Gills are often thought of as a fish's lungs. They pump oxygen in and carbon dioxide out. But that's not all gills do. They also balance out a fish's electrolytes - the ions that we lose when we sweat. The gills of modern fish evolved from structures that their ancestors - small, worm-like animals - used to pull little specks of food from the ocean. It's not entirely clear when that change occurred, but it's believed to have happened no earlier than when the first fish appeared. Michael Sackville and Colin Brauner from the University of British Columbia, Canada, teamed up with colleagues from the University of Montreal, Canada, and the University of Cambridge, UK, to test this. They looked at the gills of Pacific lamprey (Entosphenus tridentatus) larvae, Florida lancelets (Branchiostoma floridae) and acorn worms (Saccoglossus kowalevskii) to figure out when fish gills took on their modern role.
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