The Laetoli Footprints

A trail of 69 footprints captured in volcanic ash. Discovered by a team led by Mary Leakey in 1978 in Laetoli, Tanzania. They are around 3.6 million years old. 

They are a rare treasure in the record of human ancestry, as the fossils can be given an absolute date.  Around 3.6 million years ago, a volcano now called Sadiman puffed out a cloud of ash that blanketed the surrounding area. A light rain then turned the ash into a sort of cement that recorded thousands of tracks of antelopes, rhinos, guinea fowl, and monkeys, as well as the footprints of our ancestors. Volcanic rock ­ like the trail at Laetoli ­ can be dated by a method called potassium-argon dating. Hot, newly erupted lava and ash contain a form of the chemical element potassium (called potassium-40) that is radioactive. Over time, potassium-40 changes, or decays, into a different material, called argon-40. By comparing the ratio of potassium to argon, scientists can gauge how long the age of the ash or rock. This can be "pinpointed" to within roughly twenty thousand years.

Whoever left the footprints moved forward of two feet, probably with a smooth, stable way of walking. The left and right footprints lie lose together, as they would for a human, but not a chimp. Weight fell first on the heel, then on the outside of the foot, and then finally on the ball and big toe for the push-off to the next step.