Australopithecus afarensis
3.9 to 2.9 million years ago the Australopithecus afarensis evolved. These are the oldest documented bipedal footprint trails.
Source: Carl Sagan, Cosmos (New York: Random House, 1980)
Image by Tim Evanson under CC BY-SA 2.0 license
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Full disclosure, I may occasionally borrow a sentence from Will Durant's Story of Civilization. I absolutely love that collection!