Dear A Moment of Science: does a bigger butt mean a better strut?
I Like Big Butts!
The muscle that makes up your rear end is called the gluteus maximus. Humans have big ones. Humans also walk on two legs instead of four.
Now the gluteus maximus in apes is much smaller than in humans, and ends at the hipbone. Apes can stand for a little while, but they soon get tired and go back on all fours.
A small gluteus does fine at positioning the hip bone for tree climbing, but not for walking. Human glutes, though, extend farther up the back to a bone called the illium. That extra muscle is necessary if you’re going to catch yourself from falling to either side when walking.
We can assume that our common predecessor on the evolutionary tree didn’t just stand up one day and start to walk. It wouldn’t have been possible with a small gluteus. There has to have been other reasons for trying out the upright posture–such as, say, getting food out of trees.
Gradually the gluteus evolved into a larger and larger muscle, until actual walking became possible. So a bigger butt does mean a better strut!