Researchers studying prehistoric human migration patterns in northern Chile's
Atacama Desert have concluded that a mysterious pattern of early occupation and
abandonment of the region was related to climate change.
Humans first occupied the high-altitude desert, which extends from the
Pacific Ocean to the base of the Andes, from around 13,000 to 9,500 years ago.
The region was then more or less abandoned until around 4,500 years ago, when
people began returning.
This occupation, abandonment, and recolonization pattern has long intrigued
archaeologists. One possible explanation was that the region may have had an
inhospitable environment. Or the pattern could simply reflect holes in the
archaeological record or some cultural bias at the time.
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.