Clues from human waste could shed light on climate change and decline of Maya population

Extreme climate changes, both wet and dry, corresponded with the population decline of a Maya settlement in central America according to new research out of McGill University that looked at indicators left behind by ancient human waste.  

In a study published in April in the journal Quaternary Science Reviews, researchers looked at concentrations of human waste biomarkers, called fecal stanols, found in a geologist's core samples from a small lake near Itzan, a former Mayan settlement in what is now Guatemala. 

Benjamin Keenan, a PhD candidate in McGill's Department of Earth and Planetary Sciences and the study's first author, says human remains don't last very long in tropical rainforest environments â€" but the molecules present in human waste do.

By looking at the concentrations of fecal stanols that were preserved in the mud of the lake adjacent to the settlement, the team was able to paint a picture of population change over a period of 3,300 years.

Itzan is the "perfect setting" to use fecal stanols to glean information on the relationship between changes in population over time and extreme climate events, Keenan told CBC News. 

That's because waste from the escarpment-based settlement would have run down the slope and directly into the lake. 

"The lake is so small, and it's kind of like a waste bin for everything produced around it," he said. 

PhD Candidate Benjamin Keenan pictured with a core sample of sediment on Lake Izabal, the largest lake in Guatemala. Keenan says mass migration might have been behind the population decline his lake samples revealed. (Elisandra Hernández)

The study found Itzan's population declined during periods known to have suffered droughts that lasted for decades or even centuries. 

Previous studies have suggested drought might have led to the collapse of the Maya civilization's population. 

But the McGill research also found there seemed to be population decline in Itzan during a very wet period; some 400 to 200 years BC.

Keenan says that at both extremes â€" prolonged severely dry and severely wet periods â€" the population dropped, likely due to mass migration to greener pastures. 

Ample water is important for a society to thrive, but Keenan says very wet conditions can hurt agriculture. Excess water can erode soil and jeopardize a community's food source. It could have caused crops like corn, for example, to fail.

"You see this societal response to both extremes of climate," he said.

The climate change of the past was different from today's, which is due to human-caused greenhouse gas emissions, says Peter Douglas, an assistant professor with McGill's Department of Earth & Planetary Sciences and the senior author on the study.

While it may have been human-induced to some degree â€" by heavy deforestation that caused soil erosion or dried-out soil, for example â€" many experts say climate extremes in the Maya era were likely produced by El Niño.

The societal response, however â€" mass migration out of a settlement â€" is analogous to what we might see in the future, said Douglas.

"Based on what we know, the climate change that we can expect in the next 100 years is going to be, at least on a global scale, much stronger," says Douglas. 

"[This study] builds up the case that yes, when climate change happens people have to adapt and they adapt by changing their societies and moving."

Samples from the lake also revealed that the land had been settled by Maya earlier than previously suggested, about 650 years before what the archeological record suggested, and that smaller numbers of Maya continued to live there after experts believe the entire civilization collapsed around AD 900. 

Environmental historian Alan MacEachern a professor at Western University, who was not involved in the study, says it fills in some gaps left by the Maya.

"The [Maya], they never really talked about the weather as being particularly unusual," says MacEachern, who adds that this makes it challenging for scientists to determine whether climate events caused their population decline. "There's no written sources about them saying 'Man, it sure is getting dry or anything here.'" 

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