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