Why Do We Have Butts?

Why Do We Have Butts?

Welcome back to Ask Giz, the fortnightly series where we answer your burning questions from all over the tech and science world.

If you’d like to submit a question, then head on over to the submissions page. We’d love to hear it and give you an answer.

Today’s question is one you might not have considered before: Why do we have butts?

Let’s get to the bottom of this one.

Why do we have butts?

For this Ask Giz, we reached out to a number of experts who, in the course of their careers, have for one reason or another come to intimately know the evolutionary history and present-day biology of the human asscheek. It’s a story that starts 350 million years ago, featuring massive primordial tails, and – winding its way through the millennia — ends on the ergonomic flesh-and-muscle you’re probably sitting on right now.

Professor and chair of Stony Brook University, Susan Larson, had this to say on the matter:

“The contour of the human buttock is mainly due to a large muscle aptly named gluteus maximus (gluteus is from Greek gloutos meaning buttock). It is the largest muscle in human anatomy, and its size and configuration is distinctive in humans compared to our primate relatives. It acts to extend the thigh (i.e, move it backwards) and its large size is generally related to the fact that humans stand with a thigh that is already more extended than you see in other primates, and yet we are able to extend it even further, that is, past vertical. The force it produces is very important during running and climbing stairs as well as standing from a sitting position.”

She added:

“That being said, there is more fat in the gluteal region than you might expect, which contributes to the contour of the buttock. It has been suggested that this may be due to sexual selection, that is, we find a rounded gluteal region “sexy.” This is of course speculation so it is hard to prove one way or another.”

So it has a lot to do with our posture, then, and is one of the things that separates us from our evolutionary primate relatives.

Here’s what Assistant Professor of anatomy and cell Biology at the Indiana University School of Medicine Jason M. Organ had to say:

“The lesser gluteal muscles of humans no longer move the thigh backward at the hip joint like they do in other Great Apes. Instead, they move the thigh outward from the side of the body. This new action of the lesser gluteals is critical to avoid falling over during walking on two legs, but it reduces the number of muscles that can move the thigh backward, which is an equally important movement when trying to move the body forward in walking. This is why the gluteus maximus is so large: it has to be in order to make up for the loss of two other muscles that had the same action! Of course, none of this explanation takes into account the possibility that our ancestors liked big butts (and they could not lie). Perhaps the size of the gluteus maximus was also influenced by what our evolutionary ancestors found attractive in their potential mates.”

Getting the vibe that evolution is quite horny from this response, but we’ll go with it.

Finally, here’s what the Professor of biological sciences and chair of the Department of Human Evolutionary Biology at Harvard University Edwin M Lerner had to say.

“If you mean the muscle, the answer is running. The muscle making up the butt is the gluteus maximus (yes, the largest in the human body) and its upper (cranial) portion is vastly expanded in humans compared to ape. We barely use it when we walk, but when we run its plays a key role in controlling the trunk, extending the leg behind you, and slowing the leg you are swinging in front of you. Interestingly we can see from the fossil record this muscle expansion occurred around 2 million years ago with the evolution of H. erectus and the origins of endurance running.”

He added:

“The other key component of our butts is fat, and even the skinniest humans are fat compared to other mammals because we need more fat for our expensive reproductive and energetic strategy.”

So it’s also to help us run by acting as a sort of counterweight. That’s terrific.

Now we can put this question behind us.

Butts, explained

If you’d like to submit a question to be answered in a future Ask Giz instalment, we’d love to hear it.

Ask Giz is a fortnightly series where we answer your questions, be it tech, science, gadget, health or gaming related. This is a reader-involved series where we rely on Gizmodo Australia’s audience to submit questions. If you have a question for Giz, you can submit it here. Or check out the answer to our last Ask Giz: How Do Spy Balloons Get There?

This article has been updated since it was originally published.

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