AN OLD THEORY AS TO THE CAUSE OF THE REVOLUTION
The idea that change would result, if the balance between nature and culture became upset, is of course not a new one. For at least twenty-five years, there has been a general theory as to how the food-producing revolution happened. This theory depends directly on the idea of natural change in the environment.
The five thousand years following about 10,000 B.C. must have been very difficult ones, the theory begins. These were the years when the most marked melting of the last glaciers was going on. While the glaciers were in place, the climate to the south of them must have been different from the climate in those areas today. You have no doubt read that people once lived in regions now covered by the Sahara Desert. This is true; just when is not entirely clear. The theory is that during the time of the glaciers, there was a broad belt of rain winds south of the glaciers. These rain winds would have kept north Africa, the Nile Valley, and the Middle East green and fertile. But when the glaciers melted back to the north, the belt of rain winds is supposed to have moved north too. Then the people living south and east of the Mediterranean would have found that their water supply was drying up, that the animals they hunted were dying or moving away, and that the plant foods they collected were dried up and scarce.
According to the theory, all this would have been true except in the valleys of rivers and in oases in the growing deserts. Here, in the only places where water was left, the men and animals and plants would have clustered. They would have been forced to live close to one another, in order to live at all. Presently the men would have seen that some animals were more useful or made better food than others, and so they would have begun to protect these animals from their natural enemies. The men would also have been forced to try new plant foods—foods which possibly had to be prepared before they could be eaten. Thus, with trials and errors, but by being forced to live close to plants and animals, men would have learned to domesticate them.