One hundred years ago, just 11% of humans had a fabella, a tiny bone embedded in the tendon behind the knee. Last year, the percentage of people worldwide with that bone spiked to 39%, and scientists are trying to figure out why a bone that doctors generally think is "pointless" hasn't fallen by the evolutionary wayside, per the BBC. In their study published in the Journal of Anatomy, scientists from Imperial College London note not only the oddly increasing prevalence of the fabella (Latin for "little bean"), but also the fact that those who have knee problems are more likely to have one: Individuals with osteoarthritis of the knee, for example, are twice as likely to boast a fabella. Per The Scientist, researchers looked at X-rays, MRIs, and dissections from 21,000-plus knee studies over the past 150 years, and within the last century, the chances of people having a fabella increased threefold.
This sesamoid bone—so labeled because it grows, much like a kneecap, in the tendon of a muscle—actually seemed to vanish completely from humans "when the ancestors of great apes and humans evolved," making its recent uptick even more puzzling, per a release. Why it's making a comeback, per Dr. Michael Berthaume, the study's lead author, could be tied to what modern-day humans are eating. "We found evidence of fabella resurgence across the world, and one of the few environmental changes that have affected most countries in the world is better nutrition," he says. Berthaume explains that because we generally eat more than we used to, we're heavier than our predecessors, meaning our knees are under more pressure; sesamoid bones like this tend to change based on the forces exerted on them. (This study surprisingly suggests that running may protect against knee issues.)