More than 7,000 years ago, in what is now northern Poland, a hardy band of farmers came up with a clever way to preserve the milk from their cows.

They poured it into special pottery vessels with holes in the bottom, thereby separating the solids from the precious fluid to make cheese.

That is the conclusion of a study published Thursday in the journal Nature based on a chemical analysis of the pottery - the earliest hard evidence for cheese-making by ancient humans.

The seeds for the discovery were planted decades ago with a string of chance encounters that began with the travels of a young Polish American college student from Philadelphia.

That man, Peter Bogucki, now an associate dean at Princeton University, collaborated on the study with chemists from the University of Bristol in England, illustrating how a simple dairy product was a major economic innovation.

These days, cheese may be a gourmet treat or simply a layer on a sandwich, but for early farmers, it was likely important for survival. The manufacture of cheese allowed for easy storage and transport of a good source of protein, the authors said.

Perhaps equally important, it was digestible. Lactose intolerance was almost universal among adults of that era, but cheese contains little lactose because it is separated during the straining process.

The study authors showed that fatty acid residue on the pottery had come from cow milk by measuring the ratios of two isotopes of carbon, a technique developed by University of Bristol chemist Richard P. Evershed. Other pottery from the same era had fatty residue with a different chemical signature, indicating they were used for cooking meat.

Oliver Craig, an archaeologist at the University of York who was not involved with the research, said the authors had made a convincing case.

"Chemically, it's very eloquent, and very nice work," Craig said.

The discovery got its start in 1973, when Bogucki, then an anthropology undergraduate at the University of Pennsylvania, went to Poland on a summer program. Seeking to impress a young woman on the trip, he invited her to an archaeological museum.

The museum exhibits piqued his curiosity, and later, while doing graduate work at Harvard, Bogucki decided to return to Poland. He participated in excavations that unearthed the pottery with the strange holes, among other artifacts, but he did not immediately know what to make of them.

Fast-forward to the early 1980s, by which time he was married to Virginia, the young woman from the summer program. They were visiting a friend in Vermont who collected 19th-century farm artifacts. One of them, curiously, was a ceramic vessel with holes.

Told that it had been used to make cheese, Bogucki said, "Oh! We have vessels like that from 7,000 years ago in Poland."

He wrote up his theory that the Polish pottery was used to make cheese, marshaling various bits of evidence. For example, he also had found cattle bones at the archaeological sites in Poland. He reasoned that they were dairy cattle because cows take a long time to reach their full weight and they give birth to one calf at a time.

"Keeping cattle was not sensible if all you wanted was meat," Bogucki said.

But it was not until Evershed's chemical evidence that the story was complete, Bogucki said. Evershed and colleague Mélanie Salque took very small samples of the pottery and crushed them, then added solvents to extract the fatty residue. Archeologists from Poland also contributed to the research.

Traces of milk fat have been found in ancient pottery from Turkey, but those containers did not have holes. In the Polish pottery, the combination of the chemical evidence with the holes is conclusive for cheese, the study authors wrote.

They said it was likely to have been soft cheese, which would have been easy to make with those tools. But they were unable to discern the precise type.

One part of the picture does not make complete sense, said Craig, the University of York archaeologist. About the same time the pottery found in Poland was made, some Europeans developed a genetic mutation that allowed them to digest lactose into adulthood. Before then, humans lost the ability to tolerate lactose after early childhood.

So why would both developments be necessary - the ability to digest lactose and the ability to remove the lactose from milk by making cheese?

Good question, Bogucki said. It is possible the two developments arose in different parts of Europe. In any event, whether people needed cheese for its digestibility, it was certainly valuable for its easy storage, Bogucki said - especially with the addition of salt.

He marveled at the progression of events that culminated with the research.

"The string of connections is like out of some Herman Wouk novel or something like that, where people keep crossing and running into things and being in the right place at the right time," Bogucki said.

Though the discovery was made in Poland, the Poles of today have no special reputation for superiority in cheese-making.

A country better known for its fromage is France, a fact acknowledged with some amusement by Salque, who hails from that country.

"Maybe the French are still the first that made cheese," she joked, "but we can't prove it."