All domestic plants today are descended from plants that were once wild. Some domestic plants are descended from poisonous plants or from plants that look much different from their domesticated cousins. Farmers managed to grow some plants much earlier than others, and even today a few prized food plants have never been successfully farmed. Diamond then questions, how did the domestication of plants happen before modern, scientific techniques were developed? Why was it possible for people to domesticate some plants in the Stone Age, whereas others are still undomesticated today?

Plants spread their seeds by any means necessary. Many are dispersed by animals either in feces or because animals carry them. When early humans gathered plants, they would have chosen the best of the available examples in their area—the largest, best tasting, most useful plants they could find. That means people were constantly dropping the best seeds near their homes, in their garbage dumps, and in their latrines. When people selected the plants they liked and unconsciously helped these seeds grow, the plants in their environments naturally evolved qualities that made them bigger, better, or easier to harvest.

In Southwest Asia, the earliest domesticated food plants were grains and legumes. These plants’ wild cousins were already edible, and they grew quickly and easily from seeds. They were easy to store, and they required only a few genetic mutations to work well as intentionally sown crops. After these first plants were domesticated, it took people 3,500 years to begin to grow the first fruit and nut trees. These trees took years to reach maturity, so only people with a settled lifestyle would have benefited from growing them. However, these plants—olives and grapes, for example—were quite easy to grow once they were planted, and their offspring grew the same way they did. Much later, people in this region began growing trees such as apples and cherries, which are much harder to cultivate.

The Fertile Crescent’s process of plant domestication was, in many ways, similar to processes that happened elsewhere. Around the world, common early domesticates included grains, legumes, and fibers used for weaving. In some places, the main carbohydrates were roots and tubers or melons and squashes instead of grains.

After these origins, agriculture developed in different ways depending on the other available plants and animals in the area. In Eurasia and Africa, animals were soon hitched to plows to aid planting. In the Americas, no draft animal was ever domesticated, so people had to do all the plowing themselves.

By Roman times, almost all of today’s familiar staple foods were being grown somewhere in the world. Some hunter-gatherer staples, however, were never domesticated. One major example of this is the oak tree, which provided rich, nutritious food to Native Americans in California, the eastern United States, and Europe. The oak tree is so difficult to domesticate that even today’s farmers have not managed it.