Multiple ways to evolve tiny knee bone could have helped humans walk upright

The evolution of bones in primates’ knees could have implications for how humans evolved to walk upright, a new study has found.

Researchers from King’s College London analyzed the presence of the lateral fabella, a bone in the knee the size of a sesame seed, in 93 different species of primates.

They found that while most primates have these bones, they are often absent in hominoids, the group of primates that humans belong to alongside chimpanzees, gorillas, gibbons, and others.

Yet humans have evolved to have this bone differently from most primates and it is not found in all people. The researchers say this distinct evolutionary pathway of the fabella in humans could point to an evolutionary change that helped the ancestors of humans walk upright.

The lateral fabella is a sesamoid bone behind the knee which is twice as common in people with knee osteoarthritis. Sesamoids, like the kneecap, are small bones embedded in tendons or ligaments that are sometimes present within mammalian skeletons, which means they can be absent in some mammals, but present in others of the same species.

In the new study, published in Proceedings of the Royal Society B, scientists used statistical modelling to examine the presence of three sesamoid bones in the knee – the cyamella, medial fabella and lateral fabella – using research published over the last 100 years.

The scientists discovered that primates with faballae were 50 times more likely to have ancestors who also had them. The team also found that the medial and lateral fabella almost always develop in pairs, except in rare cases like humans, who only have a lateral fabella.

Further analyses suggest hominoids may have evolved a way to grow fabellae different from other primates, which could explain why humans can grow a lateral fabella without a medial one, but other primates cannot.

Want the latest science news straight to your inbox? Become a SelectScience member for free today>>