By matching climate and archaeological records, the researchers concluded that environmental conditions—specifically, bouts of extreme drought—explain the unusual pattern.

An Archaeological Puzzle

The team of two archaeologists and a geoscientist reached their conclusion based on their survey of archaeological sites at three elevations: the shorelines of paleo-lakes above 3,600 meters (12,000 feet); caves at elevations between 3,000 to 3,600 meters (12,000 to 9,800 feet); and low-altitude paleo-wetlands at 2,400 meters (7,900 feet).

To fill in the archaeological record—there was only one high-altitude campsite known to have been occupied between 13,000 to 9,500 years ago—the scientists adopted a somewhat novel interdisciplinary approach: They recreated the landscape that existed around 15,000 years ago.

"We first tried to reconstruct the paleo-environmental conditions in great detail," said Martin Grosjean, a geoscientist at Switzerland's climate research center at the University of Bern.

After identifying lake levels, shorelines, springs, and other geological formations, the researchers drew a map of ancient paleo-shorelines. From this they selected the most promising places where people are likely to have lived, then dug thousands of test pits.

"The first people arrived in small groups of extremely mobile hunters. They did not build houses, had no architecture of any kind," said Grosjean.

Taking the geo-scientific approach enabled the team to find 38 previously unknown high-altitude campsites that were occupied between 13,000 and 9,500 years ago.

"The sites are really small, absolutely unspectacular," he said. "The most we might find would be some obsidian flakes, maybe some small fireplaces, usually buried 2 to 5 centimeters [0.8 to 2 inches] below the surface. This is the only way you could find archaeological sites in such a large area."

They also found evidence of human occupation in mid-altitude caves and low-altitude wetlands from 13,000 to 9,500 years ago, and then very few people until 4,500 years ago.

"Our study shows that this [migration pattern] is real—it is not due to an incomplete archaeological survey—and that it is due to hostile environmental conditions that simply were not favorable to human occupation," said Grosjean.

Atacama, Then and Now

The Atacama Desert is known as the driest place on Earth. Part of it, known as the Salar de Atacama, is a salt flat that so resembles the landscape of Mars that NASA uses it to test its other-planet dune buggies.

Today, nothing grows there below 2,500 meters (8,200 feet). "It's a matter of precipitation, not temperature," said Jason Rech, a geologist at Miami University in Ohio who has studied the paleo-environments of the Atacama.

As the desert gains altitude, it becomes slightly less arid. "Plants grow at altitudes between 2,500 to 4,300 meters (8,200 to 14,000 feet); higher than that it's just too cold," said Rech.

Starting around 15,000 years ago, the climate became much wetter; by 13,500 years ago environmental conditions in the Atacama were hospitable enough that people moved in—almost immediately.

"As soon as there were favorable environmental conditions people moved immediately and occupied this area," said Tom Dillehay, an archaeologist at the University of Kentucky who has worked extensively on archaeological sites in South America.

A major wet pulse occurred between about 12,000 and 10,000 years ago.

"Instead of 200 millimeters [8 inches] of rain annually at high elevations, you'd get around 400 millimeters [15 inches]," said Rech. "The lower boundary of vegetation expanded as much as 1,000 meters [3,300 feet] downslope. Regions near the Salar that today are barren would have been grasslands between 13,000 to 9,500 years ago."

These broad grasslands supported the camelid populations—vicuñas and guanacos—that the Paleoindians hunted for roughly 3,500 years.

A drastic drop in precipitation starting around 9,500 years ago triggered what archaeologists call the Silencio Arqueològico, a cultural hiatus in the archaeological record.

"When vegetation and animal life were abundant, mobile hunters moved into higher elevations," said Grosjean. "They followed game seasonally, establishing camps along the shorelines of higher paleolakes in the summer, and moving to the lower wetlands in the winter. When the lakes started drying up, they left the area until conditions got better—around 4,500 years later."

More humid areas in the rain forests of central and southern Chile and the central highlands of Peru did not experience a Silencio.

Peopling of the Americas

A second question puzzling archaeologists was why the initial occupation of the Atacama region occurred 1,500 to 2,000 years later than in regions farther south.

The "blitzkrieg" hypothesis for the peopling of the Americas proposed in the early 1970s suggested that the first people to arrive in the Americas were big game hunters who moved swiftly, following their prey, to populate the two continents.

"If you assume that the Americas were colonized as the result of people crossing the Bering Strait land bridge and then moving down through North America, Central America, and into South America, you would expect at first glance to see people in northern Chile before southern Chile," said Grosjean.

Taking into account the different paleo-climate histories of different areas would lead to a different hypothesis, however.

"Then you would expect the first people to occupy the most favorable habitats at the time, namely, southern South America—Patagonia and south-central Chile," he said.

The findings of the Atacama study, published in the October 25 issue of the journal Science, lend support to an alternate hypothesis suggesting that occupation occurred at a slower pace.

"In the last 15 years or so, archaeologists have been questioning the rapid-movement model," said Dillehay. "We've come to realize that it takes time to adapt to new environments, and that as people moved into more lush environments, they became more adapted to a broader spectrum of edible plants and smaller animals. The motivation to move on in pursuit of big game just isn't there."

Paleo-archaeologists have the task of figuring out where, when, and how prehistoric peoples lived, "frequently based on very little or ephemeral information—a little like connecting the dots," he said.

The study's findings argue for more caution in extrapolating possible explanations from one early site to another, and an increased focus on interdisciplinary collaborations, said Dillehay, who wrote a commentary piece on the study in Science.

"There are a range of possibilities—cultural, ideological, technological, and ecological—that need to be considered when interpreting archaeological finds," he said.

The researchers urged the use of interdisciplinary approaches in future studies. Archaeologists and geoscientists have always worked together, but typically the geoscientists are called in to render expert opinions on issues like soil or pollen analysis, said Dillehay.

"It took a long-term research approach and commitment to come up with these findings," he continued. "The value of paleo-ecology studies is that they tell us much more about the kinds of landscapes that existed when early peoples were trekking across them and how to go about finding the places where these people might have lived."

"Finding 38 sites with very weak human signatures is remarkable," he added. "In a more lush environment where more people lived, we'd probably find 138 sites that would take an eagle's eye to spot."

The study of migration patterns of Paleoindians in the Atacama Desert was partly funded by the National Geographic Society.

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