About 10,000 years ago, a group of hunter-gatherers settled on a floodplain in modern-day Turkey and stayed for a millennium. You can still see remnants of the houses they built. Archaeologists have mapped out alleyways and uncovered intact skeletons under ancient plaster floors. After all this time, Aşıklı Höyük is remarkably well preserved.
But Jordan Abell did not come for these sights when he last visited Aşıklı Höyük in 2017. He came to look for something invisible: ancient urine.
The people of Aşıklı Höyük all, presumably, peed. So did their sheep and goats. By estimating the quantity of ancient urine deposited at Aşıklı Höyük, Abell and his collaborators reconstructed the population of humans and animals at the site 10,000 years ago. You might call it urine archaeology.
“The method is, as far as I see, totally new and creative,” says Benjamin Arbuckle, an anthropologist at the University of North Carolina at Chapel Hill, who studies animal domestication in Turkey during the same period. Sheep and goat domestication is what got Arbell and his co-authors interested in urine in the first place. Animal bones and even dung at Aşıklı Höyük suggest that its occupants were among the first people in the world to domesticate sheep and goats. They penned the once-wild horned creatures near their homes. They learned to cull young males to maximize the size of their herds.
It was the discovery of unusual nitratine crystals that prompted the team to think about sheep and goat pee. “There’s very few places on Earth that have these nitratine crystals forming,” says Abell, who is now a paleoclimate researcher at Columbia. These places tend to be very dry, and they have high concentrations of salts. Abell, along with his collaborators at the University of Arizona and Istanbul University, wondered if urine was the source of those salts. So they went out and collected 113 samples from Aşıklı Höyük. They were especially interested in “middens,” ancient refuse heaps where human and animal waste may have piled up. And they made sure to collect samples from different layers in and around the middens, spanning the 1,000 years that people lived at Aşıklı Höyük.
Back in the lab, Abell looked for the chemical signatures of urine—sodium, nitrate, and chlorine—in each of these samples. The tricky part is that these salts can come from elsewhere, too. They are also found in various concentrations in rainwater and in the natural sediment around Aşıklı Höyük. So Abell built a model attempting to account for those sources. To make sure his assumptions weren’t totally off base, he compared the urine salt concentrations at Aşıklı Höyük with that of modern livestock feedlots, and found they were similar. The model ultimately estimated that an average of 1,790 humans and animals were peeing per day in Aşıklı Höyük during the 1,000 years of settlement.
As the team went up the dirt layers and through time, they found 10- to 1,000-fold increases in the concentration of urine salts at the latter end of the millennium-long period of occupation. This suggests that the human and/or animal population of Aşıklı Höyük was getting bigger and denser. (Unfortunately, archaeologists don’t have a way to distinguish between ancient human and animal urine using this method.) Assuming the model holds up, these urine deposits can be seen as a record of humanity’s transition from hunters to animal farmers.
Bones, Arbuckle points out, are evidence of animals being eaten by humans. “It’s really hard to tell if they’re being hunted or if they’re being herded or if some of them are being hunted and some are being herded,” he says. Vast quantities of urine, on the other hand, would suggest that animals and the people herding them were in fact staying and peeing in one place.
At this point, using urine salts to understand Aşıklı Höyük’s population relies on a lot of assumptions. Canan Çakirlar, a zooarchaeologist at the University of Groningen, calls the technique “very promising,” but she also points out that not very much is known about how urine deposits might have chemically changed over the millennia. Other factors may have changed too: People and livestock had different diets 10,000 years ago than they do now, which could produce different concentrations of salts in their urine.
Rainfall patterns over Aşıklı Höyük could have changed as well. Today, it’s a fairly dry place. The region gets about 400 millimeters (15 inches) of rain a year. It would be harder to study urine deposits in wetter places, says Abell, where rainfall and a changing water table would blur the fine layers of urine salts. He hopes to get more data from Aşıklı Höyük next year, to sample more sediment from more areas and study what little rain falls over the site. He would also like to get some urine from the local sheep that still roam the fields. Ten thousand years after humans first learned to raise their flocks here, they’re still at it.