"Late Pleistocene Demography and the Appearance of Modern Human Behavior"
by Adam Powell, Stephen Shennan, Mark G. Thomas
Science 324.5932 (5 June 2009): 1298-1301.
Abstract: 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.
Greater population density → increased "cultural [symbolic and technological] complexity" or "development." Social networking, i.e. the transmission of ideas and skills, fostered the emergence of more and more complex innovations.
It has often been thought that boosts in brain power or advances in language in homo sapiens led to modern human behaviour. But it seemed to take a long time for these resources to pay off.
That is, "you can have individuals who are really great at inventing ideas and concepts and ways of approaching the world, but you need a certain population density to be able to have that stuff catch hold and spread," as Richard Potts of the Smithsonian told LiveScience.
Is this a clue that reciprocity, i.e. sociability, might be the precondition of human cognition, that humans are smart(er) because they collaborate (better than other animals) [= embedded cognition]? (It's not because we are smart and decide to work together.)
For a critique, see John Hawks on "Learning, Population Size, and 'Modern Human Behaviour.'"