Mystery behind disappearance of ancient American city finally unravelled

Archaeologists suspect city’s disappearance was gradual, not sudden as previously thought

Cahokia, one of the world’s largest ancient cities in present-day St Louis, did not suddenly disappear due to climate change at the end of the 13th century as previously thought.

The pre-Columbian settlement, which housed over 50,000 people with complex roads, public plazas, and even an astronomical observatory, was gradually abandoned as its residents moved in search of better opportunities, the study published in the journal The Holocene, suggested.

Until now, scholars had thought Cahokia residents abandoned the settlement after climate change triggered prolonged drought and a massive crop failure.

The remains of the most sophisticated prehistoric native civilization north of Mexico are preserved at Cahokia Mounds State Historic Site (Joe Angeles/Washington University)

However, researchers from the Washington University in St Louis found that Cahokians were skilled and resilient to adapt to climate change and likely had other reasons to leave town.

In the new study, scientists assessed carbon atoms left behind by fossil plants that grew when the ancient city’s population collapsed and drought was common across the Midwest.

Drought-adapted plants like prairie grasses and maize incorporated carbon into their bodies at rates that left a tell-tale signature compared to the crops Cahokians harvested for food such as squash, goosefoot, and sumpweed.

By analysing carbon atom signatures at the Cahokian archaeological site, scientists found that there was no radical shift in the types of plants growing in the area during what was thought to be the drought period.

“We saw no evidence that prairie grasses were taking over, which we would expect in a scenario where widespread crop failure was occurring,” study co-author Natalie Mueller said.

“It’s possible that they weren’t really feeling the impacts of the drought,” Caitlin Rankin, another author of the study said.

Researchers now suspect the sophisticated society almost certainly had a storage system for grains and other foods which could have powered them through such drought periods.

They say the pre-Columbian society had also adapted to a diverse diet including fish, birds, deer, forest fruits and nuts, which would have kept them nourished throughout even if a few sources disappeared.

Scientists say the abandonment of the ancient city was a gradual process.

“I don’t envision a scene where thousands of people were suddenly streaming out of town. People probably just spread out to be near kin or to find different opportunities,” Dr Mueller said.

“The picture is likely complicated,” the archaeologist added.