The greatest baby booms in North American history occurred between 500 and 1300 A.D. among southwestern Native Americans. The population soared and then crashed eight centuries later.
"We can learn lessons from these people," says Tim Kohler, an anthropologist at Washington State University (WSU), who co-authored the paper with WSU researcher Kelsey Reese.
Researchers say the Neolithic Demographic Transition, when people began eating more grain and less meat, is responsible for the population increase and the eventual demise of the ancient Puebloans.
The population boom came at a time when the early features of civilization—including farming and food storage—were at their peak, the researchers report in this week's issue of the journal Proceedings of the National Academy of Sciences.
The study examined a century's worth of data on thousands of human remains found at hundreds of sites across the Four Corners region of the Southwest. Scientists then 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," says Kohler.
Maize was grown in the region as early as 2000 B.C. By 400 B.C., Kohler says, the crop provided 80 percent of the region's calories.
Crude birth rates–the number of newborns per 1,000 people per year–were by then on the rise, mounting steadily until about 500 A.D.
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.
Kohler said that the Sonoran and Tonto people eventually would have had difficulty finding new farming opportunities for many children, since corn farming required irrigation. Water from canals also may have 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.
Around 900 A.D., populations remained high but birth rates began to fluctuate.
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," says Kohler. "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."
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, says Kohler.
Whatever the reason, he says, the ancient Puebloans show that population growth has clear consequences.