The first change occurred 180 million years ago during the Jurassic Period, when the femur angled in toward the midline and brought the knee’s orientation to the front. With this change, walking became more efficient. The next event occurred in the Cretaceous period, during the time of the emergence of the earliest mammals (protomammals). The fibula (the outer and smaller of the two bones between the ankle and the knee) moved farther away from the front and farther below the joint line. The next two changes came at the extremes of the Cenozoic Era, the current geological era. First was the development of the bony kneecap (the patella), about sixty-five to seventy million years ago and not in a common ancestor but apparently in reptiles, mammals, and birds independently. Second was the development late in the Cenozoic Era of the bipedal gait in humans’ ancestors. This change incorporated a deviation of the femoral epiphysis from the general angle of the long bone itself, making it possible for the knees to come closer to midline.
To understand this in crude terms, think of how we imitate the gait of an ancient creature in telling a story: “BOOM, BOOM, BOOM,” we say to the preschooler, as our heads go side to side abruptly. That’s because we think of the legs as coming straight down, much as the legs of a robot or those in a child’s primitive drawing do, without any indication of the angulation of the bone from the hip to the knee.
In chimpanzees, walking upright produces the same back and forth movement because the thigh bone does not angle toward the midline as it does in humans. To keep their balance and stay upright, they must move radically side to side to keep their center of gravity over the weight-bearing leg. As with the robot, their wide-apart feet generate what might be called more properly a bipedal waddle rather than a walk.
Though all four changes over the last 300 million years are extremely important, so are the constants. The striking similarities between the knees of diverse species remain astonishing. The knee has worked for more than 300 million years with few anatomical changes despite dramatic changes in functional demands.