Pre-human male hominids, we infer from observing bonobos and chimpanzees, were dominated by one alpha male per group, who got the best food and most of the females.

Um. Bonobos don't work that way. They're "dominated", if you can use the term, by less-vertical coalitions of comparitively high-status females. Food is shared widely, paternity certainty is a nonissue.

We're equidistant from them and chimps, talking divergence from common ancestors, so it's really less clear-cut than you think how well chimp analogies suffice to model proto-hominids -- both are equally-close relations, but their lifestyles and social strategies differ tremendously.