New Evidence: The human middle ear evolved from the gills that fish breathe

The human middle ear is an important auditory organ, and the exploration of its evolutionary source in nature has always attracted much attention.

At present, there is sufficient embryonic and fossil evidence to prove that the human middle ear evolved from the blowholes of fish, and where did the blowholes of fish come from?

It is a century-old problem that has plagued academia for hundreds of years.

Recently, Chinese scientists led an in-depth study of ancient fish fossils collected more than 400 million years ago from Changxing, Zhejiang and Qujing, Yunnan. It was confirmed that the fish blowholes evolved from the first pair of gills. This is also the scientific community's fossil evidence. For the first time, it has been revealed that the middle ear used by humans to hear originated from the gills used by fish to breathe.

This important achievement paper on the evolution of vertebrate paleontology, which solves the century-old scientific problem, was jointly organized by researcher Gai Zhikun of the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences (Institute of Paleoanthropology, Chinese Academy of Sciences), Zhu Min, academician of the Chinese Academy of Sciences, and Philip Donoue, academician of the Royal Academy of Sciences of the United Kingdom. It was jointly completed by Philip CJ Donoghue and Per E. Ahlberg, academician of the Royal Swedish Academy of Sciences, and has been published in the international professional academic journal "Frontiers of Ecology and Evolution".

For the first time, gill silk imprints were discovered in the early Devonian soft-shelled turtle fossils in Qujing, Yunnan, revealing that the middle ear of humans was once the gills used by fish to breathe.

How did the century-old scientific problem arise?

Researcher Gai Zhikun recently said in an exclusive interview with a reporter from China News Agency in Beijing that the tympanic chamber of the human middle ear, the three auditory ossicles that conduct sound, and the Eustachian tube leading to the oral cavity are the secrets of human beings with sensitive hearing. .

The blowhole of fish is located behind the eyes, an incomplete gill slit containing only false gills between the jaw arch and the lingual arch, which is a very important respiratory organ for fish.

In cartilaginous fish, the blowhole is mainly used to inhale the water current, which is the survival secret of the benthic cartilaginous fish to adapt to the life on the seabed, while in the primitive teleost fish, the blowhole is mainly used to breathe air, which is an early stage of research. An important reference for how meat-finned fish land on land to breathe air and adapt to life on land.

He pointed out that in terms of the evolution of fish blowholes, following the French anatomist Saint Cyran's classic book "The Philosophy of Anatomy" published in 1822, and the German anatomist Karl Gungebauer's theory of vertebrate head segments in 1872 , British paleontologist Watson further proposed the "free lingual arch theory" in 1937, arguing that a primitive state of the early jaws was that there should be a complete, not yet degenerated gill slit between the jaw arch and the lingual arch, rather than spray holes.

This theory inspired twentieth-century paleontologists to search for an unregressed branchial cleft between the jaw and lingual arches of early jawed species.

But unfortunately, after a century of searching, paleontologists from all over the world have carefully examined the fossils of all placodes, spiny fishes, cartilaginous fishes and teleosts in the extant jawless and early jawed species. No definitive fossil evidence has been found.

3D reconstruction of the skull of the oldest true armoured fish from the Silurian period in Zhejiang, China.

How was the fossil evidence in China found?

Gai Zhikun said that since 2002, the team of the Institute of Ancient Spine of the Chinese Academy of Sciences has carried out field work in the marine red layer of the early Silurian period (about 438 million years ago) in Changxing, Zhejiang, and found one of the most primitive fossils of the true armored fish. Named it "Shuyu".

These fossils, which are only the size of fingernails, are precious specimens with three-dimensional cartilage skull preservation. The research team carried out 3D non-destructive scanning in 2006 and then performed 3D virtual restoration of the skull. It took 5 years to complete a total of 7 Drosophila specimens. The three-dimensional reconstruction of the fossilized fish brain and skull, through in-depth research, it is found that the so-called interbranch ridge of armored fish is actually the dorsal part of the gill arch, that is, the gill arch of armored fish and the brain skull are merged into a head that mainly plays a protective role A, this is similar to the expansion and healing of the ribs of turtles into tortoise shells.

Further comparative study showed that the entire gill arch of D. sibiricus still maintained the original state of the entire vertebrate, and its jaw arch was located just behind the orbital foramen, forming the posterior orbital wall.

