The human middle ear—which houses three tiny, vibrating bones—is key to
transporting sound vibrations into the inner ear, where they become nerve
impulses that allow us to hear.
Embryonic and fossil evidence proves that the human middle ear evolved from
the spiracle of fishes. However, the origin of the vertebrate spiracle has
long been an unsolved mystery in vertebrate evolution.
Some 20th century researchers, believing that early vertebrates must possess
a complete spiracular gill, searched for one between the mandibular and
hyoid arches of early vertebrates. Despite extensive research spanning more
than a century, though, none were found in any vertebrate fossils.
Now, however, scientists from the Institute of Vertebrate Paleontology and
Paleoanthropology (IVPP) of the Chinese Academy of Sciences and their
collaborators have found clues to this mystery from armored galeaspid
fossils in China.
Their findings were published in Frontiers in Ecology and Evolution on May
19.
According to Prof. Gai Zhikun from IVPP, first author of the study,
researchers from the institute successively found over the last 20 years a
438-million-year-old Shuyu 3D braincase fossil and the first
419-million-year-old galeaspid fossil completely preserved with gill
filaments in the first branchial chamber. The fossils were found in
Changxing, Zhejiang Province and Qujing, Yunnan Province, respectively.
"These fossils provided the first anatomical and fossil evidence for a
vertebrate spiracle originating from fish gills," said Gai.
A total of seven virtual endocasts of the Shuyu braincase were subsequently
reconstructed. Almost all details of the cranial anatomy of Shuyu were
revealed in its fingernail-sized skull, including five brain divisions,
sensory organs, and cranial nerve and blood vessel passages in the skull.
"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 Zhu Min,
academician of the Chinese Academy of Sciences.
The spiracle is a small hole behind each eye that opens to the mouth in some
fishes. In sharks and all rays, the spiracle is responsible for the intake
of water into the buccal space before being expelled from the gills. The
spiracle is often located towards the top of the animal allowing breathing
even while the animal is mostly buried under sediment.
In the Polypterus, the most primitive, living bony fish, the spiracles are
used to breathe air. However, fish spiracles were eventually replaced in
most non-fish species as they evolved to breathe through their noses and
mouths. In early tetrapods, the spiracle seems to have developed first into
the Otic notch. Like the spiracle, it was used in respiration and was
incapable of sensing sound. Later the spiracle evolved into the ear of
modern tetrapods, eventually becoming the hearing canal used for
transmitting sound to the brain via tiny inner ear bones. This function has
remained throughout the evolution to humans.
"Our finding bridges the entire history of the spiracular slit, bringing
together recent discoveries from the gill pouches of fossil jawless
vertebrates, via the spiracles of the earliest jawed vertebrates, to the
middle ears of the first tetrapods, which tells this extraordinary
evolutionary story," said Prof. Per E. Ahlberg from Uppsala University and
academician of the Royal Swedish Academy of Sciences.
Reference:
Zhikun Gai et al, The Evolution of the Spiracular Region From Jawless Fishes
to Tetrapods, Frontiers in Ecology and Evolution (2022).
DOI: 10.3389/fevo.2022.887172