The changes in knee bones of primates may shed light on how humans adapted to bipedal walking.
Research reveals that the knee bone evolution in primates may influence our understanding of human upright walking development.
A team from King’s College London studied the lateral fabella, a tiny bone, about the size of a sesame seed, located in the knee joint, across 93 primate species.
They found that while many primates possess this bone, it is often missing in hominoids—the group that includes humans, chimpanzees, gorillas, gibbons, and several other species.
Interestingly, humans have a unique evolution of the fabella compared to other primates, and it is not universally present among the population. The researchers suggest that this particular evolutionary trend regarding the fabella in humans could indicate a pivotal change that aided our ancestors in walking upright.
Dr. Michael Berthaume, an Engineering Reader at King’s College London and the study’s author, stated: “Our findings indicate that this frequently misunderstood bone may have evolved in various forms among primates, with humans showcasing a unique evolution traceable back to the earliest hominoids.”
“Utilizing this bone differently could have facilitated early humans, such as Australopithecus, to transition from quadrupedal locomotion to walking on two legs.”
The lateral fabella is a sesamoid bone located behind the knee and is found in higher frequency among individuals with knee osteoarthritis. Sesamoid bones, like the kneecap, are small bones that can be embedded in tendons or ligaments, and their presence may vary among different mammals.
Despite extensive research over centuries, the evolution and development of sesamoid bones remain largely unclear, as does the reason their presence can differ among individuals.
The recent study, published in Proceedings of the Royal Society B, utilized statistical models to explore the occurrence of three sesamoid bones in the knee—the cyamella, medial fabella, and lateral fabella—drawing on research conducted over the last century.
The researchers discovered that primates with fabellae are 50 times more likely to be descended from ancestors that also had them. Additionally, they noted that the medial and lateral fabellae typically form in pairs, except for unusual instances like humans, who generally have only the lateral fabella.
Further investigations suggest that hominoids might have developed a distinct method of fabella growth compared to other primates, potentially explaining why humans can develop a lateral fabella without a medial counterpart—something other primates are unable to do. This could clarify over a century of discussions regarding the evolution of these bones.
The unique evolutionary path of the fabella resurfacing in humans may highlight a critical evolutionary transition that supported the bipedal movement of our ancestors.
Dr. Berthaume remarked: “We know that in nearly all instances, except for humans, primates develop fabellae in pairs. Moreover, the movement patterns of primates do not necessarily influence how these bones evolve, so what makes humans different?”
“Excitingly, this ‘decoupling’ might indicate an exaptation—when a pre-existing body feature begins to serve a new function, like bipedalism. More detailed research on the biomechanics of the fabella is required to confirm this, but initial findings are encouraging.”
