Probably my comments will not fall exactly inline with the others in here. However, for the sake of knowledge here it is...
The fact is that it's hard to explain the evolution of the butts. These muscles behave differently from Humans to our near relatives the Apes. We are two-footed apes. We have a large amount of muscle and fat in the butt. The muscle is the the gluteus maximus and it's supplemented by a layer of fat that differs from person to person.
The fat in the butt is hard to explain as it isn't needed as a biped. There's no connection of having fat in the butt and standing upright.
This makes the anthropologists to think that it's a food reserve. During difficult times and during breast feeding this reserve comes into play. In fact. there are some animals who store fat in their tails to get them through during the hard winter months.
To add further, recent research shows that the development of babies’ brains relies on fat supplies stripped directly from the mother’s thighs and bottom, especially during breastfeeding. This can directly affect the child's intelligence. This is probably why some of the males are genetically hard-coded to prefer women with larger butts.
There's another theory that says the bigger the butt, the bigger the brain. Now don't come up with Kim Kardashian as an example. No... Fake butts doesn't make you more intelligent.
The extensor muscles in this area does have a neurological connection to the brain. But it's not the size but the function stimulates brain activity.
