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

The evolution of primate knee bones could have implications for how humans evolved to walk upright, a new study finds.

Researchers from King’s College London analyzed the presence of the lateral fabella, a knee bone the size of a sesame seed, in 93 different primate species. The results are published in the journal Proceedings of the Royal Society B: Biological Sciences.

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

However, human evolution has made this bone different from most primates and it is not present in all individuals. Researchers say this distinct evolution of the fabella in humans could indicate an evolutionary change that helped human ancestors walk upright.

Dr Michael Berthaume, Senior Lecturer in Engineering at King’s College London and author of the study, said: “Our study shows that this often misunderstood bone could have evolved in multiple ways in primates, with humans having a distinct form of evolution that began just as early as all hominoids.

“Using this bone in a new way could have helped early humans, such as Australopithecus, transition from walking on all fours to walking upright.”

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

Despite hundreds of years of research, little is understood about how sesamoids evolve, how they develop, and why they are present in some people and not others.

In the new study, scientists used statistical modeling to examine the presence of three sesamoid bones in the knee (the cyamella, medial fabella, and lateral fabella) based on research published over the past hundred years.

The scientists found that primates with fabellae were 50 times more likely to have ancestors who also had them. The team also found that medial and lateral fabella almost always develop in pairs, except in rare cases such as humans, which have only one lateral fabella.

Other analyses suggest that hominoids may have evolved a different way of developing fabellae than other primates, which could explain why humans can develop a lateral fabella without a medial fabella, but other primates cannot. This could sum up more than a century of research, during which scientists have debated how these bones evolved.

The distinct evolutionary pathway of the fabella’s “re-emergence” in humans could indicate an evolutionary change that helped human ancestors walk upright.

Dr Berthaume said: “In almost all other cases, apart from humans, we know that primates have evolved fabella in pairs. We also know that the way primates move does not necessarily affect the evolution of these bones, so why are humans different?”

“Interestingly, this ‘uncoupling’ could be a sign of exaptation, when something in the body already exists and begins to be used for other purposes – in this case, bipedalism. Further work on the biomechanics of the fabella is needed to confirm this, but the preliminary results are promising.”