The study adds a new, early chapter to the evolutionary tale of gills.
Most fish species depend on their gills to breathe underwater. Less widely known is the fact that, like kidneys in other animals, fish gills control the pH and salt balance of their blood. This lesser-known gill function referred to as “ion regulation,” has historically been assumed to have evolved in tandem with breathing.
However, a surprising new study published in Nature is introducing a new, early chapter to the evolutionary history of gills.
“Our work suggests that the early, simplified gills of our worm-like ancestors played an important role in ion regulation. And that role might have originated as early as the very inception of gills, well before they played any role in breathing,” says Dr. Michael Sackville, a zoologist who led the study while with the University of British Columbia (UBC).
“This really does flip the script on our understanding of how gills and gill function evolved.”
For more than a century, scientists, including Darwin, have been captivated by the evolution of gills and lungs. Prior to this research, it was believed that gills were initially utilized for breathing and ion control near the beginning of vertebrate life. These two functions changed from the skin to the gills in tandem in this traditional timeline, helping vertebrates in their transformation from tiny, worm-like organisms to larger, active fishes. This transformation from “small and wormy” to “big and fishy” is a defining moment in vertebrate evolution.
The research compared three species that are still alive today but belong to distinct lineages: lampreys, which are vertebrates, and amphioxus and acorn worms, which are close relatives of vertebrates. The researchers reasoned that any gill functions shared by the animals were acquired from a common ancestor, which is thought to have existed well over 500 million years ago.
“We found that gills were used for breathing in only our vertebrate representative, and only with increasing body size and activity,” says Dr. Colin Brauner, a UBC zoologist and senior author on 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 gills all the way back to early deuterostome animals when very simple gill structures are thought to have first evolved. The finding supports the classic story that gills were first used for breathing in early vertebrates, but adds an exciting new, earlier chapter to the story, clearly worthy of further study.”
Reference: “Ion regulation at gills precedes gas exchange and the origin of vertebrates” by Michael A. Sackville, Christopher B. Cameron, J. Andrew Gillis and Colin J. Brauner, 19 October 2022, Nature.
The study was conducted in collaboration with researchers at the University of Montreal and Cambridge University.
The study was funded by the Natural Sciences and Engineering Council of Canada and Royal Society.