Early Americans followed the water. (Ancient Lure of the Lakes).

After living for nearly 2 millennia in Chile's lowland jungles, South American settlers first braved the region's Atacama Desert around 13,000 years ago. Modern archaeologists would like to know why.

New evidence may explain this puzzling migration and also account for an extended abandonment of the 2-mile-high desert several thousand years later.

It boils down to climate changes, say Martin Grosjean of the University of Bern in Switzerland and his two Chilean colleagues. Hunters sought Atacama game only during rainy, humid times, when high-altitude lakes were plentiful, the researchers conclude. During droughts, most of those lakes evaporated, and prehistoric hunters headed for the low country.

In the Oct. 25 Science, the researchers describe ancient camp sites situated next to now-dry lakebeds in the Atacama Desert and nearby occupation sites at lower elevations. "They have verified a link between substantial climate changes and early settlement patterns in South America," remarks archaeologist Betty J. Meggers of the Smithsonian Institution in Washington, D.C.

The new findings bolster the theory that early New World settlers moved slowly through hospitable environments, adapted to local conditions, and avoided or fled harsh locales, says archaeologist Tom D. Dillehay of the University of Kentucky in Lexington. A competing theory posits that immigrants to the New World rapidly spread southward as they hunted big game species to extinction within about 2,000 years.

At sites in and around the Chilean desert, Grosjean's group first conducted soil and pollen analyses that identified a transition to a humid climate between 11,800 and 10,500 years ago. Those conditions would have supported extensive grasslands and lake formation within the desert.

The scientists then unearthed spear points, hearths, and bones of camel-like creatures and other animals at 39 Atacama Desert camps, located on the shores of 20 dry lakebeds. Similar artifacts were found in six intermediate-altitude caves just outside the desert and in several lower-altitude sites in what had been marshy wetlands.

According to radiocarbon dating of charcoal at these sites, people first lived in the caves from about 12,900 to 9,400 years ago. Low-altitude occupations cover roughly the same period. The first high-altitude Atacama Desert camps were established between 9,900 and 8,800 years ago.

Soil data show that at the end of that period, the lakes dried up. Evidence of human occupation in the Atacama Desert disappears at the same time. Human activity also ceased in the caves, except for signs of sporadic visits to those few caves that were located near springs or marshes.

Human occupation of lakeside camps and caves resumed about 4,500 years ago, along with the return of a humid climate and a rebound in lake levels.

Overall, the findings show that the shifting availability of lakes, springs, and streams greatly influenced the movements of the first people in the Atacama region, Dillehay says. When favorable climatic conditions occurred, settlers moved to intermediate altitudes, where they were positioned to launch seasonal hunting forays at high altitudes when lakes and vegetation appeared there.

The "archaeological silence" in the Atacama region between 9,000 and 4,500 years ago may have resulted from social or power conflicts as well as arid conditions, Dillehay adds.