As Steve Shennan and his colleagues have calculated, whenever the right combination of (say) seafood, freshwater and fertile savannahs produced local population explosions, technology would have grown sophisticated in proportion to the number of people networked by exchange to sustain and develop it – in proportion to the scale of the collective intelligence. But when a river dried up or deserts advanced and human populations collapsed or shrank, technology would simplify again. Human cultural progress is a collective enterprise and it needs a dense collective brain.
