Researchers say the footprints were left in the mud by two different species, perhaps within a matter of hours or days.

Smithsonian: Fossil Footprints Reveal That Two Early Human Relatives Lived on the Same Landscape in Kenya 1.5 Million Years ...

Evolution down the Path Less Traveled," an exhibit at the Museum of Science in Boston, eyes the advantages of moving slowly.

The footprints were left in the mud by two different species “within a matter of hours, or at most days,” said ...