Evolution of the Knee
Three hundred twenty million years ago there lived Eryops, an amphibian with a long mouse-like nose, a short bulky body, and a thick but tapering lizard-like tail, which is thought to be the common ancestor of not only today′s reptiles but also modern birds and mammals. Eryops had knees whose basic characteristics can be found in those knobby protrusions you find in between your thighbones and your shins. Similar to your knee, the knee of the Eryops had two rounded projections at the knee end of the thighbone (the femur), which were for articulation with another bone, a relatively flat surface, or plateau, at the top of the shinbone (the tibia). The common characteristics of knees between most extant tetrapods have led some scientists to speculate that Eryops even had cruciate ligaments, asymmetrical collateral ligaments, and menisci more than 300 million years ago, just as human knees do today. Those ligaments are the strong bands of tissue unifying your thighbone to your shinbone and are known to many of us through news accounts of athletes who have torn them in play. The menisci are fibrous crescents in between those bones, absorbing the inevitable shocks.
Only four major changes occurred over the next 320 million years to give us the superior joint we call the knee. If you think of these 320 million years being laid out on a football field, with each foot approximately equaling a million years, three of the four changes occurred on one half of the field–in the most recent 170 million years. (You have to keep in mind that there were no modern humans–Homo sapiens–in North America until well within the last half inch of the figurative football field, when Cro-Magnon peoples crossed from Siberia and ventured southward into the American plains by way of western Canada.)
This is a section from Dr. Jack E, Jensen’s book The One Stop Knee Shop. Read the next section The Physiology of Walking.