1804: that was the year the world population reached one billion, after hundreds of thousands of years of human existence. Today, just a couple hundred years later, we’re primed to hit 8 billion.

It’s well-documented that humanity’s numbers have surged exponentially—and relatively recently. A chart from populations scientist Joel Cohen’s book, How Many People Can the Earth Support?, makes that startlingly clear:

But what about the foundations of that boom? The Industrial Revolution was at its height when we hit the one billion mark, and that’s partly why it’s often cited as the spike’s origin. Seismic changes in agriculture, manufacturing, technology, and communication facilitated unprecedented growth in average life expectancy and in the world’s population.

But a new study in PLOS One argues that the Industrial Revolution’s advances are merely proximate reasons our numbers started to scale. Aaron Jonas Stutz, an anthropologist at Emory University, points to 1,500 to 2,000 years ago as when the essential groundwork to support our growth was laid.

“If you dig further in the past,” Stutz told Emory University, “the data suggest that a critical threshold of political and economic organization set the stage around the start of the Common Era. The resulting political-economic balance was the tipping point for economies of scale: It created a range of opportunities enabling more people to get resources, form successful families, and generate enough capital to transfer to the next generation.”

Stutz created a new population growth trajectory model that differed from previous models, in that it accounted for how consumption costs (i.e., amount of resources used and how that degraded the environment) influence carrying capacity elasticity (how many people the Earth can support) over time. Stutz’s model is a variation on one by Cohen, which posits population growth is determined by an elastic carrying capacity.

Or, to draw from Nathanael Johnson’s recent post, there’s a global pie, and a growing number of people. How many people can eat up that pie (population growth) depends on how we divvy it up (elastic carrying capacity): We make a bigger pie (i.e., we increase resource availability through scientific advances), we set out fewer forks (try to reduce population), or better table manners (demand a more equitable society).

Stutz used extensive demographic and archaeological data to show consumption patterns began shifting around the dawn of the Common Era, in ways that reflected changing political-economic mores.

“The increasingly complex and decentralized economic and political entities that were built up around the world from the beginning of the Common Era to 1500 CE created enough opportunities for individuals, states and massive powers like England, France and China to take advantage of the potential for economies of scale,” Stutz said.

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