Every crab you have ever watched skitter across a beach was performing a trick that dates to the early Jurassic. A research team has now linked the signature sideways gait of true crabs to a single evolutionary origin, with molecular-clock estimates placing the divergence of the brachyuran lineage roughly 200 million years ago. That finding overturns the assumption that such a common behavior must have arisen independently in multiple lineages, though no direct fossil evidence of the gait itself survives from that era.
The study, a preprint that passed editorial review in early 2025 and is currently undergoing full peer review, scored the walking direction of more than 150 living crab species and mapped those scores onto a fossil-calibrated family tree of Brachyura, the infraorder that includes all true crabs. The result was striking: sideways locomotion sits at the very base of the brachyuran tree, meaning it appeared once in the group’s common ancestor and then held firm through the Jurassic, the Cretaceous, the mass extinction that killed the non-avian dinosaurs, and on into the present.
One gait, one origin
The phylogeny underpinning the analysis draws on a divergence-time reconstruction published in Systematic Biology, built from genetic sequences across 344 crab species and anchored by dozens of vetted fossil calibration points. When the behavioral data were layered onto that scaffold, ancestral-state reconstruction methods strongly favored a single gain of sideways walking at the root of Brachyura over any scenario involving multiple independent origins or repeated losses and re-gains.
Fossil evidence supports the timeline. Eocarcinus praecursor, a small crustacean first described by T.H. Withers in 1932 from early Jurassic rocks in England, has been reassessed as a stem-group brachyuran by Javier Luque and colleagues in a morphological study published in Arthropod Structure & Development. That placement puts E. praecursor just outside the true-crab crown group, consistent with the idea that the lineage leading to modern sideways walkers was already splitting off around 195 million years ago. Earlier molecular-clock estimates for major brachyuran clades, published in Molecular Biology and Evolution, produced broadly compatible dates.
Sideways walking is not the same as looking like a crab
One of the study’s sharpest contributions is drawing a clear line between the sideways gait and a separate phenomenon called carcinization: the repeated, independent evolution of a crab-shaped body. Carcinization has occurred at least five times in Anomura, the sister group to Brachyura that includes hermit crabs and king crabs, as documented in a widely cited 2021 analysis in BioEssays by Joanna Wolfe and colleagues. Coconut crabs, porcelain crabs, and king crabs all converged on a broad, flattened shell independently.
Yet while the body plan kept being reinvented, the sideways gait apparently was not. The new analysis suggests that the neural and mechanical coordination required for fully lateral locomotion may be far harder to evolve from scratch than a flat carapace and tucked abdomen. In other words, looking like a crab is easy; walking like one is not.
What the fossils cannot tell us
No direct evidence of how E. praecursor moved survives in the fossil record. Early Jurassic trackways are rare and nearly impossible to assign to a specific species. The 200-million-year figure therefore reflects a molecular-clock estimate cross-checked against the oldest known stem-group fossil, not a preserved snapshot of ancient sideways walking etched in stone. If future discoveries push the origin of the brachyuran lineage earlier or later, the timeline could shift.
There is also an open question about why the gait proved so durable. The study documents its persistence but does not fully explain the biomechanical or ecological pressures that prevented any true-crab lineage from reverting to forward locomotion across 200 million years. Some researchers have proposed that the mechanics of wave resistance in intertidal zones could stabilize sideways movement, but that hypothesis has not been tested through biomechanical simulation of ancestral body shapes. Whether the gait is locked in by joint architecture in the legs, by neural wiring in the motor system, or by some combination remains unclear.
Gaps in the data
The locomotion dataset covers more than 150 species, but Brachyura contains thousands of described species spanning deep-sea hydrothermal vents, freshwater streams, and tropical forests. Some cave-dwelling or burrowing crabs may use modified gaits that were not captured in the initial scoring. A handful of true crabs, notably certain spider crabs in the family Majidae, are known to walk forward or at oblique angles under some conditions. If future fieldwork reveals more lineages that routinely move forward, researchers will need to determine whether those represent rare reversals from the ancestral sideways condition or simply behavioral flexibility within species that default to lateral movement.
The behavioral scoring itself is another area to watch. Locomotion studies in crabs have a long history, but systematically categorizing walking direction across more than 150 species and mapping those categories onto a phylogeny is a relatively novel approach. Different labs may draw the line between “sideways,” “oblique,” and “forward” differently depending on species posture and the surface the crab is walking on. Until the scoring protocol is independently replicated, the strength of the single-origin conclusion depends partly on how consistently those behavioral categories hold up under scrutiny.
A core pattern with room for variety
It is worth separating “single origin” from “frozen in place.” The study argues that the basic lateral gait appeared once and persisted, but crab locomotion is far from static. Within the sideways framework, different species have evolved sprinting bursts to dodge shorebirds, stealthy stalking to ambush prey, rapid burrow excavation in mudflats, and vertical climbing on mangrove roots and sea cliffs. Evolution conserved the core pattern while still generating significant diversity in speed, agility, and maneuvering.
Ancestral-state reconstructions are also probabilistic, not absolute. The methods produce likelihood estimates that a given ancestor carried a particular trait, based on how that trait is distributed among living species and the shape of the evolutionary tree. As more taxa are sampled and new fossils refine divergence times, those estimates can shift. The current reconstruction strongly favors a single origin, but future data could modestly raise or lower that confidence.
Why three lines of evidence matter more than one
What makes the finding compelling, even at this stage, is the way it braids together three independent lines of evidence: genes supply the tree, fossils anchor the timing, and behavioral observations supply the trait. No single strand would be conclusive on its own. Together, as of June 2026, they build a case that the sideways scuttle is not just a quirk of modern beaches but one of the most enduring locomotion strategies in the animal kingdom.
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*This article was researched with the help of AI, with human editors creating the final content.
