Do you have a small hole in front of your ear? This is a memorial left by your fish ancestors

I wonder if you have noticed that there is a pair of small holes above some people’s ears, so small that they are almost imperceptible. What are these small holes?

Figure 1 The "small hole" on the human ear

(Image source: Reference [1])

In fact, this hole in the ear is a common congenital developmental defect in humans, medically known as "congenital preauricular fistula". When the human embryo develops to the fourth week, it still develops structures similar to the gill arches and gill clefts of fish, but in the later stages of embryonic development, these structures heal and develop into organs such as the jaw, middle ear cavity, three auditory ossicles, and laryngeal cartilage. However, if the "first gill cleft" is not completely closed in the later stages, such a small hole will be left in the ear.

Therefore, American developmental biologist Neil Shubin believes that the small holes on the ears can be seen as "evolutionary remnants left to us by fish gills" and as evidence of "evolution from fish to humans."

Figure 2 The small hole on the human ear is the result of incomplete healing of the first gill cleft during the embryonic period, and can be seen as an "evolutionary remnant left to us by fish gills"

Image from (reference [3])

So, did our "fish ancestor" have "ears" and small holes? The answer is: yes.

Part 1

Where are the fish's "ears" and little holes?

Fish also have "ears", but unlike humans, they do not have middle and outer ears. They only have inner ears buried deep in the skull, which are much simpler in structure. Therefore, the ears of most living fish do not "communicate" with the outside world, and sound waves are usually transmitted to the inner ear by the thin skull in the ear area.

Figure 3 The "ear" of fish - the inner ear

(Image credit: Bill Brazier)

The functions of the fish's inner ear are the same as those of humans: one is to hear sounds, and the other is to maintain body balance (yes, the human inner ear can also maintain body balance). In addition, the fish's lateral line system is relatively developed, like a radar, which can accurately determine the direction of vibrating objects and can cooperate with the inner ear to complete the fish's survival skills such as swimming, roosting, hunting and defending against enemies.

Without the middle ear and the outer ear, how does the inner ear of fish perform these functions? This is related to the structure of the inner ear of fish.

The inner ear of fish has a complex labyrinth structure, including the ellipsoid saccule, saccule, three semicircular canals and endolymphatic duct, so it is called the membranous labyrinth, which is controlled by the eighth pair of cranial nerves and contains otoliths of various shapes and sizes. The membranous labyrinth is filled with a special fluid called endolymph. When external sound waves are transmitted to the inner ear of fish, the endolymph in the inner ear will vibrate, stimulate the sensory cells of the inner ear, and then be transmitted to the brain through the auditory nerve to respond, completing the whole process of hearing.

Fish do not have the ossicles and eardrums in the human middle ear, which is equivalent to having no sound transmitter and amplifier, and the received sound cannot be amplified. However, some fish with bony swim bladders, such as crucian carp and catfish, have also developed secondarily sound transmitters and amplifiers, namely swim bladders and swim bladder bones. The swim bladder is rich in air, and the vibrations from the outside world will hit the swim bladder like a drum. Around the swim bladder wall, there are several small bones called swim bladder bones on both sides of the first few trunk vertebrae, also known as Weber's organs (Figure 3), which are used to transmit vibrations to the inner ear to complete the perception of sound vibrations. This is similar to the human ear (external ear and middle ear)!

In addition to "ears", fish also have small holes, but they do not grow directly on the "ears".

The endolymphatic vessels of tetrapods, including humans, most bony fish, and living jawless animals are all closed and do not communicate with the outside world, that is, a closed endolymphatic system. The closed endolymphatic system can maintain the stability of the internal environment of the organism. For example, if the pressure of the membranous labyrinth of the human inner ear is unbalanced, it will lead to an almost incurable inner ear disease called "Meniere's syndrome", which manifests as sudden vertigo, tinnitus, deafness, or nystagmus, which is extremely painful for the patient.

In contrast, modern cartilaginous fish, most placoderms, and some jawless armored fish have an open endolymphatic system, that is, the inner ear communicates with the outside world through a vertical tube (the endolymphatic duct) penetrating the top of the skull. The pair of open holes on the top of the skull of fish are the external openings of the endolymphatic duct, which is the only channel for the inner ear of fish to "communicate" with the outside world. Through this pair of holes, the endolymph fluid in the labyrinth of the inner ear membrane can communicate with the outside water. At the same time, mineral particles in the water can also enter the inner ear through this pair of holes and become exogenous otoliths.

Figure 4 Evolution of the inner ear and its external opening of the endolymphatic duct in early vertebrates

AC． Inner ear structures of living cartilaginous fishes such as hagfishes (D), lampreys (E), galeaspids (F), osteostracans (G), placoderms (H); (IL), endolymphatic foramen on the top of the head of Sacabambaspis (I), Shuyu (J), Ateleaspis (K), and Dicksonosteus (I).

(Image source: Reference [4])

Part 2

The "ancestor of fish" with a small hole reveals the secrets of the endolymphatic system

Why do some organisms like humans have a closed endolymphatic system, while some other organisms require external openings? A recent study may answer this question.

Recently, the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, discovered for the first time a new species of Dayong fish: Dayongaspis colubra. This is the first discovery of armored fish fossils in the Silurian Xiushan Formation.

