Late Pleistocene Demography and the Appearance of Modern Human Behavior

Modern behavior, including the development of advanced tools, musical instruments, and art, seems to have arisen in humans in stages. The earliest hints are seen in Africa about 70 to 90,000 years ago, but later in Europe about 45,000 years ago. An ongoing discussion centers on the origins and significance of human prosociality. During early human development, could the benefits of altruistic behavior have outweighed its costs (see the Perspective by Mace)? Bowles (p. 1293) constructed a model of conflict between groups of humans and extracted estimates of the critical parameters from archaeological and ethnographic data sets. Provocatively, it appears that warfare might have enhanced the emergence and persistence of altruistic behavior. Powell et al. (p. 1298) present a population model that shows that the development of modern behaviors may rely on the attainment of critical population densities and migratory patterns required for stable cultural transmission. The model is consistent with genetic inferences of population dynamics in Africa and Europe and suggests that these cultural changes may not solely reflect increased cognitive evolution.

The origins of modern human behavior are marked by increased symbolic and technological complexity in the archaeological record. In western Eurasia this transition, the Upper Paleolithic, occurred about 45,000 years ago, but many of its features appear transiently in southern Africa about 45,000 years earlier. We show that demography is a major determinant in the maintenance of cultural complexity and that variation in regional subpopulation density and/or migratory activity results in spatial structuring of cultural skill accumulation. Genetic estimates of regional population size over time show that densities in early Upper Paleolithic Europe were similar to those in sub-Saharan Africa when modern behavior first appeared. Demographic factors can thus explain geographic variation in the timing of the first appearance of modern behavior without invoking increased cognitive capacity.