Where Archaeology Can’t Go: Fecal Signals in Lake Reveal Mayan Secrets

Ruth Schuster
Ruth Schuster
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Sunset from the coring platform on Laguna Itzan, in Guatemala.
Sunset from the coring platform on Laguna Itzan, in Guatemala. Credit: Andy Breckenridge
Ruth Schuster
Ruth Schuster

Research on coprolites – that is, fossilized droppings – is all the rage, and a new study led by researchers at McGill University has gone one further. The scientists studied stanols, organic molecules found in human and animal feces, detected in sediment cores taken from the bottom of a lake near the ancient Mayan city of Itzan, in present-day Guatemala.

The study was published on Wednesday in Quaternary Science Reviews. Their finding: the size of the population varied with the climate, declining significantly both during the region’s protracted droughts and during extremely wet periods.

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Fecal stanols are fatty molecules. Their structure is a function of the animal’s diet and biome (gut bacteria). Long story short – Coprostanol (5β-cholestan-3β-ol) is produced by humans when we eat foods high in cholesterol.

Yes, coprostanol is produced by other mammals, too. But only pigs and sheep produce sufficient concentrations to potentially mask the signal of human coprostanol, the team explains. The Mayans didn’t husband pigs and sheep during the periods in question; these animals only reached that area in the 16th century.

Ergo, the presence of coprostanol in a body of water is considered indicative of human defecation, and therefore its concentration in the body of water is considered indicative of population density. One would wonder how these organic chemicals survive in the water, but the team explains that as organic molecules go, these are relatively resistant to degradation and accumulate in sediments in lakebeds.

They even have a way to correct the stanol readings for “contamination” by cows.

It bears adding that the entire thesis is based on the assumption that variability in stanol concentration at the bottom of a lake is a reliable proxy for relative human population sizes. We add that ground-penetrating radar is both new and uncommon; the archaeological evidence may be “biased” by being based on what remains visible above ground after millennia. Thus, archaeological surveys can produce “biased” results.

Collecting a lake sediment core on Lake Izabal, in Guatemala.Credit: Elisandra Hernndez

The variability of the climate in ancient Guatemala had separately been deduced through biological and archaeological studies. The researchers wondered how the changes affected the city, starting about 3,300 years ago.

The stanol analysis confirmed archaeological evidence of changes in settlement patterns as a function of climate at both ends of the scale: drought and inundation. And, they learned, the Itzan escarpment had been settled 650 years earlier than the archaeological evidence had suggested.

Nor had it been completely abandoned during the so-called “collapse” between 800 to 1000 C.E. It had been supposed that the entire population disappeared, whether because of war, drought and/or other causes, but the stanols show that it was not so.

Another surprise sprung by the stanols is a population spike 324 years ago. Why? Because, according to history, in the year 1697, the Spanish targeted the last Maya stronghold in the region at Nojpeten, or modern-day Flores in the Guatemalan lowlands, causing mass flight.

When the climate bites

We may not learn many lessons from past civilizations’ reaction to climate change because we now have technologies they didn’t, such as air conditioners. Even today, our technologies can’t just divorce us from climate, for all of Israel’s boasting that its desalination plants divorced it from drought; and the trajectory of anthropogenic climate change is worse than things civilization has experienced. But it is enlightening to deduce how past civilizations in the pre-industrial ages coped with profound change in their environment. By and large, the Mayans fled, the stanols study indicates.

Separate paleoclimatic evidence indicates three periods of intense, protracted drought: from 3,300 to 2900 years ago, from 1860 to 1670 years ago and from 1220 to 1050 years ago. During those times, the fecal proxy at the lake bottom indicates that the city population declined.

Flooding is no joy, either, and the archaeologists deduce another population decline and hiatus coinciding with a period of anomalously wet climate about a thousand years ago.

How fecal stanol molecules are transported to lake sedimentsCredit: Benjamin Keenan et al.

To be sure, this method of measuring stanols in ancient sediments is an imprecise tool. The archaeologists point out that the Mayans may have overcome soil degradation by using their own stool as fertilizer. Why do they think that? because of the paucity of fecal stanols in the sediment of Laguna Itzan, corresponding to a time when the archaeology indicates the densest human occupancy. So where did their droppings go if not washed into the lake? Into the fields, they suggest.

It may also not be a huge surprise that even when the population flees, some remain behind. We may not have found their homes, but we “see” their mark in the lake sediment.

“This research should help archaeologists by providing a new tool to look at changes that might not be seen in the archaeological evidence, because the evidence may never have existed or may have since been lost or destroyed,” stated Benjamin Keenan, a PhD candidate at McGill and the first author on the paper, adding: “The Maya lowlands are not very good for preserving buildings and other records of human life because of the tropical forest environment.”

So, any lessons for today? The American Midwest is experiencing megadrought of the kind that sent the ancient Mayans fleeing their lakeside home in Guatemala. Canada is experiencing deadly heat waves, with temperatures as much as 40 degrees above their yearly average at this time. The Middle East and North Africa are expected to experience further desertification; in the Indian subcontinent, the monsoon is becoming erratic, and the list goes on and on. If there’s a takeaway here, it’s that when the climate becomes unbearable, our species tends to flee. But we can’t flee Earth for a better planet, so we should endeavor not to make the whole thing uninhabitable.

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