Why Do We Have Butts?

Humans have bigger butts than any other animal! Here's why we have so much junk in the trunk.

Consider, if you will, the matter of the human buttocks. They're practical, functional and often aesthetically pleasing. They're also essentially unique among all animal species. Other creatures kind of have butts, but none are as big, strong and frankly glorious as ours.

Jules Suzdaltsev goes behind the science, as it were, in today's DNews special report.

The short version: Human butts are unique in form and function principally due to the fact that, unlike other primates, we Homo sapiens prefer to walk upright.

The major muscle in the posterior is the gluteus maximus, one of three gluteal muscles -- along with the gluteus medius and gluteus minimus -- that provide extension for the hip and thigh. All primates have these muscles, but they're shaped and connected in very specific ways, depending on the species. For instance, on chimps the gluteus maximus muscle is connected to the ischium, or the lower portion of the pelvis. This makes climbing trees much easier.

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On humans, the gluteus maximus attaches to the upper part of the pelvis, the ilium. This placement allows for trunk stability and helps keep us balanced. Most researchers believe that we have big butts because it helps us stay upright, and helps balance us when walking and running.

But which came first, the big butt or the upright walking? A research project published in the journal Human Evolution used an evolutionary simulator to examine how changes in the shape of our bones affected muscles. The computer model showed, among many other things, that early hominids would have had to start walking upright before developing large butts. If the butts had come first, they would have actually impeded the act of walking.

And there's your delicious irony: Even though we sit on them for alarming periods of time each day, our buttocks actually evolved from standing and walking.

-- Glenn McDonald

Learn More:

BBC: The Origin Of The Anus

Discover Magazine: These Butts Were Made For Walking

Live Science: When Humans And Chimps Split