By around 1500, the farming that had begun in Southwest Asia and the Fertile Crescent ten thousand years earlier took over the world. The human population grew to a half billion, 100 times what it had been at the dawn of agriculture. Life, at least for city dwellers, was miserable: streams of human waste gushed down city streets. Air was tainted by coal smoke and nearby rivers and lakes ran with blood, fat, hair, and acids from manufacturing runoff. Infections, disease, and plague were a constant menace.
