The 2,500-year-old city of Monte Albán sits on a mountain in the middle of the Valley of Oaxaca, a high plain in the mountains of southern Mexico. And according to archaeologists Linda Nicholas and Gary Feinman, both of the Field Museum in Chicago, the city’s founding represents a 1,000-year renaissance in the valley, kicking off an era of explosive population growth, ceremonial buildings, and even the adoption of the tortilla.

Why the city became so popular is a mystery: Monte Albán isn’t a particularly inviting place to grow food. It’s far from the rivers and aquifers elsewhere in the valley that serve thirsty corn, so its residents would have had to rely on unpredictable rainfall. 

What Monte Albán lacked in agricultural opportunities, it might have made up for in cultural cachet. A high standard of living, and lack of despotic rulers, is the best explanation for the region’s explosive growth, as Nicholas and Feinman documented in research published this week in the journal Frontiers in Political Science. 

After its founding in about 500 BCE, people flocked to the city and to the surrounding valley. “It tripled in size within a couple of centuries,” says Feinman. By around the year 0, it had about 17,000 inhabitants. Even migration from outlying settlements in the valley can’t explain that growth—people must have moved in from over the mountains. 

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Archaeologists are still learning how to study societies like Monte Albán. Studies of societies with autocratic dynasties—like Egypt, or the city-states of Mesopotamia—dominate archaeology’s past in part because their status symbols are easy to see. Graves full of jewels, weapons, and sacrificed humans, the remnants of palaces, and art that glorifies rulers all fit nicely in museums. And, for a long time, archaeologists treated hierarchy and “complexity”—the ability to build ritual sites, heave stones around the landscape, or dig miles of irrigation canals—as part of the same package.

So what would evidence of equality look like? Feinman breaks it down into two parts: governance, or how people make decisions, and economics. 

Monte Albán seems to have grown for more than a 1,000 years without dynasties or centralized power—or at least, the city never put any effort into glorifying its leaders. Only one statue of a leader exists from the city’s first 400 years, and that person is wearing a mask. “Power was not concentrated in a single individual or family line,” says Feinman.

The reality was probably messier than either a centralized state or a democracy, says Deborah Nichols, an expert in early Mesoamerican cities at Dartmouth College. Nichols says she’s wary of the term “egalitarianism,” because it can suggest a society governed entirely by consensus. “Some people have used the term oligarchy—an elite that recognized that they also had to negotiate with their subjects.”

The Oaxacan city’s decentralized power contrasts with that of the neighboring people, the Maya, says Feinman. There, “you have the rulers, and you can trace their ancestry and their descendents” from written chronicles, he says. Those rulers built the limestone pyramids of the Yucatan rainforest, which held cramped ritual spaces that could fit only a select few.

And Monte Albán lacked huge economic extremes. The hilltop city towers over the surrounding plain. At the city’s highest point, where other societies might have built a palace, was a paved plaza, big enough for most of the city’s residents to gather. There are no palaces at all, in fact. Whole neighborhoods are built on terraces cascading down the hillside, which would have taken huge amounts of collective labor to flatten, drain, and buttress. At the base of the hill was heavily-used farmland, irrigated with a network of dams and deep wells.

“The housing situation of everyday people is telling,” says Feinman. “When some people live in a castle and others live in a shack, you probably have very little equity in that society.” As people moved into the city, the material differences between households shrunk, and more houses had access to high-quality goods. In the older towns of the valley, a certain kind of glossy, water-tight pottery was only used for ceremonial bowls. But that pottery was widespread in Monte Albán. And Monte Albán’s townhouses had plaster floors and stone foundations, often around an interior courtyard—the kind of dwelling where, in older cities in the region, only the richest few lived.

Those features add up to a picture of a city with a “social charter,” the archaeologists argue. Monte Albán certainly had wealthier people, who lived in bigger homes, sometimes with in-house steam rooms or tombs. But in contrast to their neighbors, the sheer abundance of wealth suggests that the needs of everyday people were a priority. Neighbors must have worked together to maintain their homes and water systems.

This is part of an emerging global picture of how civic projects could be managed—recent research in Mesopotamia has shown that irrigation canals, which could take thousands of people to build and maintain, were developed thousands of years before the region’s first kings.

[Related: The Maya dealt with a form of climate change, too. Here’s how they survived.]

In a previous study coauthored by Feinman on 26 pre-colonial cities in Central America, 12 appear to have been similarly organized, without deference to a king. The most famous of them, Teotihuacan–in the Valley of Mexico, where Mexico City is now located–built extensive apartment complexes that housed the city’s common people.

Nichols agrees that collaboration on housing, could be evidence of a city with at least some hyperlocal governing structures. “Think of a place like Chicago, with wards and ward bosses,” she says. “The mayor is obviously having to negotiate his position with these folks.”

Compared with other cities in the area, people didn’t seem to wear the consequences of inequality on their bodies. The skeletons of relatively poor people in Monte Albán were much less likely to show evidence of malnutrition than the poor of surrounding kingdoms. 

And Nichols says the skeletons of men and women contrast with bodies from the other kingdoms, too. Sex hierarchies were “much less exaggerated” in Monte Albán than in its neighbors, Nichols says. In another analysis of nearby states, “sex differences were huge and marked. Women were clearly not getting the same access to food,” says Nichols. Their skeletons showed evidence of anemia, and had thinner bones. Those differences weren’t so stark at sites like Teotihuacan and Monte Albán, she says.

A lack of kings doesn’t mean a utopia—the landscape around Monte Albán was cleared of its forests, and erosion began to strip productive soil from hillsides. And stone carvings on top of the hill show war captives, naked, waiting to be killed. It’s likely that the city was also popular because it was easily defended during raids. By 800 CE, some elite families began to carve statues glorifying their lineages.

But the lack of evidence for any kind of dictator or king flips long-standing archaeological wisdom on its head: “For close to 75 years, the idea is, with the exception of Athens and Republican Rome, all pre-modern societies were despotic,” says Feinman. “But what archaeology has told us is that’s just not right. Just like today, there are more autocratic and democratic societies, and they can fluctuate up and down over time.”