How did a scruffy grass become maize?

Maize was domesticated by indigenous peoples in southern Mexico about 9,000 years ago, from a wild grass called teosinte. It is hard to believe at first. Maize has a single tall stalk with big ears, while teosinte is a short, bushy plant with branches. Yet the Russian botanist Nikolai Vavilov in 1931 and the American George Beadle in 1932 proposed that one gave rise to the other, and geneticists have since traced much of the difference between them to just two genes.

Teosinte was likely used by hunter-gatherers because it added security to their food supply: it was adaptable to changes in climate and environment. Its seeds sit in a very hard fruitcase, which protects them as they pass through grazing animals. Domesticated maize turned this around. The kernels of teosinte fall off the cob by themselves, but those of domesticated maize do not, so maize cannot spread without people.

The place and the number of times are now fairly clear. In 2004 John Doebley identified Balsas teosinte, native to the Balsas River valley in southwestern Mexico, as the wild relative genetically closest to maize, and he and colleagues showed that maize was domesticated only once. It then spread across the Americas. The earliest maize plants grew a single, small ear each, and the oldest ears found in caves are 6,250 and 5,450 years old.

Maize also taught people something about cooking. Indigenous Americans soaked it in alkali water, made with ashes and lime, a process called nixtamalization. That freed the vitamin niacin, and so protected people from the disease pellagra.