Nature: New research rewrites the evolutionary story of gills

The gills of fish are best known for helping most fish breathe
underwater.
But what is less known is that fish gills regulate the balance of salt and pH in fish blood, while kidneys in other animals play a vital role
.
This lesser-known gill function is collectively known as ion modulation and is traditionally thought to
have evolved in tandem with respiration.

But surprising new research adds an early chapter
to the story of gills' evolution.

"Our study shows that early simplified gills play an important role
in ion conditioning.
And this effect may have started as early as the gills appeared, long before they played any role in breathing," said
Dr.
Michael Sackville, a zoologist who led the study at the University of British Columbia (UBC).
"This really upends our understanding of
how gills and gills function evolved.
"

The evolution of the gills and lungs has captivated 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
at the earliest beginning of vertebrate life.
In this traditional timeline, these two functions connect from skin to gills, helping vertebrates transition from small worm-like organisms to larger, more active fish
.
This transition from "small and worm" to "big and fish" is a landmark event
in vertebrate evolution.

The study traced the evolution of gills by comparing three extant animals belonging to different lineages: lampreys are vertebrates, and Wenchang fish and acorn worms are close relatives
of vertebrates.
The researchers believe that any gill functions shared between these animals were inherited from a common ancestor, which is thought to have
been when simple gills first appeared more than 500 million years ago.

"We found that gills are only used for breathing in vertebrates, and only when the body is getting bigger and more active," said
Dr.
Colin Brayne, a zoologist at the University of British Columbia and senior author of the paper.
"But we found ion-regulatory cells
in the gills of all three animals.
" This allows us to trace the origins of ion regulation in gills all the way back to early posterior animals, when very simple gill structures are thought to have first evolved
.
This finding supports the classic story of gills originally used for early vertebrate breathing, but also adds an exciting new early chapter to the story that clearly deserves further study
.
”

Ion regulation at gills precedes gas exchange and the origin of vertebrates