Evolution has enabled us to walk upright in a locomotive form known as bipedalism. While this form of motion is quite energy-efficient, it is not what our muscular-skeletal system is best suited for—something that many people with back and/or knee problems can attest to. Bipedalism evolved in human predecessors somewhere around 6 million years ago, and there are a number of theories as to why bipedalism became the dominant mode of locomotion. The shift to walking upright occurred before our ancestors' brains started to grow to anywhere near the size of modern humans, and before they had begun to use rudimentary stone tools.

This change in locomotive methodology brought with it a host of anatomical changes; our feet, knees, hip, spine, and even our skull all had to change to accommodate this mode of moving. This week's edition of Science features a cover article describing the find of some very old hominid footprints. Dating to around 1.5 million years ago, the series of footprints found at Ileret, Kenya gives anthropologists a clear view of what our ancestors feet looked like at that time, and it turns out they looked quite... human.

Two surfaces in the Okote Member of the Koobi Fora Formation near Ileret, Kenya were found to contain the fossilized footprints of early hominids. The upper surface contains three hominid footprint trails— two trails containing two prints, and one containing seven distinct prints. Five meters below this surface, another set of prints was found in a single trail of two footprints, and a single isolated hominid footprint fossil. By examining the geological neighborhood of the sedimentary layers that contain the footprints, researchers have been able to determine that the upper prints date to 1.51 to 1.52 million years ago, while the lower set of prints dates to 1.53 million years ago. These represent the second oldest known set of hominid footprints, the oldest being the 3.75 million year old footprint found by Mary Leakey in Laetoli, Tanzania that is thought to have been made by an Australopithecus afarensis.

Footprints reveal a good deal about how a creature walks—they graphically illustrate the pressure distribution generated during a step or stride. Under normal walking conditions, there is a highly predictable movement and pressure distribution. Modern humans begin with our heel making the first contact with the ground; the pressure is then transferred along the outside of the foot, travels across the ball of our foot, and we finally push off using our hallux—the big toe (the one that went to market).