Fossilized Footprints
How do we know if an early ape-man or woman walked upright? An examination of certain bones -- a tibia (leg bone) or a pelvis, for example -- can reveal the answer. So can fossilized footprints.
In 1976, members of a team led by Mary Leakey discovered the fossilized footprints of human ancestors in Laetoli, Africa. The footprints were formed 3.5 million years ago when at least two individuals walked over wet volcanic ash. The wet ash hardened like cement and was then covered by more ash.
The footprints show that the individuals had perfect, two-footed strides. They also reveal that one hominid was larger than the other. And because the footprints fall next to each other, they indicate that the two hominids were walking side by side and close enough to each other to be touching.
Apes sometimes walk on two legs. How, then, can we be sure that the footprints weren't left by a couple of apes that decided to walk upright for a few yards? When an ape walks upright, weight is transmitted from the heel, along the outside of the foot, and then through the middle toes. A human foot transmits weight from the heel, along the outside of the foot, across the ball of the foot, and finally through the big toe -- this is a much more efficient way to transfer energy when walking upright. The imprints left behind at Laetoli clearly show the weight distribution of true upright walkers.
The footprints also look remarkably like a human's. In fact, they looked so human-like, some scientists had a hard time believing that they were made by Australopithecus afarensis (Lucy's species), the only human ancestor known to have lived at the time.