The human posterior is rather peculiar compared with the backsides of our close primate cousins. Its unusual form tells the story of our evolution like no other part of the human body.
Few parts of the body elicit such a mixed reaction as the human backside. We have an uneasy relationship with this most fundamental of human structures, as is clear from the many colourful terms we have for it in the English language: words like bottom, bum, rear end, fundamentum, posterior, buttocks and arse.
In many cultures, taboos about it preclude us from exposing this part of our anatomy in public unless it is thinly veiled beneath swimwear or, for the more daring, divided up the middle by a thong.
Only in the privacy of our homes, a gymnasium change room or at a nudest beach can we dare to expose that reviled and most mocked of all human features, the gluteal cleft, also known as the bum crack.
We are attracted to it and yet also repelled by it. The shape of our bum distinguishes us as male or female, and has historically been an object of sexual desire used to manipulate the opposite sex. We compare its shape to fruit – as in a woman’s peaches – and talk about men as having no bum at all.
We even use it as a term to describe someone who is a useless layabout or, worse still, somebody who treats people rather badly. At the same time it provides the opening for our gastrointestinal tract, for the removal of solid and gaseous body waste, and is perilously close to our genitals, barely hiding them from behind.
The more talented among us can even play the national anthem or the latest pop music using its more vocal-like qualities: usually a male pastime, I might note.
Yet, in the story of our evolution it seems the backside is an afterthought, rarely mentioned in the textbooks, given a bum steer if you will, neglected despite its vast store of information about our evolution.
Well I’m here to fight a rearguard action, to cast your eyes below the waistline sunshine, to defend the bum!
Why do we even have a bum?
Our backside is literally just that, part of the back, posterior or dorsal side of our bodies. At the lower end of our trunk, the bones of the hips or pelvis connect the spine to the lower limbs. They allow movement via our hip joints, and many of the muscles that move the thigh are strung from the pelvis to the femur.
Without them we couldn’t walk, run, climb or jump. The standout anatomical feature for all members of the human evolutionary branch is our upright posture and twofooted walking. It defines us as a group of primates, as hominins, or bipedal apes.
The human pelvis is fundamentally different to other apes because our ancestors evolved to walk upright: it is shorter (top to bottom), wider (side-to-side) and deeper (front to back) than a chimpanzee’s or gorilla’s, species evolved for four-limbed (or quadrupedal) locomotion.
Unsurprisingly, many of our walking and running muscles attach to it in ways that are different to other apes. Thus, the shape of the pelvic bones and the different positioning of these muscles, especially our gluteals, is the beginning point for the difference in the shape of our backsides compared with a chimpanzee’s.
Moreover, chimp bums have built-in cushions of fat called ischial callosities, for them to sit on; these features are lacking in us and our gorilla cousins.
A difficult birth
The pelvis also provides a protective enclosure for some of our vital organs as well as the nerves and blood vessels that pass to and from our lower limbs. It provides the attachments for the pelvic sling that suspends our external reproductive organs and the opening for our gastrointestinal tract and urethra emptying our bladders. And it also provides passage for babies when we give birth.
When brain size began to increase in our ancestors from around 2.0 million years ago, a further complication emerged for the pelvis: the need to give birth to larger-headed babies would have led to more complicated and dangerous deliveries.
To allow this to happen more safely, the pelvic outlet needed to become wider. There are a several ways in which natural selection could make this occur: first, through widening the pelvis from side-to-side or front-to-back; or second, by enlarging the average body size, and with it pelvis size, for the adults of this new, larger-brained species.
Both adaptations would have had costs, of course, and would also have affected both sexes because of shared genes. A larger pelvis would result in a less energy-efficient style of walking, but in a larger body, especially one with longer legs, this can be offset with a longer stride and lower metabolic costs. A short-statured, wide-hipped biped can’t cover much ground when it walks, and would use a lot of energy doing so.
