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Plenty of animals have butts, but we humans have quite the butt cheeks, too.
By
Jack Scanlan

8 Jul 2016 - 9:05 AM  UPDATED 8 Jul 2016 - 9:07 AM

It’s quite natural to be curious about butts. We all have them, and many of us think about them pretty frequently. But have you ever considered why we have such sizeable butt cheeks? Sure, they’re useful for sitting on, but do they serve any other purpose?

Human butts are pretty special: modern society makes a good deal of fuss about the size of people’s butts, but big or small, yours is almost guaranteed to be larger than that of any non-human primate. Gorillas are pretty flat back there, and the only way a chimp could “break the Internet” is if it were let loose in one of Google’s server farms.

Muscle butts

The anatomical basis for the exceptional size of human butts is due to both a large amount of fat and a large amount of muscle. The latter — the gluteus maximus — adds most of the default bulk, while the layer of fat that sits over it varies a lot more from person to person.

Explaining the size of our butt muscles is reasonably straightforward, at least for evolutionary anthropologists like Associate Professor Darren Curnoe from the University of New South Wales.

“The major differences between humans and other apes are the result of our evolution as bipeds, or two-footed apes,” he says. “The muscles we have in common with apes actually often function quite differently in humans, moving our bodies about on two feet instead of four.”

Large, thick gluteal muscles help us remain stable while walking upright, and our pelvises have been moulded by evolution (wider side-to-side, but also shallower front-to-back) to ease the transition to moving about on two legs, which combine to produce a distinctive curve to our posterior, as well as give us much wider hips. 

Fat butts

The abundance of fat on human butts is a little harder to explain. There’s no clear connection between walking upright and needing a thicker layer of fat on the behind, so anthropologists have turned to other hypotheses.

One idea is that “fat around the hips, buttocks and thighs represent a safe storage space to help humans survive episodes of food shortage, which were probably regular for our hunter-gatherer ancestors,” says Curnoe. “But also, because breastfeeding is very demanding in terms of energy consumption, the extra fat is probably a kind of insurance policy for women to ensure both their survival and that of their vulnerable infants during the first few years of life.”

In other words, big butts might be a byproduct of the general fattiness of humans — we’re the some of the fattest primates around, (although mind the fat-tailed dwarf lemur, which stores fat in its tail to get through the winter) even before you consider the world’s current obesity crisis.

Unfortunately, we might never know for sure why our ancestors evolved to have so much fat. “Theories about the role fat may have played in this area in our evolution are terribly difficult to establish,” says Curnoe. “We don't have evidence from the past, of course, because we don't have any soft tissue — only bones fossilise.”

“My hope is that maybe with the remarkable new science of ancient human DNA, we may be able to trace the evolution of human body fat in other species, like the Neanderthals and Denisovans,” says Curnoe. Finding genetic information linked to fat deposition in the buttocks in the ancient DNA of our extinct cousins might be the only way of shedding any light on the issue. The history of our cheeks might be obscured until then.

But regardless of what happens, sit down and consider your butt with pride. It turns out to be a rather big part of what makes you human.  

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