A study of 400 million-year-old fish fossils might seem an unexpected place to learn about human anatomy, but a paper in Frontiers in Ecology and Evolution claims to do just that. The authors argue the fossils provide evidence a long-lost fish gill eventually came to form part of the modern vertebrate ear.
The bones of the middle ear convert vibrations of the eardrum into more powerful pressure waves that allow us to hear even quite small disturbances across a wide range of frequencies. Their evocative names – translating from Latin to the hammer, the anvil, and the stirrup – are familiar even to many people who remember little other anatomy from school. Like almost everything in the human body, they evolved from something else.
Some scientists proposed the original was what they called a spiracular gill, which they thought sat between the mandibular and hyoid arches of early vertebrates. For those of us not very familiar with the body parts of fish, this means it sits behind the eye and in front of surviving gills.
Sharks and rays use the spiracle to draw in water where it can be stored before being expelled through the gills with its oxygen captured. Fish that like to hide from predators largely submerged in sediment have their spiracle on top so they can still access clean water. Some fish even use the spiracle to breathe air in shallow water, and get most of their oxygen this way.
Land-living vertebrates mostly abandoned the spiracle as a breathing device to focus on the nostrils and mouth as channels for air, and instead found another use in the canal through which sound is transmitted to the brain. Indeed reptiles, mammals, and amphibians independently re-purposed the opening this way, albeit with differing modifications.
We know what the spiracle became, but where it came from was harder to determine. A lost gill, proposed in the early 20th century, was a plausible explanation.
There was just one problem for proponents of the gill-to-ear pipeline: no fossilized example of a fully functional spiracular gill had been found. That's not entirely surprising – our stock of well-preserved fossils generally gets sparser the further back one goes – but it obviously remained a problem for supporters of the idea.
Now the gap has been filled by 28 Chinese specimens of a type of jawless fish known as Shuyu, aged up to 438 million years old. Reconstruction of the Shuyu braincase reveals the presence of a spiracle gill.
“These fossils provided the first anatomical and fossil evidence for a vertebrate spiracle originating from fish gills,” said first author Dr Zhikun Gai of the Chinese Academy of Sciences in a statement.
"Many important structures of human beings can be traced back to our fish ancestors, such as our teeth, jaws, middle ears, etc. The main task of paleontologists is to find the important missing links in the evolutionary chain from fish to humans. Shuyu has been regarded as a key missing link as important as Archaeopteryx, Ichthyostega, and Tiktaalik," said co-author Professor Zhu Min.