How Knee Bone Changes Helped Humans Walk Upright

A recent study has uncovered intriguing details about how the evolution of bones in primate knees might have influenced human bipedalism. Researchers from King's College London investigated the lateral fabella, a tiny bone in the knee, across 93 primate species. Their findings are detailed in the journal Proceedings of the Royal Society B: Biological Sciences.

The study revealed that while many primates have the fabella, it is often missing in hominoids, which include humans, chimpanzees, gorillas, and gibbons. Interestingly, humans have a unique evolutionary path for the fabella, which is not present in all people. This distinct evolution might have played a role in the shift of our ancestors from walking on all fours to walking upright.

Dr. Michael Berthaume, a Reader in Engineering at King's College London and the study’s author, explained that the research shows the fabella could have evolved differently among primates. In humans, this bone might have been adapted to support upright walking early in our evolutionary history.

The lateral fabella is a sesamoid bone located behind the knee and is twice as common in people with knee osteoarthritis. Sesamoid bones, like the kneecap, are small bones embedded in tendons or ligaments. They can vary among mammals, being present in some but not others.

The study employed statistical modeling to analyze the presence of three sesamoid bones in the knee: the cyamella, medial fabella, and lateral fabella. The researchers found that primates with fabellae were 50 times more likely to have ancestors with these bones. Additionally, while the medial and lateral fabellae usually develop together, humans often only have the lateral fabella.

Further analysis suggests that hominoids might have evolved a different pattern of fabella development compared to other primates. This could explain why humans can develop a lateral fabella without the medial one, unlike other primates. This discovery helps to clarify over a century of debate about the evolution of these bones.

Dr. Berthaume proposed that this unique development of the fabella in humans might be an example of exaptation, where a body part originally evolved for one function is repurposed for another—in this case, aiding in bipedalism. Although further research is needed on the biomechanics of the fabella, the initial results are promising and could shed light on how our ancestors adapted to walking on two legs.