Schematic diagram of the four stages of the evolution of fish water jet holes to the human middle ear. Shi Aijuan drawing
BEIJING, May 29 (Xinhua) -- The human middle ear is an important organ of hearing, and its exploration of the source of evolution in nature has been attracting much attention. At present, there is sufficient embryonic and fossil evidence to prove that the human middle ear evolved from the water spray hole of the fish, and where does the fish water spray hole come from? It is a century-long problem that has plagued the academic community for a hundred years.
Recently, Chinese scientists led the completion of an in-depth study of ancient fish fossils collected from Changxing, Zhejiang and Qujing, Yunnan, confirming that the fish water spray hole evolved from the first pair of gills, which is also the first time that the scientific community has revealed through fossil evidence that the middle ear used by humans to listen originated from the gills used by fish to breathe.
This paper on the evolution of paleonttomy, which solves a century-old scientific problem, was jointly completed by Researcher Gai Zhikun of the Institute of Paleontology and Paleoanthropology of the Chinese Academy of Sciences (Institute of Paleovertebrates of the Chinese Academy of Sciences), Zhu Min, academician of the Chinese Academy of Sciences, Philip C.J. Donoghue, academician of the Royal Academy of Sciences, 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.
Gill silk marks were first found in the fossils of the early Devonian broad turtle in Qujing, Yunnan, revealing that the human middle ear was once a gill used by fish to breathe. Photo by Gai Zhikun
How did the 100-year-old scientific conundrum come about?
Researcher Gai Zhikun recently said in an exclusive interview with a reporter from the China News Agency in Beijing that the human middle ear capsule ear ear capsule ear membrane chamber, the three listening small bone that conduct sound, and the Eustachian tube leading to the mouth are the secrets of human beings having a sensitive sense of hearing. The water spray hole of the fish is located behind the eye, and between the jaw arch and the tongue arch is an incomplete gill fissure containing only false gills, which is a very important respiratory organ of the fish.
In cartilaginous fish, the water jet hole is mainly used to inhale the water flow, which is the survival secret of benthic cartilaginous fish adapting to seafloor life, while in the primitive teleost fish, the water spray hole is mainly used to breathe air, which is an important reference for studying how early meat fin fish landed on land to breathe air and adapt to land life.
He pointed out that in terms of the evolution of fish water jet holes, following the French anatomist Saint Silan's classic book "Anatomical Philosophy" in 1822 and the German anatomist Carl Gengbauer's theory of vertebrate head segments in 1872, the British paleontologist Watson further proposed the "free tongue arch theory" in 1937, believing that a primitive state of early jaws should have a complete, undegraded gill fissure between the jaw arch and the tongue arch, rather than a water spray hole.
The theory inspired a twentieth-century craze for paleontologists looking for an undegraded gill fissure between the early jaw-bearing jaw arch and the tongue arch. Unfortunately, after a century of searching, paleontologists have carefully examined the fossils of all shieldfish, spiny fish, cartilaginous fish and teleost fish in the living jawless and early jawed species, and have not found exact fossil evidence before.
A three-dimensional reconstruction of the skull of the shuridge, the oldest true armored fish in the Silurian period in Zhejiang, China. Courtesy of Gai Zhikun
How was chinese fossil evidence found?
Gai Zhikun said that since 2002, the team of the Institute of Paleovertebrates of the Chinese Academy of Sciences has carried out field work in the marine red layer of the early Zhilurian period in Changxing, Zhejiang (about 438 million years ago), and found the most primitive true armorfish fossil, which was later named "Akebono". These only fingernail sized Akebono fossils are precious specimens with three-dimensional cartilage skull preservation, the research team in 2006 after a three-dimensional non-destructive scan and then a three-dimensional virtual restoration of its skull, before and after 5 years to complete a total of 7 pieces of Aquake skull fossil three-dimensional reconstruction, through in-depth research found that the so-called gill ridge of armor fish is actually the dorsal part of the gill arch, that is, the gill arch and brain skull of the armor fish healed into a main protective head armor, This is similar to the enlargement of the ribs of turtles and turtles to heal into turtle carapace.
Further comparative studies show that the entire gill arch of the aerophumi remains in its original state as a whole vertebrate, and its jaw arch is located just behind the orbital foramen, forming the posterior orbital wall. At the same time, the accurately reconstructed Aquamanian brain nerve has many evidences that one gill sac behind the eye of the armorfish is the tongue and jaw sac located between the jaw arch and the tongue arch, which is no different from the 5 gill sacs in the back, and it is opened on the ventral surface of the head nail, and it is not opened on the back of the head armor like a water spray hole, so it is basically judged that the tongue and jaw sac of the armor is still an undegraded gill sac.
