The population of what is now the American midwest boomed before the widespread adoption of agriculture, a new paper has revealed. The findings suggest that increasing numbers were a contributing factor to the adoption of settled lifestyles, rather than a consequence of them. The evidence sheds light on arguably the most important event in human history, although different factors may have been at play in other parts of the world.

"Domesticated plants and animals are part of our everyday lives, so much so that we take them for granted," said Dr Brian Codding, of the University of Utah, in a statement. "But… they allowed for large numbers of people to live in one place. That ultimately set the stage for the emergence of civilization."

The birth of agriculture has been associated with population explosions wherever it has occurred, but these might be expected to be a consequence rather than a cause. There has been a long-standing debate as to whether agriculture arose through necessity when resources were scarce, or as an experiment in good times.

In the Mississippi and Ohio valleys, this process began about 5,000 years ago, but in Royal Society Open Science, Codding reports that regional population density was rising for almost 2,000 years beforehand.

"It's really difficult to arrive at measures of prehistoric populations. So archaeologists have struggled for a long time coming up with some way of quantifying population levels when we don't have historical records," said lead author Elic Weitzel. 

^{The study area, as well as the sites where evidence of early agriculture has been found. Elic Weitzel/University of Utah}

The authors reached their conclusions by plotting the ages for all the artifacts found in the region since human arrival, assuming that more buried tools meant more tool makers. They found a doubling between 6,900 and 5,200 years ago. In eastern North America at least, it seems it was the only way to feed a rapidly growing population.

Agriculture's origins did not necessarily arise in a single step. Within the studied area, squash, sunflower, marshelder and the grain pitseed goosefoot were the key cultivated species. There is a gap of more than 1,200 years from the first evidence of squash domestication to the earliest signs of large concentrations of the quinoa-like pitseed goosefoot. Oddly, the domestication of the first plants brought no cornucopia, and populations appear to have remained largely stable through this period.

However, there may have been a synergistic benefit from the four plants; multiple sites with evidence of all four appear around the same time, suggesting the idea spread rapidly once the quartet were in use.

Weitzel argued the fertile valleys offer a better indication of agriculture’s origins than some other locations. "This is the region where these plant foods were domesticated from their wild variants," he said. "Everywhere else in North America, crops were imported from elsewhere." 

The authors acknowledge that “domestication events in the Neotropical lowlands of Central and South America and Northern China do not reveal evidence for high or increasing human populations prior to domestication,” but argue that environmental changes that reduced hunting opportunities forced the diversification of food supplies.

^{Population density of the Mississipi and Ohio valleys based on the number of sites measured for each century. Weitzel and Codding/Royal Society Open Science}