[♪INTRO].

Why do we have butts? It might seem like a cheeky question, but if you look around the animal kingdom, even at our closest relatives, big butts are pretty uniquely human.

So why would evolution grant us such an asset? We have a few hypotheses. So, our butts are made primarily of two things: fat and muscle.

And each likely has its own evolutionary role to play. Like, in general, fat is a resource that’s useful for our brains as they develop. A 2016 study proposed that development of both our large brains, and our fat stores, coincided with the time our ancestors came down from the trees.

See, it takes less energy to transport all that fat when you’re walking versus when you’re climbing. So by walking on two legs, we could carry more fat, allowing us to develop and power a bigger brain. Or, so the hypothesis goes.

There’s still plenty we don’t know about the evolution of fat, and bum fat in particular. It’s tough to track, because it isn’t preserved very well in fossils. However, we’re in slightly firmer territory with the muscle part of all this: the gluteus maximus.

It’s the biggest muscle in our bodies -- which is only the case for humans, not our ape relatives. We’re also the only primates that habitually walk on two legs, so for nearly 200 years, scientists have thought that these two observations are probably related. Now, your gluteus maximus isn’t actually used much when you walk on flat ground.

But it sure is used when you run. As your heel hits the ground with each stride, your body angles forward. And it’s that big gluteus maximus that’s partly responsible for stabilizing your body and pulling it back.

Like fat, muscles aren’t usually preserved as fossils — but the bones they attach to are. And the functions of bones and muscles are closely related, so we can make inferences about the evolution of one from the other. We know that of our various ancestors, Homo erectus had a pretty human-looking pelvis by around two million years ago -- which could mean a human-looking butt.

This came along with a bunch of other features that make us good runners. Things like long Achilles tendons, short toes, and long legs. Humans are the only primates that can run long distances.

And scientists think distance running helped us evolve new strategies for obtaining food -- by chasing prey to exhaustion, or quickly getting to carcasses to scavenge them. So butts would have been a key advantage for hunting. In support of this idea, the evolution of the butt roughly corresponds to a time anthropologists think our diet increased in protein — possibly because we were better at running after our food.

And in turn, all that protein was once again helpful for developing those big brains of ours. So the next time you’re out for a run or even thinking through a tough problem, remember… thank your butt. It’s a big part of being human.

Thanks for watching this episode of SciShow. If you enjoy completely earnest discussions of the science of butts, you’ll probably like the podcast with a dedicated butt-related segment: SciShow Tangents. It’s a lightly competitive knowledge showcase where the hosts try to outdo each other with esoteric science knowledge.

Be sure to stay until the very end to hear Butt One More

Thing: the weekly butt fact that ends each episode. Get it wherever fine podcasts are available! [♪OUTRO].