Hi! I'm interested in the transition from hunter-gatherer societies to Neolithic cultures. We know that the first organized states of the classic early civilizations appeared in the river valleys of the Indus, Nile, Yellow, and between the Tigris and Euphrates rivers. We know that the overflow from these rivers created conditions for agriculture and irrigation that made for a stable food supply, and a surprise, which brought along differentiation of labor and eventually organized polities.

But why in those places? I was watching a lecture by Columbia Professor Richard Bulliet and he said that the emmer wheat, millet, barley, and rice grown in these valleys provides fewer calories per acre than wild-growing crops like bananas or potatoes. So why didn't civilization, which requires a permanent food source, start in the Americas or in more tropical climates where those crops are native? I'd be interested to hear the explanation! Thanks!