By Guy Gugliotta
 

The hunters arrived on the high plains of northern Chile about 13,000 years ago, settling in caves, rock shelters and encampments along the shores of brackish lakes
fed by the melting glaciers of the high Andes.

Despite heart-thumping altitudes -- frequently approaching 14,000 feet above sea level -- the living was relatively easy: "There were grass, shrubs, humidity," said
Swiss climate scientist Martin Grosjean, of Bern University. "There were wild animals" such as vicuñas and guanacos -- cousins of the domestic llama -- or the
ostrichlike, flightless bird called the ñandú.

Things are different now. Today northern Chile's high plains are part of the Atacama Desert, a hostile, arid wilderness of cloudless horizons, brown dirt hills,
dwindling salt lakes and naked drifts of Andean scree.

It was the high desert's very austerity that attracted Grosjean to the Atacama in 1990, and 12 years later what started as an exploration of paleoecology in the arid
tropics has become a model for how climate science can help archaeologists find evidence of human habitation in environments so hostile that many researchers
would not even know how to look.

Last week in the journal Science, Grosjean and Chilean archaeologist Lautaro Nuñez reported the discovery of 39 lakeshore campsites in the Atacama dating from
about 13,000 years ago to 9,000 years ago.

"We found no human remains, but there were plenty of [stone] points and arrowheads and a large number of other functional tools and knives," Grosjean said. The
sites were radiocarbon-dated from the charcoal found in subsurface fire pits.

The discoveries and the climatic research that accompanied them have added a new perspective on the migrations of the earliest Americans and shed light on the
Silencio Arqueológico -- the period from about 9,500 years ago to 4,500 years ago, when humans disappeared from the region.

The arrival time of humans in the Western Hemisphere has been a matter of bitter dispute between archaeologists who espouse the traditional view that the famous
13,000-year-old site in Clovis, N.M., represents the first in-migration from Asia, and those who have found evidence of earlier human habitation.

One of the lightning rods in this debate is the lowland site of Monte Verde, in southern Chile, which a team of archaeologists finally agreed was at least 14,000 years
old or several hundred years older, as lead excavator Tom D. Dillehay has contended.

Dillehay, of the University of Kentucky, commented on the Atacama research in an accompanying Science paper, noting that the Grosjean team had found a direct
correlation between climate and human habitation in the Atacama.

The climate had been hostile before 13,000 years ago, and became hostile again during the Silencio, he noted. Around 4,500 years ago, humans returned to a more
hospitable Atacama before virtually abandoning it forever.

Human habitation of the Atacama "could not have been a blitzkrieg movement," he wrote, "but a stutter-step." What occurs, Dillehay said in an interview, "is that
people hover, then move in and move out depending on the conditions."

Grosjean said the Atacama project began as an effort to document climate change at the end of the tropical ice age around 12,000 years ago. The Atacama, spread
across a vast region of northern Chile and parts of southern Bolivia and northwestern Argentina, was a natural choice.

"If you look at humidity changes, you better look in dry places," Grosjean said. "All the fingerprints of past humidity changes are very apparent in the landscape. It's
stable under current dry conditions."

Finding undisturbed terrain is a critical and difficult problem for archaeologists studying early human habitation in the Americas, for "if a site goes back to the last
glacial maximum, the soils have been scoured out or blown away," said Joseph M. McAvoy, the archaeologist who oversees the excavation of Cactus Hill in
southeastern Virginia, another controversial pre-Clovis site.

Grosjean teamed up with Nuñez, and together they were able to pinpoint caves, rock shelters and campgrounds where ancient Americans came to hunt and live.
Most of the sites were high above the natural tree line, but the region was reasonably water-rich because of seasonal monsoons, and there were fish in the lakes.
When there was no rain, the hunters moved downhill.

All this Grosjean could read by examining the outlines of the old lake shores. Prospecting along these ancient high water marks, the team looked for flakes of
obsidian -- black, amorphous volcanic glass used extensively by prehistoric humans for tools ranging from spear points to needles.

"Imagine an area the size of Switzerland, and you have no clue where to find early Holocene and late glacial sites," Grosjean said. "They are just below the surface.
By reconstructing lake levels, we were able to make a detailed subsurface survey."

Then it was a matter of hard work. Once obsidian was found, the team dug tiny test pits, "hundreds of them," Grosjean said. And in several, there was charcoal --
carbon deposits that yielded much more rigorous dates than any artifacts or animal remains could provide.

By testing sites at different elevations, the team was able to document the movement of people in and out of the region -- higher elevations in the warm season, lower
elevations in the colder season, abandonment of the highest sites when drought came. During the best times around 11,000 years ago, shorelines were more than
200 feet above today's salt lakes, Grosjean said.

The artifacts at the sites were instructive for what they included and did not include. Points and tools of obsidian and local stone were interspersed with a variety of
animal bones, which besides the llama-like species, also included rodents, deer and an ice age horse. Once the lakeside sites were abandoned, however, they were
never reoccupied, for there is no evidence of ceramics or tools from a later time.

It is impossible to know the origin of the Atacama colonists, whether it be from central Chile, the eastern slope of the Andes or from the northern reaches of the
Andes themselves.

"Often if there is absence of a human record, we interpret it as an absence of humans," Dillehay said. "But maybe they were living somewhere else, and we just don't
know about it. My sense is they were pretty much everywhere early."