What Ancient Aztecs Shared With Modern New Yorkers

There wouldn’t seem to be a whole lot that Coeur d’Alene, Idaho (pop. 44,000) shares with Shanghai, China (pop. 24 million), but size isn’t everything. A new study published in PLoS One now shows that all cities, regardless of age and population, grow in pretty much the same way.

As urban scientists have known for a while, densely packed cities can, in some ways, be a bargain. The more infrastructure you build, the less you need of it per person—having twice the number of people means less than twice as much train track. And the amount of money generated by the city’s economy per capita, like other socioeconomic measures including number of patents produced and violent crimes committed per capita, grows in the other direction. Twice as many people means a GDP that’s more than twice as high as before, more than twice as many patents, more than twice as many crimes.

Recently, a team of investigators who sifted through data on 1,500 ancient towns, villages, and cities that flourished in the Basin of Mexico over the course of 2,000 years sought to determine how long ago these laws of scaling emerged. Since the basin is where Mexico City—one of the most densely populated places in the world—stands today, the site could yield some powerful lessons.

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Archaeologist and lead author Scott Ortman, a professor of anthropology at University of Colorado, began the work by digging out the data collected by researchers in an epic burst of fieldwork in Mexico in the 1960s and re-examining it. Superficially, the scaling laws seemed to apply. “I got on my computer and opened up the data set,” he recalls, “and I saw mathematical patterns that seemed to correspond to the models.”

It was electrifying, and he, physicist Luis Bettencourt of the Santa Fe Institute, and their other collaborators started to delve into the data in earnest, but they quickly ran up against a problem: figuring out the population of different towns from just the archaeological traces left behind is tough since they could not be sure of the methods earlier investigators used to crunch their numbers, which could cast doubt on any analyses that might be conducted later. So Ortman wound up doing a kind of archaeology of archaeology, painstakingly retabulating the raw data to make sure it stood up.

It did. With their re-examined data set, the team was able to say that the settlements did get denser as they grew in just the way that the models of modern cities suggest, and most likely not because of some quirk of how populations were estimated. This means that some ancient cities might have had more inhabitants than previously thought, and might have been more culturally or technologically sophisticated as well.

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“What we’re working on is a framework or perspective in which all human societies, past and present, actually work in the same way,” says Ortman. “They appear radically different on the surface due to the scale of coordination that’s involved, but the fundamental processes that create those patterns are the same.”

Just why cities are so productive is easy enough to understand. For all the stresses of urban living, the very proximity of so many other people starts a virtuous cycle.“Your life is a path through a city,” Bettencourt explains. “You go to work, you take your kids to school, you go to the grocery store. ” As you move through that densely populated space, you have more interactions with less effort than you would in the countryside, and the result, on a large scale, is more business deals closed, more inventions produced, more brainstorms that otherwise would have stayed quiet. Yes, the subways screech and the taxis ignore you in the rain, but the payoff—once you get home and dry off—can be considerable.

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