Clarifying the time frame and units of selection in the cultural group selection hypothesis

We raise and discuss two issues for clarification. First, over what timescale is cultural group selection proposed to have been active? Unprecedented cooperation is inferred to have arisen in hunter-gather life well before the recent period on which Richerson et al. focus. Second, what is the unit of selection? Groups (of human individuals)? Or cultural entities along the lines of “meme-complexes”?