Lucy's Knees

By Richard P. Holm MD

In 1974 in Ethiopia’s Awash Valley, a 3.2 million year-old skeleton of an ape was discovered that was different than other ape skeletons. The knee bone shape, along with pelvic architecture, indicated that this ape walked upright. As the Beatles music “Lucy in the Sky with Diamonds” was playing in the background, archeologists speculated that this could be the long sought link between apes and humans, and the upright walking individual was famously nicknamed “Lucy.” Her brain was small and ape-like but she walked upright.

This was evidence that the upright position might have come first. They speculated that standing tall allowed for the evolutionary advantage of having a better view of approaching enemy or mate and all the rest followed. Could it be that the special design of an upright knee allowed for the first big step toward the evolution of humanity? And what is so special about this design?

The knee is a hinge joint mostly held together with four ligaments. The two “collateral” ligaments run along the inner and outer sides of the knee keeping our legs from bending inward (knock-kneed) or outward (bowlegged).

The more noteworthy structures however are the two tough fibrous ribbon ligaments, which cross each other, front to back, on the inside of the knee forming an "X.” This explains why they are called the cruciate or cross-like ligaments.

The anterior cruciate ligament or ACL starts at the back of the thighbone or femur above, crosses to connect at the front of the shinbone or tibia below, and keeps the lower leg from sliding forward.  The posterior cruciate ligament or PCL starts at the front of the thighbone, crosses to the back of the shinbone, and keeps the lower leg from sliding backward.

What is so ingenious is how these crossing ribbons provide for such stability, and yet at the same time, allow for the bending of the knee. So it is as Gerard Manley Hopkins the Priest poet said: “The world is charged with the grandeur of God.”