Soil from a Copper Age site in northern Kazakhstan has yielded new evidence for domesticated horses up to 5,600 years ago. The discovery, consisting of phosphorus-enriched soils inside what appear to be the remains of horse corrals beside pit houses, matches what would be expected from Earth once enriched by horse manure. The Krasnyi Yar site was inhabited by people of the Botai culture of the Eurasian Steppe, who relied heavily on horses for food, tools, and transport.

"There’s very little direct evidence of horse domestication," says Sandra Olsen, an archaeologist and horse domestication researcher at the Carnegie Museum of Natural History in Pittsburgh, Pa. That’s because 5,600 years ago there were no saddles or metal bits to leave behind. Equipment like bridles, leads, and hobbles would have been made from thongs of horse hide, and would have rotted away long ago. Likewise horses themselves have not changed much physically as a result of domestication, unlike dogs or cattle. So ancient horse bones don’t easily reveal the secrets of domestication.

With research funding from the National Science Foundation, Olsen’s team took a different tack. They looked for circumstantial evidence that people were keeping horses. One approach was to survey the Krasnyi Yar site with instruments to map out subtle electrical and magnetic irregularities in the soils. With this they were able to identify the locations of 54 pit houses and dozens of post moulds where vertical posts once stood. Some of the post moulds were arranged circularly, as would be most practical for a corral.

Next, geologist Michael Rosenmeier from the University of Pittsburgh collected soil samples from inside the fenced area and outside the settlement. The samples were analyzed for nitrogen, phosphorus, potassium, and sodium concentrations by Rosemary Capo, University of Pittsburgh geochemist, and her students. Modern horse manure is rich in phosphorous, potassium, and especially nitrogen, compared to undisturbed soils. But because nitrogen is mobile in soils, it can be lost to groundwater or transferred to the atmosphere by organic and inorganic processes. Phosphorus, on the other hand, can be locked into place by calcium and iron and is more likely to be preserved in the soils for millennia