For most of the time our species has existed, nobody planted anything. Then, starting roughly 11,500 years ago, people in several parts of the world began to tend plants and animals on purpose, and over many generations a new way of life spread across the planet. Archaeologists call this shift the Neolithic Revolution, or simply the agricultural revolution. Almost everything that came after it, from cities and writing to taxes and armies, rests on it.

What domestication means

Domestication is a long process in which people select plants or animals until they come to depend on human care. It was not a single invention. Wild wheat scatters its seeds as soon as they ripen, which helps the plant but frustrates a harvester. A few mutant plants held their seeds on the stalk. People who cut stalks with sickles carried those seeds home and planted some of them, and without meaning to, they bred a wheat that waits to be gathered. Over centuries domestic crops grew bigger seeds, ripened together and lost their natural defenses. Many could no longer survive without farmers, and farmers could no longer survive without them.

Animals went through similar changes. Sheep, goats, cattle and pigs were domesticated in southwest Asia, llamas and alpacas in the Andes, chickens in Southeast Asia. Herders kept the calmer animals and ate or culled the aggressive ones, and over time domestic animals became tamer, smaller-brained and more varied in coat color than their wild relatives.

Many beginnings

Farming did not start in one place and spread from there. Agriculture arose independently in several regions, among them the Fertile Crescent of southwest Asia (wheat, barley, lentils), China (rice along the Yangzi, millet along the Yellow River), Mesoamerica (maize, beans, squash), the Andes (potatoes, quinoa), the New Guinea highlands (taro, bananas) and parts of Africa (sorghum, yams). Scholars still argue about the exact count, but the pattern is clear: people on different continents, out of contact with one another, reached the same solution within a few thousand years.

Why then, after so long? One leading explanation points to climate. The end of the last Ice Age brought a warmer and, above all, more stable climate, and planting a crop only makes sense if next year will look roughly like this year. Others stress rising population, which pushed foragers to squeeze more food from less land, or social motives such as feasting, in which leaders gained prestige by hosting large meals.

Was it progress?

The traditional story treats farming as a triumph. Skeletons tell a more mixed tale. In many regions early farmers were shorter and less healthy than the foragers before them, with more tooth decay, more signs of childhood malnutrition and more infectious disease. A diet built on one or two grains was less varied than a forager's. Crowded villages, living beside animals and their waste, became breeding grounds for germs. Work hours probably rose.

In a 1987 essay the geographer Jared Diamond went so far as to call agriculture the worst mistake in the history of the human race, arguing that it brought inequality, disease and despotism. Critics answer that the picture varies by region, that farming allowed vastly more people to live, and that every later achievement, including modern medicine, depended on the food surpluses it created.

Surplus, storage and power

The deepest change was social. Grain can be stored, counted and taxed. A storable surplus allowed some people to stop producing food: priests, soldiers, craftsmen, scribes and kings. The earliest writing in Mesopotamia was largely bookkeeping, records of grain, beer and livestock. Surplus also gave people something to steal and to guard, and the gap between those who controlled the granary and those who filled it grew wide. Jean-Jacques Rousseau saw this in 1755, writing that "the first man who, having enclosed a piece of ground, bethought himself of saying This is mine, and found people simple enough to believe him, was the real founder of civil society." Whether or not one shares his gloom, the link between farming, property and political power is hard to deny.

Worked example: maize from a weedy grass

Maize, the most produced grain on Earth today by weight, looks nothing like its ancestor. That ancestor is teosinte, a wild grass of southern Mexico whose ear holds only about a dozen hard kernels, each locked in a stony case. A hungry person would get little from it.

Genetic studies point to the Balsas River valley in southwest Mexico, around 9,000 years ago, as the place where people began selecting teosinte. A handful of genes made the biggest difference. One controls the hard case around each kernel; another controls how the plant branches. Farmers who saved seed from plants with softer kernels and fewer branches pushed those genes in one direction, generation after generation. The result, thousands of years later, was a cob with hundreds of exposed kernels that cannot spread its own seed at all. Drop a whole ear on the ground, and the sprouting kernels crowd together and choke one another. Maize needs people as much as people came to need maize.

The story shows the main ideas of this level at once: domestication was slow and partly unintended, it happened in the Americas without any help from the Old World, and it made a plant and a people dependent on each other.