At the same time, various evidences from the accurately restored Drosophila cranial nerves show that a gill sac behind the eyes of the armored fish is the lingual-maxillary sac located between the jaw arch and the lingual arch, which is no different from the five gill sacs behind, and It is open on the ventral surface of the head armor, not like the water spray hole on the back of the head armor, so it is basically judged that the lingual-maxillary sac of the armored fish is still an undegraded gill sac.

"However, in order to prove that it is a gill with normal breathing function, the last link in the whole chain of evidence is still missing, that is, the fossil evidence of the existence of gill filaments in the gill sac."

Gai Zhikun said that for this reason, the research team turned their attention to Qujing, Yunnan, which is known as the "kingdom of ancient fish" and "the birthplace of ancient human ancestors". After several years of field excavation work, in 2017, I was fortunate to collect the first new fossil material of the broad turtle that completely preserves the gill silk imprint in the first gill sac behind the eye.

hrough in-depth analysis, the research team further proved that the first gill sac behind the eyes of the armored fish is a gill with normal breathing function, rather than a degenerate blowhole, thus providing evidence for the origin of the blowhole of vertebrates such as fish from the degenerate gill. The most definitive anatomical and fossil evidence.

Researcher Gai Zhikun was interviewed in the Beijing office, showing the fossils of ancient fish collected from the "Kingdom of Ancient Fish" in Qujing, Yunnan, with gill silk imprints intact.

How was the evolution sequence from gill to ear established?

Gai Zhikun said that on the basis of the fossil evidence to solve the scientific problems of the century, the research team clearly proposed the evolution of water blowholes from jawless to tetrapods, thus establishing the evolution of water blowholes from jawless gills to humans. The evolution sequence of the ear:

With the splitting of the paired nasal sacs of the armored fish, an abnormally large middorsal foramen (single nostril) developed in front of the head carapace to serve as the main respiratory organ for inhaling water, so the lingual-maxillary sac between the jaw arch and the lingual arch was the first time. It develops into a complete gill sac, which, like the five normal gill sacs behind, has complete front and rear half gills, and the half gills have gill filaments, which are the main places for oxygen exchange.

Researcher Gai Zhikun was interviewed in the Beijing office, showing the fossils of ancient fish collected from the "Kingdom of Ancient Fish" in Qujing, Yunnan, with gill silk imprints intact.

With the origin of the jaws and double nostrils, the jaws have successfully evolved double nostrils, but the double nostrils are not connected with the mouth, and have no breathing function, only the olfactory function.

However, the respiratory needs of fish have not decreased, so the first gill sac behind the eyes (lingual-maxillary sac) was transformed into a water jet hole and became the main respiratory organ, which was the case in the most primitive jawed placodes. It has happened because the most primitive placodes, the carcass carpids (Doublefish) and the most advanced placodes, the unicornus, already have blowholes, so blowholes are likely to accompany the jaws The appearance of the jaws and double nostrils originated at the same time.

Blowholes are mainly used to inhale water in cartilaginous fish, while in early bony fish they are mainly used to breathe air, which provides a prerequisite for fish to land on land to breathe air.

With the origin of the internal nostrils in mollusk fish (such as Kentucky fish), the channel between the nasal cavity and the oral cavity was successfully opened, and the nostrils became the main respiratory organs, which further laid the foundation for the fish to use the lungs to breathe on land.

Schematic diagram of the evolution of blowholes from jawless gills to the human middle ear.

Quadrupeds that landed on land were faced with a whole new environment and had to develop new sensory organs to better survive in the air.

In this context, the water jet holes that have lost their breathing function were recycled and gradually evolved into the tympanic chamber of the human middle ear cavity, while the square and articular bones of the hyoid and jaw bone and its joints gradually degenerated and became smaller, and finally entered the tympanic cavity of the human middle ear cavity. The middle ear evolved into three ossicles, which were renamed the stapes, malleus, and incus, responsible for transmitting sound to the brain, allowing humans to eventually have sensitive hearing.

He emphasized that if it weren't for the bold experiments of these prehistoric fish to breathe air through the blowholes above their heads, humans may never have evolved such a keen sense of hearing. The continuous evolution of ancient fish fossils has brought people the opportunity to enjoy a feast of hearing.

"If you pinch your nose and close your mouth and exhale hard, you will feel the tympanic membrane of the ear is very swollen, which shows that our ears and mouth are still connected, and the connection between them is the original from the water hole to the mouth. The Eustachian tube in the mouth is also one of the evolutionary remnants left to us by fish," said Gai Zhikun.

Written by Sun Zifa for China News Service, Beijing