One of the most important transitions in human history was when we stopped hunting and gathering for food and instead settled down to become farmers. Now, to reconstruct the history of one particular archaeological site in Turkey, scientists have examined a pretty unexpected source – the salts left behind from human and animal pee.

The dig site of Aşıklı Höyük in Turkey has been studied for decades, and it's clear that humans occupied the area more than 10,000 years ago, where they started experimenting with keeping animals like sheep and goats. But just how many people and animals occupied the site at different times has been trickier to track.

For the new study, researchers from the Universities of Columbia, Tübingen, Arizona and Istanbul realized that the more humans and animals there are on a site, the higher the concentration of salts in the ground. The reason? Everybody and everything pees.

The team began by collecting 113 samples from across Aşıklı Höyük, including trash piles, bricks, hearths and soil, from all different time periods. They examined the levels of sodium, nitrate and chlorine salts, which are all passed in urine.

Sure enough, the fluctuating levels of urine salts revealed the history of human and animal occupation of Aşıklı Höyük. Very little salt was detected in the natural layers, before any settlement existed. Between about 10,400 and 10,000 years ago, salt levels rose slightly, as a few humans began settling. Then things really took off – between 10,000 and 9,700 years ago the salts saw a huge spike, with levels about 1,000 times higher than previously detected. That indicates a similar spike in the number of occupants. After that, concentrations go into decline again.

That large spike, the team says, suggests that domestication of animals in Aşıklı Höyük occurred faster than was previously thought.

Using this data, the researchers estimated that over the 1,000-year period of occupation, an average of 1,790 people and animals lived in the area per day. At its peak, the population density would have reached about one person or animal for every 10 sq m (108 sq ft).

The estimated inhabitants of each time period can't be all human – the houses found on site indicate a smaller population. But the team says this is evidence that salt concentrations can be a useful tool to study the density of domesticated animals over time.

The researchers say this technique could be used in other sites, to help find new evidence of the timing and density of human settlement.

The research was published in the journal Science Advances.

View gallery - 3 images