Harris explains that for most of human history, people moved in small groups and followed food rather than staying in one place. In a few places, including the Nile valley, humans began gathering in large numbers and cooperating. Each year, rains in the East African Highlands sent huge amounts of water rushing down the river, creating a flood that swelled the river through the desert.
The flood carried volcanic rock picked up from the Ethiopian mountains. Harris describes this sediment as rich in phosphorus, potassium, iron, magnesium and calcium, which acted as free fertilizer delivered every year. In the desert, where the sun was constant, farmers only needed to drop a seed into the soil for crops to grow. While most farmers elsewhere depended on rainfall that sometimes failed, Egyptians had a yearly flood, which they worshipped as the god Hapi.
Harris adds that surplus food freed some people to do other work, such as building monuments, and that the Nile supported not one major civilization but two, with Nubia upstream built on the same flood.