How Did Their Gardens Grow?

A Weather-Based Theory of Early Farming Sprouts Amidst the Prehistoric Beans and Squash

by Robert Sanders

The independent domestication of plants around 10,000 years ago in at least four widely separated areas--Mexico, Peru, China and the Near East--was an indirect response to climate change at the end of the last Ice Age, says Roger Byrne, associate professor of geography.

Byrne, a paleoecologist specializing in the analysis of fossil pollen, argues that increased seasonal variation of temperature and in particular rainfall at the end of the Ice Age led to the evolution of new potentially useful plants.

The shift toward a more seasonal climate began around 18,000 years ago, but peaked in intensity during a cool phase known as the Younger-Dryas.

The Younger-Dryas lasted roughly 1,000 years, from 10-11,000 years BP, or before the present, based on uncalibrated radiocarbon dating. Byrne argues that the climate change led to the proliferation of annuals--plants that grow, set seed and die in a single year--at the expense of herbaceous perennials, which survive throughout the year and do not produce many seeds. The shift in climate also favored tuber-producing plants that are able to mature quickly during short growing seasons, he says.

Humans took advantage of the abundant annuals--cereals such as wheat, corn and rice; legumes such as beans and peas; and tuberous plants such as the potato and yam--and eventually became dependent upon them, Byrne argues.

The new foods allowed people to live in one area throughout the year, and led to increased human population, Byrne says. Quite probably the population explosion led to over exploitation of available plant resources and necessitated planting and cultivating.

"Environmental change presented our ancestors with opportunities, and they took them," Byrne says. "If plants and animals had never been domesticated the human population would probably be no more than 10 to 20 million today. The development of agriculture was of tremendous importance for the human species because it made possible the growth of cities and civilized society in the broader sense."

Byrne set forth his arguments in a session on global climate change and agricultural origins Feb. 20 at the annual meeting of the American Association for the Advancement of Science in Atlanta.

Climate change has long been discounted as a cause for the origins of agriculture, Byrne says, with most archeologists and anthropologists preferring human causes--increasing population pressure, for example. Some scientists have claimed there was no significant climate change at the time agriculture arose, and that even if there were, why had similar climate changes in the past not led to plant domestication? Others doubted that agriculture in these four areas developed at the same time.

These objections have been shot down, Byrne says, as more evidence accumulates about ancient climate and better dates are obtained for the domestication of plants. Current evidence indicates that during the period 9000-7000 BP, wheat and barley were domesticated in the Near East, corn and squash in Mexico, rice and millet in China and beans and potatoes in the Andes.

Plants with underground tubers or bulbs are seldom preserved in archeological deposits, so dates for domestication are poorly known.

It is no coincidence that the wild relatives of these domesticated plants evolved in areas with strongly seasonal rainfall, Byrne says. These seasonal patterns persist today: the Near East has hot, dry summers and wet, cool winters, while China, Peru and Mexico have the opposite.

Seasonally wet and dry climate conditions favor annuals--plants that complete their life cycle in one growing season--but make it difficult for herbaceous perennials like bunch grasses. Annuals must produce abundant seed to survive, and in some places with a marked dry season plants also have evolved to produce larger seeds, which can sprout more quickly when rain does come. It is this edible and nutritious seed or grain that early humans learned to forage.

Tuber bearing plants probably also proliferated because tubers can survive harsh winters and dry summers underground, storing enough energy to sprout quickly during short growing seasons.

Such seasonal extremes of temperature and rainfall were not typical of most of the last glacial period, Byrne notes. At that time in many parts of the world summers tended to be cooler and winters warmer than today. Only when the ice began to melt toward the end of the last glacial period 18,000 years ago did the seasonally wet and dry climates begin to develop.

Accentuating this change was the Younger-Dryas Event. As the ice sheets covering large areas of the northern hemisphere melted, a large volume of fresh water rushed into the Atlantic and formed a brackish layer over the ocean that stretched from New England to northern Spain.

Because of differing densities the brackish water on top did not mix easily with the salt water below, and the ocean became stratified. This accentuated winter cooling and as a result the North Atlantic pack ice extended as far south as the coast of Spain, Byrne says.


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