Researchers find the sands of time are soaked in urine
Analysis of soil salts provides startling estimates for the beginnings of farming. Richard A Lovett reports.
Scientists sifting through dirt from a 10,000-year-old archaeological site in central Turkey are seeking a rare treasure: traces of ancient urine.
Their goal? To find evidence of when people first began domesticating farm animals, specifically sheep and goats.
Prior studies had found signs that the residents of the ancient village, known as Aşıklı Höyük, had kept animals on the site, mostly sheep. Evidence for this comes from discoveries of dung piles and the fact that bones of slaughtered animals indicate that males were dispatched at substantially younger ages than females — a sign that the villagers were probably saving breeding stock.
But how many animals were present at any given time was an open question.
To address the matter, researchers led by geochemist Jordan Abell of Columbia University, US, got to wondering: if the villagers were indeed raising significant numbers of animals, what could the animals have been doing that might leave traces of their numbers?
“So we thought, if they’re on the site, they have to pee,” says Abell. “And if they pee, what do they leave behind? Salt.”
Previous studies had found that the soil at Aşıklı Höyük was unusually salty. So Abell and colleagues collected 113 soil samples and measured the concentrations of various ions commonly associated with urine, including sodium, nitrate, chloride, potassium, magnesium, and sulfates.
All of these, they found, were present in much higher concentrations than in the native soils underlying the site.
They also looked at nitrogen isotopes in the nitrate, a measurement sometimes used by environmental researchers concerned about water pollution downstream of modern feedlots.
“High nitrogen isotope values are indicative of animal waste,” Abell says.
All of the measurements revealed that the soils of Aşıklı Höyük were highly enriched with what the researchers called “urine salts”, suggesting the presence of a great many animals during the 1000 years or so the site was occupied.
To figure out how many animals had contributed to the accumulated pee the findings represented, the scientists turned to modern measurements of the salt content of urine. They also accounted for the fact that some of the salt would have been deposited by rain, or by ash from the residents’ wood fires.
All told, they calculated that, on average, somewhere between 1300 and 2300 animals had been peeing on the site daily, throughout its history.
More interestingly, the levels of urine salts rose markedly from the time the first villagers arrived to when the site was abandoned, about 9700 years ago.
One caveat is that the researchers can’t distinguish sheep and goat urine from human urine, although there are hopes that this might be able to be done in the future.
But there’s no way, Abell says, that humans could have accounted for all of it. The village simply wasn’t that big.
“There’s probably human input to these salts,” he says, “[but] the salt values predict a number of organisms well outside any [possible] human occupation.”
Meanwhile, he adds, the study offers a new way to peer into a crucial period in human history, when our ancestors were making the transition from hunter-gatherers to villagers who not only planted crops, but made their first forays into animal husbandry.
One theory, which was in vogue a few generations ago, Abell says, held that this transition began in a single spot somewhere in the Middle East’s Fertile Crescent — a region that spans parts of modern-day Iraq, Syria, Lebanon, Israel, Jordan, the Palestinian Territories, and Egypt — and then expanded outward, as people observed what their neighbours were doing and emulated them.
But more recent theory holds that animal husbandry popped up independently in many different areas, as various nomadic groups settled down and needed to find a way to supplement depleting game resources.
Furthermore, Abell says, the transition appears to have occurred with surprising speed.
“Previous zooarcheological research has shown they might have kept a few animals around, still probably hunting a majority of the time,” he says. “But [our] idea is that they’re switching pretty rapidly.”
Old World pre-historian and zooarchaeologist Sandra Olsen of Kansas University, US, who was not part of the study team, calls it an important find.
“I am blown away by this paper,” she says. “I have used slightly different methods to identify horse urine and dung in corrals at Botai, in Kazakhstan, but this takes it to the highest level I have seen. Very impressive and convincing.”
Furthermore, she says, the new study “expands the geographic region of the so-called Fertile Crescent into a more diverse and broader region” thereby increasing understanding of the early expansion of animal husbandry.
“I find this research to be top notch, and a major contribution to archaeological science,” she says.
Meanwhile, the new study also indicates that there is a lot to be learned from the soils that archaeologists once ignored in the search for artefacts and other objects large enough to be captured by their sieves.
“The sediments between these larger objects have largely been ignored,” Abell says. “I am hopeful that our study will aid in bringing attention to the potential of applying geochemical techniques to these sediments.”
The research is published in the journal Science Advances.