Human knees kind of suck — here's why we haven't evolved better ones

The groans of pain as we get up from the sofa or the sound of crunching cartilage when taking the stairs are all too familiar. Many of us look down at our aching knees and curse them — wondering why they seemingly evolved to hurt so much. But the human knee has a complex evolutionary History. And new research is showing how misunderstood it is.

The knee has undergone major changes to its size and shape, not only to allow early humans to walk upright, but also to differentiate us (Homo sapiens) from our extinct genetic relatives, such as Homo erectus and Homo neanderthalensis (Neanderthals).

Natural selection, acting with other evolutionary forces, like random mutation or genetic heritage, probably shaped the knee to help us walk on two legs more efficiently and for longer than our relatives.

Many of the knee problems we face today are new problems our ancestors did not experience. For example, in 2017, research suggested that the sedentary Lifestyle of the post-industrial world may have led to a 2.1-fold increase in the rate of knee osteoarthritis, the most common form of arthritis of the knee.

When the researchers studied the remains of hunter gatherers who lived up to 6,000 years ago, they discovered that knee osteoarthritis was probably not a problem at all back then. In the U.K. today, over a third of people over 45 have sought treatment for osteoarthritis — primarily for the knee.

Weaker muscles for stabilizing and protecting joints and relatively weaker cartilage to cushion the scraping of bones are probably the result of humans moving a lot less than they used to — sitting in an office or running on a treadmill builds less muscle than hunting deer for most of the day in challenging terrain. For us to evolve osteoarthritis-free knees, sedentary people with "good" knees would need to have more children than sedentary people with "bad" knees for many generations.

But it gets more complicated. The knee is an intricate piece of biological machinery that scientists don't fully understand.