The Walk That Happened Once
A crab scuttles sideways across a beach, legs pumping in that distinctive lateral shuffle that seems to define what a crab is. That movement, the thing that makes a crab look like a crab to anyone who's watched one, evolved exactly once, roughly 200 million years ago, and then spread to nearly 8,000 species across every marine habitat on Earth [2][3].
The finding, published as a reviewed preprint in eLife on April 21, 2026, challenges a basic assumption about how evolution works [1]. While crab-like body shapes have evolved repeatedly through a process called carcinization, different lineages independently arriving at the same flat, wide-bodied form, sideways walking took a different path [2][4]. It happened once, in a single ancestral group called Eubrachyura, and became so successful that it defined the entire lineage of true crabs for the next 200 million years [2][4].
Associate Professor Yuuki Kawabata of Nagasaki University's Graduate School of Integrated Science and Technology led a research team that analyzed locomotion patterns in 50 crab species, combining behavioral observations with genetic data from 344 species and 10 gene sequences [1][2]. Of the 50 species observed, 35 moved sideways and 15 moved forward [1]. The genetic analysis revealed something unexpected: every sideways-walking crab inherited the behavior from a common ancestor, rather than evolving it independently [2].
The Jurassic Innovation
The timing matters. Two hundred million years ago places the origin of sideways walking in the earliest Jurassic period, just after the Triassic-Jurassic mass extinction [2]. Pangaea was rifting apart, shallow marine habitats were expanding, and one group of forward-walking crustaceans made a switch that would define their descendants for the next 200 million years [2][4].
The biomechanical advantages explain why the innovation stuck. Sideways walking allows crabs to move at similar speeds in both lateral directions without turning their bodies, making their escape direction unpredictable to predators [2]. For a wide-bodied animal, lateral movement is more efficient than forward locomotion, a principle confirmed by experiments with crab-like robots [2].
From that single origin, sideways walking spread to approximately 7,904 species of true crabs, colonizing terrestrial, freshwater, and deep-sea environments [2]. The sister group to true crabs, Anomura, primarily moves forward, as do the closest relatives of true crabs, Astacidea [2]. The behavioral split is sharp: crabs form two distinct groups, sideways walkers and forward walkers, with very few showing mixed behavior [2].
The Exceptions That Prove the Rule
At least six lineages broke the pattern, reverting from sideways to forward walking after inheriting the sideways gait from their ancestors [2]. Spider crabs, soldier crabs, and pea crabs all walk forward despite descending from sideways-walking ancestors [2]. But these are reversions, not independent inventions, crabs that lost the sideways walk rather than evolving it fresh [2].
The contrast with carcinization is stark. Becoming crab-shaped happened multiple times across different decapod lineages, suggesting that a flat, wide body is an easy evolutionary solution to certain environmental pressures [2]. But walking sideways, despite its advantages for that body plan, happened once and then became locked in [2].
What the data reveals is a pattern of evolutionary constraint masquerading as evolutionary success. Sideways walking has remained highly conserved across true crabs since its single origin, not because each species independently discovered its advantages, but because 7,904 species inherited it from one ancestor and couldn't escape it [2]. The innovation was so effective that it became the default, spreading faster than alternatives could emerge.
Lock-In Versus Repetition
Evolution's most durable innovations aren't always the ones that repeat. Carcinization shows what happens when a solution is so broadly useful that different lineages converge on it independently. Sideways walking shows the opposite: a solution so successful in one lineage that it defines the lineage entirely, spreading through inheritance rather than rediscovery.
The Jurassic ancestor that first walked sideways left no descendants that walk any other way, except for the handful that reverted. For 200 million years, through mass extinctions and radical habitat shifts, the sideways walk persisted, not because each generation reinvented it, but because each generation inherited it from parents who had no other option to pass down.
That's the evolutionary lesson embedded in the lateral scuttle of a crab crossing a tide pool. Some innovations become so fundamental to a lineage that they stop being innovations and become identity. The crab-like body evolved over and over. The crab-like walk evolved once, 200 million years ago, and never let go.
In evolutionary terms, that makes sideways walking not a triumph of convergent adaptation, but a monument to the power of inheritance, a single ancient innovation that proved impossible to improve upon, escape from, or forget.