Ahhh, the noble savage.

Pretty sure this line of thinking started with Lorenz. The basic idea is that we were all so much happier when we were barely-sapient hunter-gatherers with a lifespan of 28. The downside is that we're competitive breeders and once we'd figured out how to get all the calories we needed for idle time, the stuff we filled our idle time with was stuff that increased our social standing. From that point civilization is inevitable.

The big news to emerge from recent archeological research concerns the time lag between “sedentism,” or living in settled communities, and the adoption of agriculture. Previous scholarship held that the invention of agriculture made sedentism possible. The evidence shows that this isn’t true: there’s an enormous gap—four thousand years—separating the “two key domestications,” of animals and cereals, from the first agrarian economies based on them. Our ancestors evidently took a good, hard look at the possibility of agriculture before deciding to adopt this new way of life.

It's equally possible that it took 5000 years to evolve the social structures necessary to irrigate.