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Andrew Somerville made
an unexpected discovery while studying the origins of agriculture.
Photo by Christopher Gannon
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AMES, Iowa An unexpected discovery by an Iowa State University
researcher suggests that the first humans may have arrived in North
America more than 30,000 years ago nearly 20,000 years earlier
than originally thought.
Andrew
Somerville, an assistant professor of anthropology in world
languages and cultures, says he and his colleagues made the
discovery while studying the origins of agriculture in the Tehuacan
Valley in Mexico. As part of that work, they wanted to establish
a date for the earliest human occupation of the Coxcatlan Cave in
the valley, so they obtained radiocarbon dates for several rabbit
and deer bones that were collected from the cave in the 1960s as
part of the Tehuacan Archaeological-Botanical Project. The dates
for the bones suddenly took Somerville and his colleagues in a different
direction with their work.
The date ranges for the bone samples from the base of the cave
ranged from 33,448 to 28,279 years old. The results are published
in the academic journal Latin
American Antiquity. Somerville says even though previous studies
had not dated items from the bottom of the cave, he was not expecting
such old ages. The findings add to the debate over a long-standing
theory that the first humans crossed the Bering Land Bridge into
the Americas 13,000 years ago.
We werent trying to weigh in on this debate or even
find really old samples. We were just trying to situate our agricultural
study with a firmer timeline, Somerville said. We were
surprised to find these really old dates at the bottom of the cave,
and it means that we need to take a closer look at the artifacts
recovered from those levels.
Somerville says the findings provide researchers with a better
understanding of the chronology of the region. Previous studies
relied on charcoal and plant samples, but he says the bones were
a better material for dating. However, questions still remain. Most
importantly, is there a human link to the bottom layer of the cave
where the bones were found?
To answer that question, Somerville and Matthew Hill, ISU associate
professor of anthropology, plan to take a closer look at the bone
samples for evidence of cut marks that indicate the bones were butchered
by a stone tool or human, or thermal alternations that suggest the
bones were boiled or roasted over fire. He says the possible stone
tools from the early levels of the cave may also yield clues.
Determining whether the stone artifacts were products of
human manufacture or if they were just naturally chipped stones
would be one way to get to the bottom of this, Somerville
said. If we can find strong evidence that humans did in fact
make and use these tools, thats another way we can move forward.
Year-long journey to even find the bones
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One of the rabbit bones
dated for the study. Photo courtesy of Andrew Somerville
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Not only was this discovery unexpected, but the process of tracking
down the animal bones to take samples was more than Somerville anticipated.
The collection of artifacts from the 1960s Tehuacan Archaeological-Botanical
Project was distributed to different museums and labs in Mexico
and the United States, and it was unclear where the animal bones
were sent.
After a year of emails and cold calls, Somerville and his collaborator,
Isabel Casar from the National Autonomous University of Mexico,
had a potential lead for a lab in Mexico City. The lab director,
Joaquin Arroyo-Cabrales, agreed to give Somerville and Casar a tour
to help search for the missing collection. The tour proved to be
beneficial. Among the countless boxes of artifacts, they found what
they were looking for.
Having spent months trying to locate the bones, we were excited
to find them tucked away on the bottom shelf in a dark corner of
the lab, Somerville said. At the time, we felt that
was a great discovery, we had no idea it would lead to this.
Once he located the bones, Somerville got permission from the Mexican
government to take small samples about 3/4 inch in length
and 1/4 inch in width from 17 bones (eight rabbits and nine
deer) for radiocarbon dating. If closer examination of the bones
provides evidence of a human link, Somerville says it will change
what we know about the timing and how the first people came to America.
Pushing the arrival of humans in North America back to over
30,000 years ago would mean that humans were already in North America
prior to the period of the Last Glacial Maximum, when the Ice Age
was at its absolute worst, Somerville said. Large parts
of North America would have been inhospitable to human populations.
The glaciers would have completely blocked any passage over land
coming from Alaska and Canada, which means people probably would
have had to come to the Americas by boats down the Pacific coast.
Isabel Casar, a professor at the National Autonomous University
of Mexico, and Joaquín Arroyo-Cabrales, a researcher with
the National Institute of Anthropology and History in Mexico, contributed
to this research. The work was funded by the National Science Foundation
and the Wenner-Gren Foundation.
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