Today, everyone from city planners to the World Bank uses data to map the spread of urbanization. But tracking the expansion of past civilizations is trickier. Sure, historians have created snapshots of ancient societies, but no one has ever had a comprehensive record of when and where those societies emerged—until now.

Thank Yale urbanization researcher Meredith Reba. This month, she and her colleagues published a paper in the journal Scientific Data that plots 6,000 years of global, city-level population history. Their data lists not only the size of past cities, but how, when, and where they emerged. That's a big deal—and not just for historians. “The more complete the record is, and the more accurate it is, the better we can look at these current and past relationships and try to see certain trends,” Reba says.

Reba's isn't the first historical record of city populations, but it is the most extensive, and user-friendly. Her team's research builds on work by historian Tertius Chandler and political scientist George Modelski, who dedicated their careers to charting the global spread of urbanization. But Chandler and Modelski’s original data sets, published respectively in 1987 and 1970, are hard to parse—they’re tabulated, contain no geographic information, and are only available in print. Reba's team digitized the data, cleaned it up, and assigned each of the 1,700 cities a longitude and latitude, transforming Chandler and Modelski’s unwieldy 6,000-year data set into something more useful and digestible. You can even download it for free.

__ __Reba thinks this new set of historical data will help researchers ask fresh questions about contemporary cities. In 2014, the United Nations estimated that 54 percent of people lived in urban areas, and that the number would rise to 66 percent by 2050. Reba says knowing what happened in the past—and where—can help researchers who study urbanization identify new patterns, and address questions about everything from sustainability, to the sociopolitical ramifications of urban growth.

“Cities require different things to develop and expand, so if we can look at how the centers of cities change through time, it can tell us something about the relative power of places, how resources are used, the level of governance, or institutional or economic development in these places,” Reba says.

In fact, people are already putting Reba's data to use. "It’s not often that you see a data set that goes back 6,000 years,” says data analyst Max Galka. In fact, he adds, "most of the data you find goes back a few decades at most.” (The United Nations has the only other geolocated data set on global urbanization, and it only goes back to 1950.) In a recent blog post, Galka translated Reba's data set into a map—a great service to anyone too intimidated to dig through a journal with a name like* Scientific Data*. His visualization, which you can watch above, traces the growth and spread of cities from 3700 BC to today. For each city, a dot appears at the date of the earliest recorded population. The size of the dot corresponds to its population at that time.

Reba, for her part, will use the data to investigate the impact that fertile ground has had on urban development, and how it's changed over time in relation to the emergence of trade routes and transportation networks. Reba can’t predict what questions other researchers will ask about the factors that shape cities, but she’s confident her team's database is a good first step to finding them.