In 1978, fossilized footprints preserved in volcanic ash were discovered in Laetoli, Tanzania, by legendary anthropologist Mary Leakey, proving that our human ancestors were already striding across the landscape 3.6 million years ago.

The discovery was monumental, because it contradicted a long-standing idea about human evolution, perhaps debunking the theory that early humans developed a large brain before they stood up. Host Kallie Moore explains in this episode of PBS Eons.