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
Another point against the sexual-selection hypothesis is that the modern/western emphasis on big boobs is far from universal. Consider the famous Venus de Milo statue for an idea of what the Classical civilizations of the Mediterranean considered sexiest.
One somewhat crackpot idea I’ve yet to see fully refuted suggests human breast size is a spandrel. Nearly all other female mammals have a pronounced estrus cycle such that they’re either sexually unavailable or “in heat”. Adult human women’s hormonal cycles, though still quite powerful, are more evened out and lack such absolute peaks and valleys. Whatever the advantages to that (assuming this itself is not a spandrel due to other causes), could it be that the more constant presence of estrogen and related biochemicals during adolescence affects breast growth to cause permanent enlargement?
Indeed. Have a man take estrogens for a while, or just suppress androgen action during adolescence, and what you will have is gynecomastia ( real male boobs).
Primates are unique in the mammalian world for having high circulating levels of DHEA during adulthood, starting at 7-8 years old (that is called the adrenarche). The increase and decrease in DHEA levels also correlates with the maturation and decline of the immune system.
One of the interesting side effect of high DHEA levels is that even with the gonads removed or inactivated, 50% of active sex hormones remain to act in the tissues, synthesized on site (that is called auto- and intra- crinology).
With all the sources and pathways to sex hormones in humans, I wonder how our physiological tissue (breast) concentrations compare with those of other mammals ?
I don’t have anything scientific to add to this discussion, only an observation that in our sexually schizophrenic society female breasts sell.
Just look no further than the Sports Illustrated Swimsuit Issues.
In the animal kingdom there are plenty of examples of exaggerated morphology (the peacock’s tail, for instance), generally thought to be related to sexual signaling. You don’t need something as specific as Morris’ idea about encouraging a sexual position; slightly enlarged breasts could simply signal reproductive ability, and from there you get an arms race leading eventually to Dolly Parton.
True, this doesn’t happen in other mammals, but then again most birds don’t have massive useless tail feathers. Breasts could simply be a somewhat uniquely human signaling mode. Remember that humans have a distinctive sexual cycle, with human females sexually receptive even when they aren’t fertile, which is unusual.
The answer is pretty obvious to me:
- You have a breast-feeding baby.
- You need to walk for long distances.
- You need to have your hands free, and be able to crouch to gather roots, skin animals and what have you.
- You need to be able to put your hands around it, to protect it.
Is it more advantageous to have that baby on your belly, or near your chest?
Well, I’ve never seen a strap-on baby-carrying contraption that positions the baby below the chest, so I guess free market has already answered it.
Even if it were the same, there are probably additional advantages when you have the baby closer to your face. You can sooth it easily, use your breath to keep it warm, it can see your face etc.
Another viable position for the nipples would probably be the top of our back, or maybe even the shoulders (saw an old picture of an Inuit the other day, with her baby on her back, his cheek almost touching hers).
Their size and shape is another thing, and I guess it has a lot to do with both comfort and how easy it is for the baby to feed.
Oh, and another obvious one. Our ancestors would not even be clothed, so females would need to hold their babies in their arms while walking. With the breasts positioned where they are, the baby could breastfeed without the mother ever stopping.
On a slightly tangential note about the video, if someone already knows that they prefer fake or natural breasts, why would they need instructions to tell them apart?
I suspect that the human visual processing system and desire for surrounding ourselves with aesthetically appealing things has a lot to do with the sexual selection aspect of breasts. By this I mean the more general concept of niceness, which spans all sizes except perhaps the most grotesquely huge.
I think what is key here is the confidence of the person wielding the breasts, which is of course influenced by the culture that dictates which size is a source of embarrassment and which is a source of pride. The more confident breast-owners will be more likely to show them off (and this can be done tastefully), and thus more likely to capture male attention (and seed). This confidence can also capture the awe and respect of fellow females, mixed with a little bit of competitiveness, and thus there are societal advantages given to those who are endowed and proud of it, which again increases the chance of capturing male attention (and seed).
The main point though is that there are a lot of interacting reasons–maternal, sexual, and societal–for the breast to continue to evolve to become even more beautiful, no matter what the size. As my breast-loving husband once said, “We don’t care what size they are; we just want to see ‘em.”