Laetoli footprints

At Laetoli in northern Tanzania, a trail of footprints about 27 meters long preserves the steps of early humans who walked across wet volcanic ash roughly 3.66 million years ago. The trail includes about 70 prints made by at least two and probably three individuals walking in the same direction, and the prints show a humanlike stride. The tracks are generally attributed to Australopithecus afarensis, the species of Lucy, whose teeth and jaws were found nearby.

The prints were made after a nearby volcano, Sadiman, ejected ash. A light rain dampened the ash, and the walkers left their prints in the soft surface. Then more ash fell and the surface hardened like cement, and the prints were buried and preserved for millions of years. The first tracks were spotted in 1976 by members of an expedition led by Mary Leakey, and the main trail was excavated in 1978 and 1979. Studies of their shape show a heel strike first, then a rolling of weight to the ball of the foot and a push off the big toe, with the big toe lined up with the others, not splayed outward like an ape's.

The tracks showed that walking upright on two legs came long before large brains or stone tools. At the time, many scientists expected that big brains came first, so evidence of a small-brained, upright walker was a surprise. Some researchers argue the walkers still moved with bent knees and hips, a more ape-like gait than ours, but the main conclusion that these hominins were committed bipeds is widely accepted. The site was reburied in 1979 to protect it, and it has been studied again with modern techniques.