Birth rates that probably "exceeded the highest in the world today" took place among Native Americans in what is now the U.S. Southwest between A.D. 500 and 1300, researchers say.
The centuries-long "baby boom" was the result of the maturing of agriculture and accompanying food storage technologies, a study published in the Proceedings of the National Academy of Sciences says.
"It's the first step toward all the trappings of civilization that we currently see," says Washington State University anthropologist Tim Kohler.
However, the "growth blip" was followed by a population crash that should be taken as a warning signal about the risks of overpopulation in the modern world, the researchers say.
"We can learn lessons from these people," Kohler says.
For the study, Kohler and fellow researcher Kelsey Reese analyzed a hundred years' collection of data derived from human remains discovered at numerous locations around the Four Corners region of the U.S. Southwest, the point where modern day New Mexico, Arizona, Utah and Colorado meet.
"This research reconstructed the complexity of human population birth rate change and demographic variability linked with the introduction of agriculture in the Southwest U.S.," says Alan Tessier of the National Science Foundation's Directorate for Biological Sciences, which provided support for the research.
"It illustrates the coupling and feedbacks between human societies and their environment," he said.
Agriculture, along with food storage, meant plenty of sustenance was available, so birth rates -- newborns per each 1,000 people annually -- rose steadily, the researchers said.
And yet, after reaching a population estimated at around 40,000, it all crashed around the year 1300 -- probably because of a major drought -- and within three decades the northern region of the southwest was pretty much uninhabited, they said.
Kohler says he suspects the population grew too large for the dwindling food supply as climate changed and growing conditions deteriorated.
As drought drove farmers away, the population likely plummeted to a point where it was difficult to maintain a social unity necessary for defense and to create new infrastructure or even maintain the existing infrastructure, Kohler says.
And yet as conflicts between peoples began to rage through the northern Southwest, birth rates stayed high, he points out.
"They didn't slow down," Kohler says. "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," he adds.
Their eventual fate proves that "population growth has its consequences," he says, a possible warning for today.
Even with giant advances in farming efficiency and food distribution methods, we are still at risk from catastrophic changes, and the climate and resources necessary for our survival shouldn't be taken for granted, Kohler says.
