CB makes a good point here; however, it's orthogonal to the point of my earlier post. Regardless of family structure, offspring are more likely to survive if there are adults around to care for them, including the father. In the kind of society that CB describes, a man who fucks and leaves would be socially stigmatized, marginalized, and unlikely to have much sexual access to women, wouldn't he?
Miles
^^^^ CB: Probably. They had ways of enforcing taboos.
However, there wouldn't really be such a thing as "leaving". Leaving the group would mean death. There were no Robinson Crusoes. That's a bourgeois myth. There was no leaving a "couple" , because they weren't organized in couples.
Then - this is critical - there was no special responsibility of the "biological" father for his biological children. Even biological fathers who didn't "leave", in whatever sense, didn't have a special responsibility for their bio children. Uncles, all men had male responsibility for all "nieces and nephews".
If one doesn't break away from the notion of special responsibility of biological parents, one doesn't get it. There was no special relationship between biological father and children. The biological father wasn't known. Any of a group of men could be the bio father. The special relation of the biological mother is to put the child in a lineage. Otherwise, a whole group of women are "mommy".
The whole concept of adaptive advantage from biological fatherly care vs disadvantage from lack of biological fatherly care is inapposite, not in play, a modern projection onto the past.
Monogamy is culturally based ( socially constructed) _not_ biologically-evolutionarily based. Monogamy is not a human instinct. It did not derive out of males who "stayed" giving an advantage to their offspring producing offspring. Monogamy "against" women arises historically so men can be sure that they pass on their private property to _their_ biological offspring. It arises way after socalled hardwiring of the human species.
Of course, human babies _must_ have adult care for much longer than other species in order to survive. This is a unique human species characteristic - longest period of children's helplessness. All early humans have redundant parental care for infants, beyond one father and mother. The redundant parental care - every child has many more than two parents - is more efficient than the two parent model for children to survive until they reproduce.
The currently popular African proverb - it takes a whole village to raise a child - is an actual window on first human society in Africa. They didn't have "married couples".
By the way, I agree with you , Miles, that it is important to note that much of human "hardwiring" is hardwired from the time of many ancestor species. Eyes in the front of the head came about in "earlier" primates. We have a lot of hardwiring from being mammals. Sexual reproduction is from a billion ( 2?) years ago. This is an important point. Much of human nature is not uniquely human nature.
The interesting question is what is , if anything, uniquely human nature, not shared with other species - upright posture, opposable thumb, specific teeth, lot less fur, etc. I'm wondering if there is a uniquely human biological "module" for our high level of sociality. Our language capacity is a candidate for this.