In each case, a small population produced a star-burst of pioneers who permanently changed our way of thinking. Genius erupted in groups as well as in individuals. It seems likely that these bursts of creative change were driven by a combination of cultural with biological evolution. Cultural evolution was constantly spreading ideas and skills from one community to another, stirring up conservative societies with imported novelties. At the same time, biological evolution acting on small genetically isolated populations was causing genetic drift, so that the average intellectual endowment of isolated communities was rising and falling by random chance. Over the last few thousand years, genetic drift caused occasional star-bursts to occur, when small populations rose to outstandingly high levels of average ability. The combination of imported new ideas with peaks of genetic drift would enable local communities to change the world.
!- explaining human history : combination of cultural and biological evolution