The footprints are the first clear evidence that the two hominin species shared a habitat, raising questions about whether their interactions shaped human evolution.

A new discovery of fossils dating back 1.5 million years is giving scientists fresh insight into the behaviors of human ancestors known as hominins. An international team of researchers said Thursday that they had found two sets of hominin footprints that were made about the same time on an ancient lake’s shore in present-day Kenya.

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