Could the Anasazi have stayed? (Archaeology).

A new computer simulation of the changing environmental conditions around one of the Anasazi cultural centers early in the past millennium suggest that drought was not the only factor behind a sudden collapse of the civilization.

Northeastern Arizona's Long House Valley was home to the Kayenta Anasazi, one of several branches of a complex culture that disappeared from the American Southwest in the 12th century. Anasazi were subsistence farmers who grew maize, beans, and squash. The rich archaeological record in the 96-square-kilometer valley, along with a wealth of data about environmental conditions there, enables scientists to construct detailed computer models of population growth in the valley, says George J. Gumerman, director of the Arizona State Museum in Tucson.

The simulation uses data about climate from tree rings, pollen in geologic sediments, and soil erosion in Long House Valley to estimate crop yields for each hectare--an area about 2.5 times the size of a football field--from A.D. 400 to A.D. 1400. Because each person would have consumed about 160 kilograms of maize each year, Gumerman could use that information to estimate the population that could be supported by the available food supply.

Long House Valley was abandoned around A.D. 1300, about the time that the region suffered a severe drought and a spate of erosion. However, Gumerman says, simulations suggest that the northern parts of the valley could have supported between one-quarter and one-third of the predrought population if families had dispersed into small settlements.

The fact that the Anasazi left the valley rather than stay in reduced numbers suggests that factors other than drought, such as disease, may have been at work. Because Anasazi culture was so complex--for instance, they had separate governments ruling in the summer and winter--Gumerman suggests that the parched valley couldn't support a critical mass of people to maintain the society.--S.P.