It is almost the 40th anniversary of one of the most pivotal discoveries from paleoanthropology: Donald Johanson's discovery of "Lucy," a 40% complete skeleton of primitive human ancestor. Why was she so exciting?

Because Lucy walked upright. Not only that, but she was by far the oldest and most complete skeleton that evidenced bipedality. Clues enclosed within the fossils demonstrate this indisputably. These clues include the angle of the femur (imagine a human's leg angle to its pelvis compared with a quadripedal monkey, such as a rhesus macaque), the shape of the kneecap (patella), the pelvis shape, and the spinal curvature necessitated by upright walking.

For further reading, check out the top 7 human evolution discoveries from Tanzania:

Or the beautifully illustrated book "From Lucy to Language," written by Johanson.