A useless bone that only causes problems is making a comeback, and doctors aren't sure why.
A hundred years ago only 11.2 percent of people had a fabella, a tiny bone in the back of the knee - but a new study has found 39 percent of us have it now.
We don't know what the fabella's function is - nobody has ever looked into it," said lead author Michael Berthaume of Imperial College London.
"The fabella may… help reduce friction within tendons, redirecting muscle forces, or - as in the case of the kneecap - increasing the mechanical force of that muscle. Or it could be doing nothing at all."
Even stranger is that the bone was previously on the way out. Analysis of knees from 1875 found it 17.9 percent of humans had it.
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But its presence is bad news. People with a fabella are twice as likely to develop osteoarthritis. It can also cause pain on its own, and can be an obstacle to knee replacement surgery.
"As we evolved into great apes and humans, we appear to have lost the need for the fabella," said Dr Berthaume. "Now, it just causes us problems - but the interesting question is why it's making such a comeback."
The best explanation he's come up with is that we're eating better.
"The average human, today, is better nourished, meaning we are taller and heavier. This came with longer shinbones and larger calf muscles - changes which both put the knee under increasing pressure. This could explain why fabellae are more common now than they once were."
He said one day, doctors may consider the fabella the appendix of the skeleton, referring to the organ which serves no purpose but can become inflamed and require removal.
The study was published this week in the Journal of Anatomy.