According to a new study conducted by Washington State University anthropologists Dr Tim Kohler and Dr Kelsey Reese, pre-Columbian Native Americans experienced very high birth rates between 500 and 1300 CE, when they possibly exceeded the highest in the world today.
Reconstruction of life on a Hohokam platform mound in the Sonoran Desert in 1300 CE. Image credit: Pueblo Grande Museum, City of Phoenix.
The study, reported in the Proceedings of the National Academy of Sciences, looked at a century’s worth of data on thousands of human remains found at hundreds of sites across the Four Corners region of the Southwest.
“It was a time when the early features of civilization had matured to a level where birth rates likely exceeded the highest in the world today. Then a crash followed, offering a warning sign to the modern world about the dangers of overpopulation. We can learn lessons from these people,” Dr Kohler said.
While many of the remains studied have been repatriated, the data let the scientists assemble a detailed chronology of the region’s Neolithic Demographic Transition, in which stone tools reflect an agricultural transition from cutting meat to pounding grain.
“It’s the first step toward all the trappings of civilization that we currently see. Maize, which we know as corn, was grown in the region as early as 2000 BC,” Dr Kohler said.
“At first, populations were slow to respond, probably because of low productivity. But by 400 BC, the crop provided 80 percent of the region’s calories,” Dr Kohler said.
“Crude birth rates – the number of newborns per 1,000 people per year – were by then on the rise, mounting steadily until about 500 CE. The growth varied across the region.”
People in the Sonoran Desert and Tonto Basin, in what is today Arizona, were more culturally advanced, with irrigation, ball courts, and eventually elevated platform mounds and compounds housing elite families.
Yet birth rates were higher among people to the north and east, in the San Juan basin and northern San Juan regions of northwest New Mexico and southwest Colorado.
The Sonoran and Tonto people would have difficulty finding new farming opportunities for many children, since corn farming required irrigation. Water from canals may have also carried harmful protozoa, bacteria and viruses. But groups to the northeast would have been able to expand maize production into new areas as their populations grew.
Populations remained high but birth rates began to fluctuate around 900 CE. The mid-1100s saw one of the largest known droughts in the Southwest. The region likely hit its carrying capacity.
From the mid-1000s to 1280, by which time all the farmers had left, conflicts raged across the northern Southwest but birth rates remained high.
“They didn’t slow down. Birth rates were expanding right up to the depopulation. Why not limit growth? Maybe groups needed to be big to protect their villages and fields. It was a trap, however,” Dr Kohler said.
The northern Southwest had as many as 40,000 people in the mid-1200s, but within 30 years it was empty, leaving a mystery. Perhaps the population had grown too large to feed itself as the climate deteriorated. Then as people began to leave, that may have made it harder to maintain the social unity needed for defense and new infrastructure.
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Timothy A. Kohler & Kelsey M. Reese. Long and spatially variable Neolithic Demographic Transition in the North American Southwest. PNAS, published online June 30, 2014; doi: 10.1073/pnas.1404367111