So, it’s probably no coincidence that significantly larger brains evolved along with increased stature and relatively longer lower limbs. Human babies are also very immature compared with most other mammals when they are born.
In part this results from the slowing down of brain growth in the womb towards the end of gestation to make birth somewhat easier. Thus the size of the maternal pelvis acts to stall growth. It also leads to a rapid period of “catch up” brain growth after birth.
It took guts
Another peculiar aspect of our bodies is our much shorter gastrointestinal tract, much of which is contained in the pelvic cavity and of course ends at the backside. While the human stomach is about the same size as a chimpanzee’s, our small intestine is more than twice as long and our large intestine only about half the length of a chimp’s.
Our gut doesn’t bulge out like an ape and our caecum and colon are small as we don’t need to ferment bulky plant food. The reason for this shift in gut proportions was a change of diet to high quality, energy-rich items like starchy vegetables and meat.
Genetic research suggests that some species of tapeworm that infest millions of people around the world each year may also have evolved at this time, making humans a definitive host that they need if they are to reproduce. It may even have been us who gave tapeworms to other mammals like cattle, pigs and sheep.
Now, among other mammals, only carnivores are also definitive hosts for tapeworms, giving some surprising insights into the meat-eating habits of our early ancestors, and emphasising the importance for meat consumption during our evolution.
Some archaeologists even think that cooking may have begun at this time, allowing early humans to sterilise meat, making it more palatable and allowing them to eat starchy foods, the Stone Age equivalent of potatoes, in order to access otherwise indigestible cellulose for energy.
Girls from the boys
The broader pelvis of human females also clearly distinguishes them from males. Females are born with a broader, relatively wider and deeper pelvis for reproduction.
Yet another, but more difficult to explain, difference between the sexes is the amount of fat we deposit beneath the skin around our hips, buttocks and thighs, as well as other part of our bodies.
These differences are present at birth and become exaggerated from puberty with the action of sex steroids, and are unique in humans among the primates.
One possible explanation is sexual selection. Perhaps in the early evolution of our species, or in another ancestral one, females with a more curvy figure were better able to compete to attract high quality mates. Or, perhaps the larger, more curvaceous bottom of females from puberty acted as a sign of fertility.
One thing we do know is that humans are the second fattest of all primates, in terms of the amount of our body comprising fat. The fattest primate is a loris species that stores large amounts of fat in its tail for annual hibernation.
Thus the larger female backside may be part of the overall fatter human body, perhaps providing a safe and physiologically efficient place to store fat, especially for times of food scarcity, which occurred frequently among our hunter-gatherer ancestors, or for extra energy during breastfeeding.
Also unlike our chimpanzee cousins, the backside doesn’t advertise when a woman is potentially able to make a baby each month.
When chimp females are ovulating – known as oestrus in apes – their rear ends, especially the skin around the external genitals and perineum, engorge with blood and distend to advertise their fertility and interest in sex.
At the end
The shape, anatomy and evolution of our backside are quintessentially human. Yet, throughout history the bum has been much maligned and misunderstood.
It is an evolutionary oddity that reflects many aspects of our complex evolution, from upright posture and bipedalism to shifts in diet and ecology through to exaggerated differences in body shape between females and males as well as our peculiar social and sex lives.
The multifunctional nature of our rear end has resulted in competing and conflicting demands over the 7.0 million years of our evolutionary history. These have led to it being a structure of compromise in terms of its shape, functions and mechanical efficiency.
The creative evolutionary force of natural selection can, after all, only work with structures that exist – it can’t invent ones like the pelvis from scratch – and even then only by minor tinkering with genes that have largely existed for hundreds of millions of years.
I think it’s time to stop giving this remarkable human feature such a bum steer. We should celebrate the bum for its fundamental place in our lives, for its beauty, its fascinating and complicated anatomy, and for the story it has to tell about who we are as a species and the remarkable history of our evolution.
Originally published by Australasian Science.