Sarah Blaffer Hrdy has written a new book, Mothers and Others: The Evolutionary Origins of Mutual Understanding to be released this spring, and the New York Times' Natalie Angier has a little story about Hrdy's thesis, that alloparenting was the key to human evolution:
Our capacity to cooperate in groups, to empathize with others and to wonder what others are thinking and feeling — all these traits, Dr. Hrdy argues, probably arose in response to the selective pressures of being in a cooperatively breeding social group, and the need to trust and rely on others and be deemed trustworthy and reliable in turn. Babies became adorable and keen to make connections with every passing adult gaze. Mothers became willing to play pass the baby.
The article makes a weak reference to the problem of human effective size:
“I’m not comfortable accepting this idea that the origins of hypersociality can be found in warfare, or that in-group amity arose in the interest of out-group enmity,” she said in a telephone interview. Sure, humans have been notably violent and militaristic for the last 12,000 or so years, she said, when hunter-gatherers started settling down and defending territories, and populations started getting seriously dense. But before then? There weren’t enough people around to wage wars. By the latest estimates, the average population size during the hundreds of thousands of years of human evolution that preceded the Neolithic Age may have been around 2,000 breeding adults. “What would humans have been fighting over?” Dr. Hrdy said. “They were too busy trying to keep themselves and their children alive.”
"Breeding adults" generally refers to the effective population size, which would have been several millions immediately before the Neolithic. Genetically, humans were relatively inbred at times before around 150,000 years ago in Africa, and less elsewhere, with an effective size on the order of 10,000 individuals. But that doesn't mean there weren't large populations competing for resources. If 10,000 people became our ancestors, we still are left with the archaeological evidence for occupation of much of the Old World. A plausible scenario is that many small groups of humans were in fact competing intensively, and many of them failed to persist over the long term. In other words, a small effective size is hardly evidence of no ancient competition or warfare. It may be the result of intense competition leading to many local extinctions.
But that doesn't detract from Hrdy's hypothesis, that selection in ancient humans caused extension of the mother-infant dynamic, with prosocial behavior resulting as a side effect. I'll be interested to read more details.