by The groan of pain as we get up from the couch or the sound of cracking cartilage as we climb the stairs are all too familiar. Many of us look at our aching knees and curse them – wondering what made them evolve to hurt so much.
But the human knee has a complex evolutionary history. And new research shows how misunderstood it is.
The knee has undergone major changes in size and shape, not only to enable early humans to walk upright, but also to distinguish us (Homo sapiens) of our extinct genetic relatives, such as Homo erectus And Homo neanderthalensis (Neanderthals).
Natural selection, in concert with other evolutionary forces such as random mutations or genetic inheritance, has likely shaped the knee so that we can walk on two legs more efficiently and for longer periods of time than our relatives.
Many of the knee problems we face today are new problems that our ancestors didn’t have. For example, research in 2017 suggested that the sedentary lifestyles of the post-industrial world may have led to a 2.1-fold increase in the incidence of knee osteoarthritis, the most common form of arthritis of the knee.
When researchers studied the remains of hunter-gatherers who lived up to 6,000 years ago, they found that knee osteoarthritis was probably not a problem at all back then. In the UK today, more than a third of people over the age of 45 have sought treatment for osteoarthritis, mainly in the knee.
Weaker muscles to stabilize and protect joints and relatively weaker cartilage to absorb the scraping of bones are likely the result of people moving much less than they used to. Sitting in an office or running on a treadmill builds less muscle than spending most of the day hunting deer in challenging terrain.
To develop knees free of osteoarthritis, sedentary people with “good” knees would have to have more children over many generations than sedentary people with “bad” knees.
But it gets more complicated. The knee is a complex piece of biological machinery that scientists don’t fully understand.
This is especially true for sesamoid bones – small bones that are embedded in tendons or ligaments, such as the kneecap. These bones can be found anywhere in the mammalian skeleton.
This means that some mammals can have sesamoid bones, while even members of the same species do not. An example of this is the lateral fabella, which is located behind the knee and is found in an average of 36.80 percent of human knees today.
Despite hundreds of years of research, little is known about the evolution, growth, development of sesamoids, and why they are present in some species and not in others. This is so bad that sesamoids are often missing from the articulated skeletons you see in museums, discarded with the muscles in which they were embedded.
New research from my colleagues and me has shown that two of these often misunderstood bones, the medial and lateral fabellae, which are located behind the knee, may have evolved in different ways in primates and helped early humans learn to walk upright.
The study was a systematic review of three sesamoid bones from 93 different primate species, including other hominids and common ancestors of humans.
Our research shows that humans have undergone a separate form of evolution for these bones. This evolution may have begun at the origin of hominoids, a group of primates that includes apes and humans.
Scientists believe that repurposing the existing fabella bone, something called exaptation, may have helped early humans go from walking on four limbs to two. Interestingly, this bone is also linked to higher rates of osteoarthritis. People who have it are twice as likely to develop the condition. Evolution is not a simple path to biomechanical efficiency.
This picture gets even more complicated when we realize that unlike teeth, knees are “plastic,” meaning they shift and change depending on factors such as diet and use. Teeth, on the other hand, (once they grow) do not adapt and simply get damaged. This is why it is so important to exercise as we age—to keep our bones strong.
Knees change and adapt in response to their use, or lack thereof. For example, a global increase in diet that causes people to grow taller and heavier is the leading hypothesis for why fabellae are becoming more common. The fabella has tripled in abundance in the past 100 years, with some variation worldwide.
We know that the evolution of the knee in humans has not been straightforward, and has instead had branching paths. We also know that we live in a way that our bodies are poorly adapted to, and lifestyle changes are likely the culprit for knee problems that have become more severe over time.
The knee was not designed for the times we live in and the bone that helped us walk in the first place may be a major cause of those problems.
So if you have wobbly knees on the treadmill or pain when you sit, think about them. Evolution is not as easy as it seems.
Michael Berthaume, Lecturer in the Department of Engineering at King’s College London
This article is republished from The Conversation under a Creative Commons license. Read the original article.