"However, to prove that it is a gill with normal respiratory function, the last link in the entire chain of evidence is missing, that is, to find fossil evidence of the presence of gill filaments in the gill sac."
Gai Zhikun said that the research team has shifted its attention to Qujing, Yunnan Province, known as the "ancient fish kingdom" and "the birthplace of human distant ancestors", through years of field excavations in the Qujing early Devonian (about 419 million years ago), in 2017, it was fortunate to collect the first new material of wide turtle fossils that completely preserved gill silk marks in the first gill sac behind the eyes.
Through in-depth analysis, the research team further proved that the first gill sac behind the eye of the armorfish is a gill with normal respiratory function, rather than a degenerated water spray hole, thus providing the most accurate anatomical evidence and fossil evidence for the origin of the water spray holes of vertebrates such as fish from degenerated gills.
Researcher Gai Zhikun was interviewed at his Beijing office to display ancient fish fossils collected from the "Ancient Fish Kingdom" of Qujing, Yunnan Province, and completely preserved gill silk marks. Photo by Sun Zifa, a reporter from China News Service
How is the evolutionary sequence from gill to ear established?
Gai Zhikun said that on the basis of fossil evidence to solve a century-old scientific problem, the research team clearly proposed the evolution of water spray holes from jawless to tetrapods, thus establishing the evolution sequence of water spray holes from jawless gills to human middle ears:
With the division of the nasal sacs in pairs of armor fish, an unusually large middle dorsal hole (single nostril) develops in front of the head to act as the main respiratory organ for inhaling water, so the tongue and jaw sac between the jaw arch and the tongue arch for the first time develop into a complete gill sac, which has a complete anterior and posterior half-gills like the 5 normal gill sacs behind it, and the half-gills have gills, which are the main places for oxygen exchange.
Researcher Gai Zhikun was interviewed at his Beijing office to display ancient fish fossils collected from the "Ancient Fish Kingdom" of Qujing, Yunnan Province, and completely preserved gill silk marks. Photo by Sun Zifa, a reporter from China News Service
With the origin of the jaw and double nostrils, some jaws have successfully evolved double nostrils, but the double nostrils do not communicate with the oral cavity, have no respiratory function, only the function of smell. However, the breathing needs of fish have not decreased, so the first gill sac (tongue and jaw sac) behind the eye is transformed into a water spray hole and becomes the main respiratory organ, which has already occurred in the most primitive jawed shield fish, because the most primitive shield fish carapacefish (ditch scale fish) and the most advanced shield fish unicorn have water jet holes, so the water spray hole is likely to originate simultaneously with the appearance of jaw and double nostrils in the jaw.
Water vents were mainly used in cartilaginous fish to suck in water streams, while in the early bony fish were mainly used to breathe air, thus providing a prerequisite for fish to land to breathe air. With the origin of the inner nostrils in meatfin fish (such as Ken's fish), the passage between the nasal cavity and the mouth was successfully opened, and the nostrils became the main respiratory organ, thus laying the foundation for the fish to land and breathe with their lungs.
Schematic of the evolution of water spray holes from jawless gills to the human middle ear. Shi Aijuan drawing
The quadrupeds that landed, faced with a completely new environment, had to develop new sensory organs in order to survive better in the air. In this context, the water jet holes that have lost their respiratory function are recycled and gradually evolved into the tympanic membrane chamber of the human middle ear cavity, and the square bones and joint bones of the tongue and jaw and its joints are gradually degenerated and smaller, and eventually enter the middle ear and evolve into three auditory ossicles, and are renamed stapes, hammer bones and anvils, responsible for transmitting sound to the brain, so that humans finally have a sensitive sense of hearing.
He stressed that if it were not for the bold experiments of these prehistoric fish breathing air through overhead water spray holes, humans might never have evolved such a keen sense of hearing, and the continuous evolution of ancient fish fossils has brought people the opportunity to enjoy the feast of hearing.
"If you pinch your nose and close your mouth and breathe hard, you will feel that the eardrum is very swollen, which means that our ears and mouth are still connected, and it is the Eustachian tube that originally led from the water spray hole to the mouth, which is one of the evolutionary remnants left to us by fish." Gai Zhikun said. (End)
【Editor: Chen Haifeng】