New research results are rewriting the evolutionary history of the gills

Gills are best known for helping most species of fish breathe underwater. Less well known, however, is the fact that gills regulate the salt and pH balance of the blood of fish, a vital role played by the kidneys of other animals. This lesser-known gill function, collectively referred to as ion regulation, has traditionally probably evolved along with respiration.

But surprising new research published in Nature adds a new, early chapter to the evolutionary history of the gills.

“Our work suggests that the early, simplified gills of our worm-like ancestors played an important role in ion regulation. And that role may have arisen as early as the gills’ inception, long before they played any role in respiration,” he tells Dr. Michael Sackville, a zoologist who led the study while at the University of British Columbia (UBC).

“This really turns the script on our understanding of gill development and gill function.”

The evolution of gills and lungs has fascinated biologists, including Darwin, for more than 100 years. Prior to this study, it was thought that gills were first used for respiration and ion regulation shortly before the beginning of vertebrate life. In this traditional timeline, these two functions simultaneously shifted from the skin to the gills, helping vertebrates evolve from small, worm-like creatures into larger, active fish. This transition from “small and wormlike” to “large and fishy” is an iconic event in vertebrate evolution.

The study traced the evolutionary journey of the gills by comparing three animals living today but belonging to different lineages: lampreys, which are vertebrates, and amphioxus and acorn worms, which are close relatives of vertebrates. The researchers hypothesized that all the gill functions the animals share were inherited from a common ancestor believed to have first appeared simple gills well over 500 million years ago.

“We found that gills were used for breathing only in our vertebrate representatives, and only with increasing body size and activity,” says Dr. Colin Brauner, UBC zoologist and senior author of the paper.

“But we found ion-regulating cells in the gills of all three of our animals. This allowed us to trace the origin of ion regulation at the gills back to early deuterostomy animals, when very simple gill structures are thought to have evolved first.” The finding supports the classic story that gills were first used for breathing in early vertebrates, however, adds an exciting new, earlier chapter to the story that is clearly worthy of further study.”