A University of Wyoming professor says, had European contact happened 200 years earlier, they would have been met by a much more difficult response from American Indian populations that may have changed the trajectory of America.

“Europeans unknowingly arrived at a time when Indigenous populations had already declined,” said Robert Kelly, professor emeritus of archaeology. “Had they come a few hundred years earlier, they would have faced much larger, well-organized societies capable of stronger resistance — potentially altering the course of North American history.”

For a long time, researchers have sought to estimate the size of North America’s Indigenous population before European colonization to fully understand its impact.

Kelly's study reveals significant fluctuations in the Indigenous population of North America before European contact, using watershed radiocarbon data from the past 2,000 years. By analyzing records from 18 major watersheds across the continent, the study found that human population peaks varied regionally, occurring as early as 800-770 A.D. in some areas and persisting until after European contact in others. However, interior regions of the continent saw declines in populations between 1080 and 1300 A.D., while populations in the Great Lakes, New England, the Mid-Atlantic, the Central Plains, the Northwest and California remained stable until after European arrival.

Pre-contact declines were most likely due to climate, especially drought, disease in large settlements, emigration and warfare. Sharp declines persisted after the arrival of European contact and in many cases, population declines were linked to migration.

In northwest Wyoming the population dropped sharply after 1150 A.D., with some people migrating north and west. In the Mesa Verde region, maize-dependent communities moved south to the upper Rio Grande Valley, while the Ute, a smaller foraging population, resettled the area.

However, migration alone does not fully explain the overall population loss, the researchers say. While European contact had a devastating impact, Indigenous populations did not simply peak in 1500 A.D. and then decline.

Previous studies by other researchers have used radiocarbon data from the Canadian Archaeological Radiocarbon Database (CARD) to identify a pre-contact population decline in North America around 1150 A.D. However, their study relied on a smaller, less accurate database that was biased toward northern states.

In this study, Kelly and his team used a larger, updated CARD to analyze Indigenous population trends in the continental U.S. over the past 2,000 years. Their findings confirm a widespread population decline, but with regional differences. Additionally, the study rules out radiocarbon sampling bias as the cause of these trends, providing strong evidence for major demographic shifts before and after European contact.

Warfare can cause major population declines, but a pre-Columbian decline in Indigenous numbers does not mean they were a "dying race," as colonialist ideology once assumed, the researchers say. Instead, like all human societies, Indigenous populations experienced periods of growth, decline, migration and resettlement.

Also involved in the study were Wyoming State Archaeologist Spencer Pelton, Madeline Mackie, of Michigan State University and Erick Robinson, of Desert Research Institute in Reno, Nevada.

Kelly has worked on the archaeology, ethnology and ethnography of foraging peoples since 1973, working on archaeological research projects in Nevada, California, New Mexico, Kentucky, Georgia, Chile and Wyoming.

The full study, “Spatiotemporal distribution of the North American Indigenous population prior to European contact,” was published Monday in the Proceedings of the National Academy of Sciences.