Traces of human faeces found in a lake have revealed what led to the downfall of one of the greatest ancient cities of the Americas.

Cahokia was once a thriving, cosmopolitan settlement, located across the Mississippi River from the modern town of St Louis.

With its large earth mounds topped with temples and mortuaries, the city was the largest urban centre north of the stepped pyramids of Mexico and Central America at the time.

While it housed a population that numbered in the tens of thousands at its peak, those who lived there appear to have abandoned the site by the time Europeans arrived on the continent.

The mystery of Cahokia’s fall from its peak in around AD1100 has long been debated by archaeologists and now new evidence has been recovered based on excrement washed from the ancient town.

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Scientists tracked the history of the area using samples taken from the nearby Horseshoe Lake in Illinois, using them to follow both historical human population changes and weather patterns.

They concluded that a period of climate change accompanied by both droughts and floods had driven people in their thousands from the city.

“When we see correlations with climate, some archaeologists don’t think climate has anything to do with it, but it’s difficult to sustain that argument when the evidence of significant changes in the climate show people are facing new challenges,” said Professor Sissel Schroeder, an anthropologist at the University of Wisconsin-Madison.

The study was based on faecal stanols – molecules produced in the human gut that can persist in the environment for thousands of years.

Taking samples of sediment from the bottom of the lake, the researchers were able to track changes in population size over the centuries the Cahokia site was inhabited.

The more people living and defecating there at a specific time, the more stanols they found in the corresponding layer of sediment.

At the same time, the same lake samples could reveal evidence of changing climate over the period – for example changes in rainfall, which can be determined by oxygen concentrations in the sediment.

Precipitation from the past can be measured by examining ratios of different forms of oxygen in samples.

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The decline in summer rainfall would have left the population vulnerable, as they would no longer have been able to grow the maize they relied on for survival. By around AD1400, the city was abandoned.

Dr Schroeder said her team’s work, published in the journal Proceedings of the National Academy of Sciences, still has relevance for communities living through climate change today.

“Cultures can be very resilient in face of climate change but resilience doesn’t necessarily mean there is no change. There can be cultural reorganisation or decisions to relocate or migrate,” she said.