Although there is circumstantial evidence that the inhabitants of southern Africa and the Nile valley sought to manipulate the growth of plants as early as eighteen thousand years ago, the systematic domestication of plants, or agriculture, evolved independently in the Middle East about 8000 bce, and by 4000 bce, there were substantial agricultural settlements along the banks of the Nile and in its delta. There is still controversy as to whether the domestic cultivation of crops in the Nile valley was borrowed from the nearby Fertile Crescent or evolved locally in the rich soil deposited on the banks of the river; it was probably a bit of both. Whatever its origins, this agricultural revolution made possible the evolution of settled communities. Hunting and gathering was an inefficient means of survival, let alone procreation. Hunting was dangerous, limited to small animals or, for the hominids, scavenging the remains left by more powerful carnivores. Gathering, although more mundane, could be an unreliable and often arduous enterprise. There are two hundred thousand species of wild plants, but most are indigestible, poisonous, without nutrition, hard to gather, or difficult to prepare. Only a very few, no more than a dozen, were edible and available for Homo sapiens. One acre of any one of them cultivated by farmers could support a hundred times the number of hunter-gatherers, who have consequently been overwhelmed over the last two millennia by the slow but steadily increasing numbers of Africans practicing agriculture.