires are built on the basis of showing them to a ravenous public. That’s not exactly what you’d call a mystery. The big question is why the female human breast evolved the way it is today. Is it a matter of comfort and security for the offspring as some studies suggest? It is an odd twist in the evolution of what began as a part of mammalian immune systems? Or is it, as some hypothesis offer, a kind of auto-mimicry that evolved as part of human bipedalism and has a lot more to do with attracting the opposite sex than nursing? In any case, it’s very hard to deny that there really is sexual selection involved since some women go so far as to get surgical help with their natural assets.

Ok, so it’s pretty obvious that natural selection helps shape breasts and keep them around, but how and why did they appear? According to the hypothesis proposed by zoologist Desmond Morris in 1967, the breasts we know today are an evolutionary side-effect of bipedal locomotion. Walking upright gave our ancestors an edge by freeing up appendages for carrying things, manipulating tools and having a higher vantage point when they walked on the ground. However, it also meant that the shape of the pelvis had changed, and so did the typical arrangement during intercourse. Mating from a rear position supposedly became mating face to face and so, the female breasts swelled to let the males know that there’s been a slight shift in the location of reproductive organs. It’s an idea, but there’s a reason why it hasn’t become the official theory and I bet you can see why.

First off, as a number of studies suggest, baby primates need softness and comfort when feeding or trying to relax. Baby humans love to cuddle with their moms and yes, they love to rest their heads in a soft, comforting cleavage. And the same goes for their evolutionary cousins, baby chimps. While they may not have the same kind of breasts to hang on to, they sure love to latch on to something soft and warm as well, showing that our common ancestors were already playing a role in shaping the evolution of breasts in hominids. That means that a purely sexually selective role for the breast just doesn’t cut it. Additionally, it seems like an adaptation to signal reproductive availability to males due to a change in hip and spine structure would require an unlikely turn of events. Plus, let’s factor in that there’s a very wide spectrum of breast sizes and that sex doesn’t always happen only in the missionary position, and we’re left with a lot of questions that the Morris hypothesis and all its variations still fall far short of answering.

But if neither the needs of baby primates or reproductive hints to males explain the whole story, leaving plenty of issues that need to be resolved, what could explain why female breasts are so distinct in the animal world? Well, there is some work which shows how mammary glands may have evolved in the first place, starting out as patches of skin secreting proteins and sugars loaded with antimicrobial agents that help innate immune systems of infants, providing chemical that evolved to kill bacterial infestations. After analyzing the pathways that regulate lactation in mammary glands, one team of biologists proposed that the breasts actually swelled due to their primary function and the rest is implied to have been up to selection. Still, there’s no answer to the question of why they swelled so much in humans but not in the vast majority of other mammals. And maybe, just like with many evolutionary mysteries, we’ll never know exactly why and the best explanation we’ll have is an educated guess with a hefty dollop of constant doubt on top…

See: Vorbach, et. al., (2006). Evolution of the mammary gland from the innate immune system? BioEssays, 28 (6), 606–616 DOI: 10.1002/bies.20423