How did the world's first great settlements mysteriously collapse?

Whether intentional or not, the way large settlements were built in southeastern Europe 6,000 years ago may have limited the spread of disease.

In new research focusing on the first farmers in Europe, researchers often wondered about a strange pattern over time: Farmers lived in large, densely populated villages, then dispersed for centuries, then formed cities, only to abandon those cities as well. Why?

Archaeologists often explain what we call urban collapse in terms of climate change, overpopulation, social pressure, or some combination of these factors. But scientists have added a new hypothesis to the mix: disease . Living in close proximity to animals leads to diseases being passed from animals to humans. Outbreaks can cause crowded settlements to be abandoned, at least until later generations figure out how to arrange their settlements to be more resistant to disease.

Excavations at Çatalhöyük show how close people lived together before the settlement collapsed. (Photo: Mark Nesbitt/Wikimedia Commons).

The First Cities: Lots of People and Animals

Çatalhöyük, in modern-day Turkey , is the world's oldest continuously inhabited farming village, dating back more than 9,000 years. Thousands of people live in mud-brick houses so tightly packed that residents have to enter by ladder through a trapdoor in the roof. They even bury selected ancestors beneath the floors. Despite the abundance of space in the Anatolian Plateau, people are still packed together.

For centuries, the people of Çatalhöyük herded sheep and cattle, grew barley, and made cheese. Evocative paintings of bulls, dancing figures, and a volcanic eruption evoke their folk traditions. They kept their homes neat and clean, sweeping the floors and maintaining storage bins near the stove, located under trapdoors to let the smoke escape. Keeping clean meant they even re-plastered the interior walls several times a year.

These traditions ended around 6000 BC, when Çatalhöyük was mysteriously abandoned. The population dispersed into smaller settlements on the surrounding floodplain and beyond. Other large farming populations in the area also dispersed, and nomadic herding became more common. For those populations that remained, the mudbrick houses were now isolated, in contrast to the centralized dwellings of Çatalhöyük.

Was disease a factor in the abandonment of crowded settlements in 6000 BC?

At Çatalhöyük, archaeologists have found human bones mixed with cattle bones in burials and garbage heaps. The crowded conditions of humans and animals may have caused zoonotic diseases at Çatalhöyük. Ancient DNA has identified tuberculosis (TB) in cattle in the area since 8500 BC and tuberculosis in the bones of infants shortly thereafter.

DNA in ancient human remains has identified salmonella dating back to 4500 BC . Assuming the transmissibility and virulence of Neolithic diseases increased over time, densely populated settlements like Çatalhöyük may have reached a tipping point where the impact of disease outweighed the benefits of living in close proximity.

Around 4000 BC, large urban populations reappeared in the large settlements of the ancient Trypillian culture, west of the Black Sea. Thousands of people lived in large Trypillian settlements such as Nebelivka and Maidanetske in what is now Ukraine.

If disease was a factor in dispersal millennia ago, how could these large settlements have survived?

Simulating socially distanced neighborhoods

To simulate the spread of the disease in Nebelivka, the researchers had to make a few assumptions.

1. First , they assumed that the diseases were initially spread through food, such as milk or meat .
2. Second, they assumed that people visited other houses in their neighborhood more often than outside houses.

Is this clustering enough to prevent an outbreak? To test the impact of different possible interaction rates, the researchers ran millions of simulations, first on a grid to represent residential clusters. They then ran the simulations again, this time on a virtual layout modeled after real-world floor plans, where homes in each neighborhood had a higher chance of coming into contact with each other.

Based on these simulations, the researchers found that if people visited other neighborhoods less frequently than they visited other homes in their own neighborhood, the clustered housing layout in Nebelivka would have significantly reduced the initial outbreaks of foodborne illness. This makes sense, since each neighborhood had its own clustered housing. Overall, the results suggest that the Trypillian layout may have helped early farmers live together in low-density urban populations, at a time when zoonotic diseases were on the rise.

The residents of Nebilevka did not need to consciously plan the layout of their neighborhood to help their population survive. But they may have done so, because it is human instinct to avoid signs of infectious disease. As in Çatalhöyük, residents kept their homes clean. And about two-thirds of the homes in Nebelivka were intentionally burned at various times. These periodic, intentional burnings may have been a pest control tactic.

Some early diseases eventually evolved to spread through means other than bad food. Tuberculosis, for example, became airborne at some point. Once the plague bacteria adapted to fleas, it could spread through rats, which didn't care about neighborhood boundaries.

The world's first cities, along with those in China, Africa, and the Americas, were the foundations of civilization. Their form and function were shaped, so to speak, by millennia of disease and human responses to it, dating back to the world's first farming villages.