Our ancient river flooded and filled the footprint with mud and sand, protecting it. So we've got our protective layer of sand. What happens next? WILL:So next thing is, sea level rose so our ...

Kimolo Mulwa at the site of the first hominin footprint discovery in 1978. Deep, sand-filled depressions to his left show hippopotamus tracks in cross section. Anna K. Behrensmeyer We collected ...