Recently I had (what I believe is) a novel idea for why human females generally have prominent, permanent breasts. Obviously they have DNA that programs breasts as a phenotype but why is that so? Why is that morphology almost unknown in other mammals? If you give it some thought, it is not immediately obvious why the human arrangement of mammary features might be superior except by tautology. Indeed it turns out to still be an evolutionary mystery.

One traditional bad answer is: because men like them. Perhaps that aspect did co-evolve — babies like them too — but that doesn’t explain why other mammals don’t bother. To say that men and women are generally attracted to each other seems unremarkable and a facile explanation for why we are the way we are in the first place. It may seem obvious to some people that men love breasts, but anthropological research (beyond the parochial realm of cultures that do anthropological research) show that attraction to breasts is hardly universal. Now that WEIRD people have plenty of obese men with breasts, I’m going to say that the hypothesis that breasts exist because men like them has been somewhat refuted.

Another common bad answer I found in the literature is very bizarre to me: something for a baby to grab on to. Obviously, many women have breasts that comprise quite a bit of their entire mass, especially during lactation; yet newborn babies have quite tiny hands, hands that are rather inept at doing anything but poking themselves in the face. I’m sorry, but I’m not buying it. Wikipedia’s only hypothesis is that they protrude to keep flat faced babies from suffocating; maybe that’s so, but why permanent? Explanations like warmth and chest protection also seem lacking if men and other mammals didn’t also get them.

So why did that weird physiognomy evolve uniquely in our species? My theory suggests that breasts co-evolved with tool use. Imagine you had to craft wearable items from dead animals. One could imagine this starting by proto-hominids simply killing animals with tools (flint spears, arrows, sharp sticks, etc) and using the same tools to help eat the game. This would naturally leave behind a pelt as one of the least edible parts of the animal. It wouldn’t take long for our ancestors to totally gross out the rest of the animal kingdom by augmenting their own caveperson fur with these pelts. At first they would just wrap themselves in them at night and enjoy lower mortality. That’s step one.

Before moving on to loincloths or heavy metal attire or any item of clothing we might consider obvious today, I can imagine those pelts being worn for a different purpose than clothing. If I had a sharp piece of flint and a recently deceased deer, and if I was cold and naked, I’m pretty sure I could transfer that deer’s hide to cover mine. What is the next easiest thing to fashion with such tools and materials?

My guess would be a bag, something to carry possessions in. Basically you lay the pelt on the ground, put your things on it, and then gather the edges together with your opposable thumbs. Now it’s easy to pick up many things while gripping one. That’s possibly useful, sometimes, but it suggests an application that is extremely useful almost always for early hominid females — carrying a baby.

Consider how closely the two developments of wearing hides over your body to give extra protection, and lifting bundled babies had to be. This is no mere pointless fashion accessory. This allowed hominid females to be much less constrained by the task of keeping their offspring safely stowed.

Since I personally can imagine myself having the capabilities to simply tie a pelt in a knot, I’m going to suggest that doing so was the next developmental step in the history of clothing. With a knotted pelt, the really big deal is that you essentially have invented the baby sling. I propose that this invention, likely first developed by females, was one of the most profound innovations of our ancestral species that led us to where we are today.

I think of the baby sling as a milestone achievement on par with fire and flint tools. It allowed hominid species to essentially extend the intimate protections of pregnancy even further. This effectively helped create a species that promoted big brains by prolonging the transfer of knowledge, warmth, protection, and nutrients.

That brings us back to boobs. Imagine a new band of especially clever apes whose new mothers now carried around their babies slung over their shoulder in dead animal pelts. (Does it help to consider the mysterious obsession some modern human women seem to have with leather handbags?) When sitting or resting the mother can position the baby in the front for feeding. Maybe protruding breasts help reach babies nestled in hide slings.

However, most of the time while foraging and migrating, women would tend to wear their babies on their backs. Why did human women evolve to have strangely large breasts? I’m going to offer the simple engineer’s explanation of counterbalance. Remember, the hide innovation, like flint and fire, must have hit the innovating species pretty quickly in evolutionary terms. Once hominids started wearing their babies, evolution was going to make some strange and radical selections to keep up with the suddenly new circumstances. That is, to me, a more satisfying explanation of why prominent permanent breasts are such a uniquely human feature.

Perhaps some hominid species were pitched forward naturally and used their hands for stability, like gorillas. As evolution filled in the gaps between them and us, some force would want to keep the delicate vertical alignment once bipeds were fully bipedal. The morphology of modern human breasts may not seem compatible with this explanation of the origin of breasts. Today breasts may well be the equivalent of peacock feathers. But it must be kept in mind that peacock feathers are still feathers and feathers are evolutionarily quite clever in general.

In short, I propose that oversized hominid breasts of some form arose to counterbalance and reach infants being worn in hide slings.

Subsequently breasts could have taken on a new additional role as a signal of reproductive fitness but it would seem arbitrary to propose that as the primary reason for their existence.

I’m not a paleontologist so I don’t know exactly what about this hypothesis is testable. (Spine wear? Skeletal posture?) If some aspects were, and the theory could be strengthened, it may actually provide a way to link hide wearing with physiological features in the fossil record. One could also reconstruct the physiological choreography of reproducing humans. As the baby grew and started to become weaned, the breasts would still serve as a counterbalance. The counterbalance would be further increased by the toddler’s younger sibling in the womb. When that baby was born, it would be time for the older kid to start to walk for themselves.

All of this brings me to my own most valuable advice for reproducing humans: slings are great! Carrying your baby in a sling, like pretty much all human women have done for pretty much all of human history, has so many advantages that it’s hard to enumerate them all. (I’ll let the rest of the internet do that job.) When my son was born I made a (canvas) sling and it was by far my most valuable piece of baby equipment.

I actually got the idea for breasts being a counterbalance when I was trying to learn how arctic natives build igloos. I came across Nanook Of The North, a 1922 documentary from before the time that documentaries really existed. At that time the film’s creators could only properly understand the genius and macho power of the male titular character. But to a modern audience who won’t take such things for granted, the women also clearly inspire awe. Here is Nyla who did an amazing job of demonstrating the most extreme form of a life physically connected to one’s baby.