About 7,000 years ago, dairy farmers in modern-day Poland used clay strainers to turn cow's milk into cheese.
The new discovery offers the earliest evidence yet of cheese-making, which began before people developed the ability to digest the lactose sugars in unprocessed milk.
Not only did cheese, which contains very little lactose, provide a valuable source of nutrition for prehistoric Europeans. It also allowed them to store milk in a form that was easy to transport and would keep for months without spoiling.
"The interesting thing is that people at that stage could not digest the lactose in the milk, so processing milk into cheese would have given them the benefit from the nutritious effects of milk without having the side-effect of being ill," said Mélanie Salque, a chemist at the University of Bristol in the United Kingdom.
"It was a very good product for them because you don't have to kill animals to get the milk out of them," she added. "Milk was a big development and cheese was as well."
Some 30 years ago, archaeologists described sieve-like pottery fragments found in a region of north-central Poland, where some of the region's earliest farmers settled. The shards dated back to between 7,200 and 6,800 years ago. And the holes in the sieves were tiny, just two or three millimeters (about a tenth of an inch) wide.
Alongside the clay fragments were lots of cattle bones, leading some scientists to speculate that the reconstructed bowl-shaped containers were cheese-strainers. But without proof, other hypotheses have endured, including the possibility that the vessels were used to strain chaff while making beer.
In an attempt to figure out once and for all what the containers were for, Salque and colleagues conducted detailed chemical analyses on 50 fragments from 34 vessels. They were looking specifically for residues of fats, which get absorbed by pores in clay during food processing and can remain trapped for millennia.
After crushing a small amount of each fragment into a powder, mixing the powder with solvents, and running the solution through their instruments, the researchers detected milk residues in all but one of the pieces.
People wouldn't have strained milk before drinking it, and butter is not processed that way. The only possible explanation for what they found, the team reports today in the journal Nature, is that people were coagulating milk and then using the sieves to separate semi-solid curds from liquid whey to make cheese.
"They have these vessels that look like they could be strainers for making cheese, but what they've really done now is confirmed that that's the case," said Mark Thomas, an evolutionary geneticist at University College London.
The people who made the vessels belonged to a culture called the Linearbandkeramik. They were the first major farmers to settle in interior central Europe. In his own work, Thomas has been able to show that these farmers had not yet developed the gene for digesting milk as adults when they were making the cheese-strainers.
Instead, the new study shows that the Linearbandkeramik people came up with a clever way to reduce the lactose content of their milk and make it nutritious and digestible, Thomas said. They also created distinct ceramic pots for solving a unique dietary problem. They weren't necessarily the first people to make cheese, but the new discovery is the earliest direct evidence of the process.
"Making cheese is not trivial — it takes a lot of skill and a lot of knowledge and there are a lot of things to get wrong," Thomas said. "These guys were a lot smarter about these things than many people have previously given them credit for."