Study reveals shifts in ancient population tied to changes in moisture, temperature

Jan 10, 2013 From staff reports

During the past 13,000 years, Wyoming's Big Horn Basin has experienced significant increases in population growth -- due primarily to periods of high effective moisture and moderate temperatures -- according to three University of Wyoming professors.

By contrast, when water levels were lower and temperatures higher, the state's populations of hunter-gatherers decreased or may have even disappeared altogether, they concur.

The research, presented in a paper titled "A Continuous Climatic Impact on Holocene Human Population in the Rocky Mountains," was published Dec. 24 in the Proceedings of the National Academy of Sciences (). The organization advises the president and Congress on scientific and technological issues that frequently affect policy decisions at the national level.

Robert Kelly, lead author of the paper; Bryan Shuman, a UW associate professor of geology; and Todd Surovell, a UW associate professor of anthropology; collaborated on the study by combining their individual research in the Big Horn Basin. Geoff Smith, a former UW graduate student and now a professor at the University of Nevada, also was a co-author.

Kelly and Surovell conducted research of the region's archaeology, studying existing records of radiocarbon data that offered clues as to how old humans were at various times in the Big horn Basin's history. Shuman studied moisture and temperature levels at different points in time in the basin.

The UW faculty members compared population, moisture and temperature records (primarily using existing data) to evaluate potential linkages between changes in climate and past human populations. Low effective moisture and high temperatures that created an arid environment are both associated with lower population levels, while cooler temperatures and wetter periods led to population growth.

Many archaeological examples emphasize the impact of severe events, but have not resolved the importance of continuous climate change in shaping cultural history, according to the UW researchers.

Humans entered the Big Horn Basin around 14,000 years ago. The UW research group identified five periods of population growth -- 9,100 years ago, 4,500 years ago, 3,800 years ago, 2,600 years ago and 1,900 years ago. When calculating the years, the researchers started with the year 1950 (the year radiocarbon dating started) as "before present" and worked backward, Surovell says.

While the study produced no exact population numbers during the various periods, Surovell says the group could determine population increases or decreases in relative terms using radiocarbon dating.

The dating method uses naturally occurring carbon-14 to estimate the age of carbon-bearing materials as far back as 50,000 years.

"We're just now able to reconstruct human population size in pre-history in a way that is reliable and accurate," Surovell says.

"For the first time, we are able to see the population of Wyoming or at least a section of Wyoming. Perhaps, it's not surprising that there are more people living in the state when it's cool or wet than when it's hot and dry."

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