Along shorelines across the world, crabs move like a familiar joke nature never tires of repeating. They do not march forward like most animals. They slip sideways, quick and precise, as if obeying an older rhythm. New research suggests that rhythm began around 200 million years ago.

Scientists studying crab evolution found evidence that sideways walking likely emerged once in a common ancestor of true crabs, rather than evolving many separate times. That makes the famous crab shuffle rarer and more significant than once assumed.

Researchers examined living species and compared movement styles with evolutionary family trees. Many modern crabs inherited sideways locomotion, while some later lineages appear to have shifted back toward forward movement.

Why would sideways walking matter so much? For a broad-bodied crab with laterally placed legs, moving left or right can be faster and more efficient than pushing forward. It may also help sudden escapes from predators.

The timing is notable. Around 200 million years ago, Earth was changing after the Triassic-Jurassic extinction period. New habitats were opening, particularly shallow marine environments where crabs thrive.

Sometimes evolution repeats body shapes, a process known as carcinization, where unrelated crustaceans become crab-like. But this study suggests the sideways walk itself may have had a single deeper origin.

That means one behavioral innovation may have helped fuel the remarkable diversity of crabs seen today, from beaches to reefs to mangroves. A simple step became a successful strategy.

What looks comic to human eyes may, in truth, be one of nature’s oldest practical decisions.

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