Crabs that inherited the trait are far more species-rich, suggesting it fueled their success A crab scurries sideways along a beach in the United Arab Emirates. Scientists have traced this distinctive gait to a single ancestor that lived roughly 200 million years ago. Creatas Video+/Getty Images Plus The iconic sideways walk of crabs may have evolved just once, in an ancestor that roamed Earth roughly 200 million years ago. That conclusion, published April 21 in eLife , comes from researchers who tracked the movement of 50 crab species and mapped the results onto a crab family tree . The lineage that inherited sideways locomotion went on to become by far the most species-rich group of crabs on the planet, suggesting the trait may have been a key driver of their evolutionary success. Crabs’ sideways walk is almost unique in the animal kingdom, yet its origin has long eluded researchers.…