Food Crops for Humanity

Human beings have been used about five thousand species of plants as food, but only 150 or so have entered world commerce and less than twenty provide most of the world's food. Just three crops--wheat, rice, and maize--account for roughly 60 percent of the calories and 56 percent of the protein that humans consume directly from plants.

Many of the most important food crops belong to just a few plant families. The grass family--including such crops as wheat, rice, maize, barley, sorghum, millet, oats, and rye--provides some 80 percent of calories consumed by humans, and the legume family has yielded soybeans, peanuts, common beans, peas, chickpeas, cowpeas, and other protein-rich crops.

Forty percent of an estimated 2,300 species of cultivated plants belong to just four families: Graminae (grasses), Leguminosae (legumes), Rosaceae (apples, pear, etc.), and Solanaceae (potatoes). The remaining species belong to a diverse array of more than 160 families.

Source Regions of Major Crops

Like economically valuable species in general, many of the major food crops originated in regions that are not particularly species rich. Crops were domesticated in warm temperate and subtropical zones and in tropical mountainous regions. Wheat and barley were first grown in the steppes and woodlands of southwestern Asia, and the origin of maize has been traced to the seasonally dry central highlands of Mexico. The highlands of Peru contributed the tomato and potato, though the tomato was probably first cultivated in Mexico.

Most important food crops appear to have originated where seasons are pronounced, so it makes sense to look there--and not in rain forests--for promising new crops. The cereals of both the Old and New Worlds come from regions with well-marked wet and dry seasons, while root and tuber agriculture, a mainstay of tropical regions, seems to have developed in tropical lowlands with distinct dry periods. As for why, scientists point in part to the tendency of plants in seasonal environments to store nutrients during the growing season; often, these reserves are what human beings eat.

Several regions--known as Vavilov Centers of Diversity after N.I. Vavilov, the Russian botanist who first described the pattern--have been identified as locations of highly diverse crop genetic resources. The centers of crop genetic diversity--including the Mediterranean, the Mexican highlands, Central China, and the Northern Andes--are characterized by a long agricultural history, ecological diversity, mountainous terrain, cultural diversity, and a lack of heavy forest cover. These centers may or may not be located where the crop was first domesticated; wheat and barley were domesticated in southwest Asia, but a current center of their varietal diversity is in Ethiopia; the tomato originated in northwest Peru, but the greatest domestic varietal diversity is in Mexico.

Introduced Crops

Much of the world's agriculture is based on introduced crops. In developing countries in the Americas, only 32 percent of production, by value, is of crops of American origin. The comparable figure for African developing countries is 30 percent. Only in Asian developing countries is most production--70 percent--by native species. Dependence on introduced species reaches its extreme in Australia, the Mediterranean, northern Europe, northern Asia, and the United States and Canada. In these regions, more than 90 percent of production is derived from introduced species. None of the world's twenty most important food crops are native to Australia or to North America north of Mexico.

Whether introduced or native, the most important crops in any region of the world originated or diversified in places with climates similar to those where they are now grown. The main crops grown in temperate zones are thus not the same as those of the tropics where most developing countries law. Rice, which has origins in either India or China, is the eighth most important crop in the developed world (by weight) but by far--a factor of two--the leading crop in the developing world, and is the most important source of calories in tropical developing countries.

Similarly, cassava (also known as manioc)--native to tropical America--is not grown in developed countries but is the fourth most important crop in the developing world (by weight) and provides more than half of the caloric requirements for over 420 million people in 26 tropical countries. In Africa, cassava is a fundamental subsistence crop, and in tropical developing countries it is the fourth most important dietary source of calories after rice, maize, and sugarcane.