Rain pattered in freshly laid volcanic ash across northern Tanzania some 3.66 million years ago. Before the ground had fully dried—the volcano perhaps still smoldering in the distance—a trio of ancient human relatives strolled across the damp sediments, casting their prints in the slowly cementing ash.

When the fossilized footprints were discovered in the 1970s, they shook the paleontology world. Left by the same species as the famed hominin relative Lucy, Australopithacus afarensis, the tracks were the first clear evidence of our early ancestors walking on two feet.

Now a fresh analysis of a set of long-forgotten prints nearby hints that these early humans weren't alone. If the scientists are right, a mysterious upright-walking hominin also left its