I'm reading through the paper by Samuel Bowles, "Did warfare among ancestral hunter-gatherers affect the evolution of human social behaviors?" I've done some work (cited in the paper) on population extinction and recolonization, so I'm reading carefully and checking parameters as I go. Personally, I'm skeptical that Pleistocene warfare would have spurred altruism. More on that later.
Meanwhile, the first two paragraphs of the discussion stick out:
The mortality data summarized in Table 1 are consistent with what is known about the Late Pleistocene from more indirect data. Frequent lethal intergroup encounters may reconcile two otherwise anomalous facts about hunter-gatherer demographics. Human population grew extraordinarily slowly or not at all for the 100,000 years prior to 20,000 years before the present (35, 36), yet under peaceful conditions foraging populations are capable of growth rates exceeding 2% per annum (37, 38).
Further, the extraordinary volatility of climate during the Late Pleistocene (39) must have resulted in natural disasters and periodic resource scarcities, known strong predictors of intergroup conflict among hunter-gatherers in the historical record (40), and undoubtedly forced long-distance migrations and occasioned frequent encounters between groups having no established political relations.
That's called the "everything but the kitchen sink" argument. Warfare stopped the population from growing! Warfare inevitably followed resource scarcities!
Didn't any of the reviewers notice that, uh, resource scarcity limits population growth even in the absence of warfare? These two factors explain each other just fine. Warfare is mediated by resource scarcity and population growth, sure, but they don't constitute arguments in favor of widespread warfare in the Pleistocene, any more than they are arguments for widespread warfare in beavers and ducks. Every species in nature can grow fast when resources aren't limited, most a lot faster than us.
References:
Bowles S. 2009. Did warfare among ancestral hunter-gatherers affect the evolution of human social behaviors? Science 324:1293-1298. doi:10.1126/science.1168112
