The Evolution of Agriculture

Nomadic immediate return, hunting and gathering supports a population of about zero point one people per square kilometer. Once you have a few cultures living primarily by intensive forms of agriculture, like rice and grain cultivation, it's off to the races. It's no coincidence that the two most populated civili ations in the world, india and china, have their roots in rice farming. Individual farmers are more productive and successful if they have more kids to do more farm labor for them. And farmer women stop breast feeding their children at about age one because of the availability of milk from domesticated herd animals.