Some people have an extra bone in their knee, known as a fabella. No one has identified a purpose for it and it increases the risk of arthritis, so it's hardly surprising it was becoming increasingly rare. Yet a study of human knees over the last century and a half has revealed the fabella is bouncing back at a rate too fast to be natural selection.
Dr Michael Berthaume of Imperial College London led a team that looked at 58 studies of human knees, including records of 21,000 individuals, starting from 1875. At the start of that period, 17.9 percent of people whose knees were examined had a fabella, which grows in a tendon behind the knee.
By 1918, this had fallen to 11.2 percent, an oddly rapid shift. Fabellae may put people at an evolutionary disadvantage, but it’s a small one. A slightly higher risk of arthritis as you age wouldn't be expected to have a big impact on the number of children you raise, certainly not enough to see a third of fabellae disappear within two generations.
What came next is even stranger. By 2018, 39 percent of the population were recorded as having fabellae, which are more common in many non-human animals.
“We don’t know what the fabella’s function is – nobody has ever looked into it!” Berthaume said in a statement.
These swings in frequency are orders of magnitude faster than would be expected from natural selection, so something else must be going on. One possible explanation lies in the population that is being sampled. Asians and Australians are more likely to have fabellae than Europeans and South Americans. The study used data from 27 countries, but century-old records are far more comprehensive for some places than others. However, Berthaume and the co-authors of a paper in the Journal of Anatomy compared studies of fabellae frequency before and after 1960 in four nations and found in all cases fabellae rates had risen.
Fabellae don't always show up on X-rays or MRI scans, so detection rates, rather than occurrence, might have changed. However, the authors looked at studies of 10 other bones that would be similarly easy to overlook, and found no equivalent change in frequency over time.
Consequently, the paper argues, the presence of fabellae must have an environmental, as well as a genetic component. Complicating matters, almost a third of people who have the bone only have it in one knee.
The authors believe the knee bone's connected to the thigh bone. They propose better nutrition may be behind the fabella’s return by increasing leg length and muscle mass, stimulating bone formation. The mystery is not solved, however, since the presence of fabellae (Latin for little bean) does not appear to correlate with height in adult humans.