Analysis of the fossilized teeth of our early ancestors living in southern Africa shows it was the women who ventured out when they came of age, while men tended to remain close to home, researchers say.
Science sais they left because they wanted to avoid inbreeding or was looking for a mate. I say, it's becaused the males wouldn't pick up there laundry and kept leaving the rock seat up.
I dont think they left voluntarily, I think a more likely scenario is one where the rival tribe stole the women. More women means more children, more children increases the numbers of your tribe while the other goes stagnant.