The human joint is a wonderfully flexible and durable evolutionary innovation, but like any good machine eventually it wears down. And in many people, this wearing is thought to cause arthritis.

Pain from arthritis strikes some 54.4 million U.S. adults, and is "one of the most common chronic conditions in the nation," according to the Centers for Disease Control website. The disease causes stiffness, swelling and pain in the joints and has been found in humans for thousands of years. (Scientists even identified evidence of arthritis in Nefertari's mummified knees.) But researchers have long assumed that arthritis rates have spiked in recent years as people live longer and populations grow heavier. Now, as Mitch Leslie reports for Science, a study of ancient knees has finally provided evidence to support the trend, and suggests that arthritis may not be an inevitable fate of old age.

To tease out the history of arthritis, Harvard University biologist Ian Wallace studied skeletons of middle-aged and elderly people from various time periods of America, including specimens from Native Americans up to 6,000 years old. He thought that perhaps in the early days of humanity—when when walking was the main way to get around and many people spent their lives hunting, farming or fighting—the rates of arthritis would actually be fairly high due to the joint stress from all this activity.

But this wasn't the case.

Instead, it appears that osteoarthritis of the knees affects far more Americans today than even just a few decades ago, Leslie reports. And after controlling for weight and age, the results suggest that these factors have no effect on how many people develop the disorder. Strikingly, the rate of osteoarthritis has more than doubled among Americans just since 1940. Wallace and his team pubished their results earlier this month in the journal Proceedings of the National Academy of Sciences.

“We were able to show, for the first time, that this pervasive cause of pain is actually twice as common today than even in the recent past," Wallace says in a statement. "But the even bigger surprise is that it’s not just because people are living longer or getting fatter, but for other reasons likely related to our modern environments.”

The study doesn't make any conclusions for why this spike has occurred, but study co-author Daniel Lieberman suggests that the epidemic of sitting in mondern-day America could be affecting how our joints are formed and maintained, leading to more arthritis, Richard Harris reports for NPR. Changing diets and the rising rates of injuries from sports among children and adults could also play a role.

Though cause is still unknown, the study's results suggest that the disease may not be as inevitable as once believed. “We should think of this as a partly preventable disease," Lieberman says in a statement.

Today, there is no true "cure" for arthritis, only management of pain, such as taking medications, wearing splints and losing weight. In 2003, Americans spent some $80.8 billion on diagnosis and treatment of the disease. But researchers hope to eventually stem the flow of that money. The latest study gives hope that with continued testing of treatments and ways to prevent osteoarthritis, we can eventually beat this ancient ailment.