Crabs’ sideways scuttle likely began with one ancestor 200 million years ago—and became a game-changing advantage. Credit: Shutterstock Scientists have uncovered new clues about how crabs developed their distinctive sideways movement. A new study, released as a Reviewed Preprint in eLife , brings together the largest dataset yet on how crabs move. By comparing many species, the researchers traced this unusual walking style back to a shared ancestor that lived roughly 200 million years ago.