… is from page 350 of Matt Ridley’s superb 2010 book, The Rational Optimist  (emphasis added):
Somewhere in Africa more than 100,000 years ago, a phenomenon new to the planet was born. A Species began to add to its habits, generation by generation, without (much) changing its genes. What made this possible was exchange, the swapping of things and services between individuals. This gave the Species an external, collective intelligence far greater than anything it could hold it its admittedly capacious brain. Two individuals could each have two tools or two ideas while each knowing how to make only one. Ten individuals could know between them ten things, while each understanding one. In this way exchange encouraged specialisation, which further increased the number of different habits the Species could have, while shrinking the number of things that each individual knew how to make. Consumption could grow more diversified, while production grew more specialised.