Another factor I think you need to look at in the development of family structures is patriarchal lines of succession. Marriage is, at its essence, a social institution designed to protect and determine bloodlines traced through fathers. Everybody can tell who the mother of a baby is, because it clearly comes out of her body. There’s never any question there. But for male-dominated societies, there’s never any way to be totally sure if you’re the “baby-daddy”. So, part of what marriage did and does is to (theoretically) guarantee for men that children his wife bears are his own. And if you don’t believe me that this is a major factor, just spend one afternoon watching Maury Povitch give out paternity tests and the enormous turmoil that results.