Humans are officially more obsessed with butts than ever before. If you need further proof, just take one look at the Belfie Stick—a selfie stick that allows you to easily take photos of your very own rear end. And hell, why not? Everyone likes a shapely butt. But (with one ‘t’), why do we have such pronounced asses in the first place? Fact: Our big butt cheeks are the whole reason we’re able to walk on two legs.
Here’s a brief anatomy lesson (to explain the anatomy in your briefs). The gluteus maximus muscles, which give our butt cheeks their shape and size, help us remain stable while walking upright. On top of that, our wide pelvises—which are the structural support behind our wide hips and butts—provided the balance necessary to transition from walking on all fours to standing upright. That’s why non-human primates like gorillas have relatively flat butts—they don’t need any extra stability, because they walk on all fours.
“The major differences between humans and other apes are the result of our evolution as bipeds, or two-footed apes,” evolutionary anthropologist Darren Curnoe told science writer Jack Scanlan. “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.”
The way the human body distributes fat also contributes to the fact that some people have plumper behinds than others: Women, for example, tend to store fat around their hips and thighs as a source of post-pregnancy fuel, as opposed to men, who tend to grow potbellies.
It’s also worth keeping in mind that while all butts are the same general shape, they vary slightly depending on your individual anatomy. The inverted butt, for instance, is fuller around the hips and the top of the butt, but narrows in size and shape towards the bottom. Then there’s the square butt, which maintains the same proportions from the waist to the hips. There’s also the round butt, aka the “bubble butt,” and finally, there’s the heart-shaped butt, which is smaller around the waist and larger towards the bottom of the thighs. Whichever specific butt shape you have is generally just a matter of how your body decides to store its fat.
It should also be noted that butts change shape over the course of our lives—especially if we work a desk job. Years of sitting can cause what’s called an anterior pelvic tilt, which is when your pelvis essentially tilts forward and makes your butt look flatter than it really is. Combine that with the fact that men lose an average of five percent of their muscle mass (muscle that once kept their behind perky) every ten years after the age of 35, and you can say bye-bye to your once-pert tush. In short? If you’re going to buy that Belfie Stick, do it sooner rather than later.