Study Traces Sideways Walking in Crabs to a 200-Million-Year-Old Ancestor
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A new study has traced the distinctive sideways walking of 'true crabs' to a single evolutionary origin approximately 200 million years ago. Researchers combined video observations of 50 crab species with an evolutionary family tree to find that sideways movement evolved once from a forward-walking ancestor. This adaptation emerged during a period of major environmental change following a mass extinction.
Facts First
- Sideways walking evolved once in 'true crabs' (Brachyura) from a forward-walking ancestor at the base of the Eubrachyura group.
- Researchers estimate this adaptation originated around 200 million years ago during the earliest Jurassic period.
- The study observed 50 crab species, finding 35 moved primarily sideways while 15 moved forward.
- True crabs are a highly diverse group of approximately 7,904 species that have colonized land, freshwater, and deep-sea environments.
- The research combined behavioral video data with a published evolutionary tree to map the trait's history.
What Happened
Researchers investigated the evolution of sideways walking in crabs by studying the movement of 50 species of true crabs (Brachyura). They recorded each species for 10 minutes in a circular arena and combined these behavioral observations with a previously published evolutionary family tree (phylogeny) of crabs. The analysis showed that sideways walking likely evolved a single time from a forward-walking ancestor. The team estimates this evolutionary shift occurred approximately 200 million years ago, during the earliest Jurassic period, which followed the Triassic-Jurassic extinction event.
Why this Matters to You
This research provides a clearer picture of how a familiar animal behavior—the crab's sideways scuttle—came to be. Understanding the deep evolutionary history of such traits helps explain the incredible diversity of life on Earth, including how species adapt to new environments. The study's findings may also offer insights for robotics or biomechanics, as engineers often look to efficient natural movements like the crab's gait for inspiration.
What's Next
The researchers studied one individual per species due to practical limitations, so future studies may involve observing more individuals to confirm these movement patterns. Further research could investigate the specific anatomical or neurological changes that enabled sideways walking and how this trait contributed to the successful global colonization of habitats by true crabs.