Roughly 3.6 millions years ago the Sadiman Volcano erupted covering the surrounding ground in ash; rain soon followed making a soft and muddy ground over which prehistoric animals trekked. More ash then erupted from the volcano covering and, in effect, preserving the prehistoric tracks.
Forward to 1978, when a group of researchers (led by paleoanthropologist Mary Leakey) discovered the preserved tracks, within which were the oldest known footprints of human ancestors. From the prehistoric footprint’s shape, length, and toes it was evident that these hominids were bipedal and were walking upright on two legs, before even the use of stone tools. The prints were left by two or three hominids (one or two adults and a child) and show that these early humans walked in a very similar manner as modern humans.
Today, the actual footprints have been covered up in hopes of protecting them, but due to fear of erosion they are planned to be uncovered again. Whether the prints remain at their discovered location (with a possible museum built over them) or are to be cut-out and moved entirely is currently under debate.
For now visitors can visit the covered up site of discovery and the nearby Olduvai Gorge Museum, which features casts of the prints and related artifacts.