For the first time, humans lived in dense clusters, and occasionally interacted with other clusters, which allowed their fragile innovations to persist and propagate. The end result was a positive feedback loop of new ideas. While it's very nice to have some statistical evidence for this idea...it's worth pointing out that the density explanation isn't particularly new. In The Economy of Cities, Jane Jacobs forcefully argued against the "dogma of agricultural primacy," which assumed that farmers and agricultural innovations made civilization possible. Jacobs argued that the dogma was exactly backwards, and that it was the density of urbanesque clusters which generated the innovations that made farming possible. As Jacobs writes: "It was not agriculture then, for all its importance, that was the salient invention...Rather it was the fact of sustained, interdependent, creative city economies that made possible many new kinds of work." After all, you can't learn how to grow food until you've got a system for transmitting knowledge, which is why population density is so essential.