Cheese found with the bodies of 4,000-year-old mummies in Xinjiang, China was likely a dietary staple for local herding tribes.

That block of cheddar gathering mold at the back of your fridge may be old, but could it survive for millennia?

Lumps of fermented cheese have been found intact in the graves of early Bronze Age herders in northwest China, buried beneath the sands of the Taklamakan Desert for nearly 4,000 years.

The 200 ancient bodies were laid beneath upturned boats covered in cowhide and topped with 13-foot wooden poles that resemble oars and may be symbols of fertility. The men, women, and children buried at the site lived at a time when there were still rivers and lakes in the region.

In research published in the Journal of Archaeological Science, Chinese and German researchers explained that the cheese in the Tarim Basin graves is the oldest ever found, though there is some evidence of cheese-making as early as the 6th millennium B.C. in northern Europe.

Archaeologists don’t know the name of the people buried at Small River Cemetery No. 5, what language they spoke, or when they arrived. But the hot, dry climate of the modern-day Tarim Basin, where the cemetery was found, perfectly preserved their clothing and grave goods—and the snacks they took with them to the afterlife.

Related News: Parasites in Crusaders’ Poop Offer Insight Into Medieval Famine »

The researchers say the cheese is similar to kefir, a fermented milk drink that can be curdled and strained to make a substance that resembles cottage cheese. Kefir is still popular throughout Eastern Europe. It’s naturally lactose-free and is traditionally made by dunking bacterial “grains” into cow, sheep, or goat milk.

According to Susan Weiner, a dietitian and certified diabetes educator in New York, kefir is a tart tasting drink that is a good source of calcium, phosphorus, protein, vitamin B12, and thiamin.

Kefir, and the cheese found in the Tarim Basin graves, also contains helpful bacteria called probiotics.

“Probiotics are ‘good bacteria,’ which can help with the heath of the human gut,” Weiner told Healthline. “Heating or baking of cheese kills most bacteria. So, grilled cheese sandwiches or pizza, for example, aren’t good sources of probiotics. Gouda cheese is a particularly good source and can ‘survive’ the journey through your [gastrointestinal] tract.”

Learn About the Surprising Health Benefits of Probiotics »

In addition to aiding digestion, probiotics like Lactobacillus can also improve the health of the human immune system, research suggests. But unlike the diets of Bronze Age herders, the diets of modern Americans tend to be low in probiotics from dairy and other sources.

“If we eat mainly processed foods in large quantities, we may not get enough probiotics. Also, many people eat processed cheese spreads or pizza which are not good sources of probiotics,” Weiner said. “Research shows that the elderly might benefit from consuming more probiotics. It seems to give a boost to the immune system.”

Are Probiotics an Effective Treatment for Ulcerative Colitis? »

The archaeologists who studied the ancient Tarim Basin people say that being able to produce cheese on a large scale would have made their animal herds more valuable, both as a food source and as a financial investment.

“Kefir fermentation of…milk by a symbiotic culture of Lactobacillus kefiranofaciens and other lactic acid bacteria and yeasts was the basis of robust, scalable, probiotic, lactose-free dairy and a key technological advance that introduced economic benefits of extensive herding into a semi-pastoral…population,” the researchers concluded.

They add that the cheese was probably not intended to last for long periods of time (the shelf-life of unrefrigerated kefir is as short as three or four days). The Tarim Basin tribes could never have known that their creamy treat would survive for the ages.

Read More: Probiotic May Prevent Acid Reflux, Constipation, and Colic in Infants »