Compared with other types of Dayong fish fossils, it reveals more primitive characteristics of the Dayong fish family, including a pair of well-preserved small holes on the back of the carapace. The holes are located just in front of the second median transverse communicating duct near its inner ear, and may represent the external opening of the endolymphatic duct in the inner ear of Dayong fish.

Figure 5 Photo of the fossil of Cobra Dayongyu and a close-up of a pair of "small holes" on its head

(Photo credit: Photo by Gai Zhikun)

Figure 6 Reconstruction of the cobra fish

(Photo credit: Guifang Hui)

Figure 7 Ecological restoration of Cobra fish

(Photo credit: Shi Aijuan)

This time, the research team discovered small holes on the back of the head armor of the Cobra Dayong fish. These small holes also exist on the back of the head armor of some early Silurian armored fish (such as Changxing fish, Shu fish, Anji fish, etc.). Therefore, this pair of small holes may represent the primitive characteristics of armored fish.

That is, organisms with external openings are more primitive, while organisms with closed lymphatic systems are more fully evolved.

But whether the "open" or "closed" endolymphatic system of the fish inner ear is more primitive has been a long-standing topic of debate in academia.

French paleontologist Janvier believes that the closed endolymphatic system of living jawless hagfish and lamprey may represent the primitive state of vertebrates, while Swedish paleontologist Jarvik believes that the closed endolymphatic duct of lamprey may be the result of secondary degeneration, because he found that the endolymphatic duct of lamprey is longer in the larval stage than in the adult stage. British paleontologist Gardiner also believes that the closed endolymphatic duct of living ray-finned fish may also be the result of secondary degeneration rather than a primitive feature, because he found that the external opening of the endolymphatic duct still exists in some primitive ray-finned fish sturgeons (Acipenser).

What is the truth?

Part 3

The wrong hole

To get to the bottom of the problem, we need to start with a small hole that was made wrong.

A pair of similar small holes have also been described on the skull of the Ordovician Arandaichthys and Sakabanichthys, but they were interpreted by the authors as pineal foramina.

The pineal foramen is another important feature of early vertebrates. It is located on the dorsal midline of the front of the head and is the opening of the parapineal or pineal gland, which protrudes from the top of the diencephalon and is basically a part of the brain. Therefore, the pineal foramen is not a completely homologous structure in various vertebrates, but rather an opening arising from either of the two processes of the parapineal or pineal gland (but the pineal gland is the main one).

Vertebrates usually have only one pineal foramen, which is the opening of the pineal gland on the parietal cartilage. It is located between the two eyes and is able to sense light, making it the "third eye" of vertebrates.

Although the pineal foramen and endolymphatic foramen are both small open holes, their locations and functions are very different.

The Ordovician Arandaichthys and Sakabanichthys are the oldest known relatively intact armored fishes in the world. They are considered to be closely related and belong to the family Arandaichthyidae. Their small holes, called pineal foramina, are very far from the eyes but very close to the inner ears, which is difficult to correspond to the normal position of the pineal foramina in vertebrates. The pair of small holes are arranged in a bilaterally symmetrical manner, which is very similar to the paired external endolymph openings in armored fishes, bony armored fishes and placoderms.

Figure 8 The pair of small holes on the top of the head of the Sacabamba fish may be endolymphatic foramina rather than pineal foramina

(Image source: Reference [2])

Therefore, the research of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences shows that the so-called "pineal foramen" in the Aranda fish family may actually be a pair of endolymphatic foramina, not pineal foramina! The real pineal foramen of the Aranda fish family may be located on the pineal plate in the middle between the two eyes.

This discovery indicates that the open endolymphatic duct system of vertebrates may have appeared as early as the Ordovician period and is not the result of secondary degeneration. It answers the question of which is more primitive, the "open" or "closed" endolymphatic system of the inner ear of fish, and shows that the open endolymphatic system represents the primitive state of vertebrates.

As to how the endolymphatic system changed from its original open state to a closed state during the evolutionary process, there is no conclusion yet. However, such a change has great benefits for organisms. As mentioned above, it can maintain the stability of the internal environment of organisms. A closed endolymphatic system may enable organisms to get rid of their dependence on water, provide conditions for subsequent landings, and enable organisms to adapt to a wider environment.

Conclusion

As one of the jawless ancestors that are the closest relatives of living jawed vertebrates, armored fish have greatly promoted our understanding of the origin of jawed vertebrates and their key characteristics in terms of anatomy. This time, the cobra fish has explained to us that the opening state of the endolymphatic system is more primitive. Paleontologists will continue to study armored fish. I believe that with the continuous discovery of armored fish fossils in the future, more secrets of the evolutionary process from "fish to man" will be revealed!

References:

[1]Cho, YJ, Min, HJ and Kim, KS, 2022. The Differences Between 2 Cases of Preauricular Fistula. Ear, Nose & Throat Journal, 101(7): NP276-NP278.

[2]Gai, ZK and Zhu, M., 2017. Evolutionary history of Agnathans and their fossil records in China. Selected studies of paleontology in China. Shanghai scientific & technical publishers, Shanghai, 314 pp.

[3]Shubin, N., 2008. Your inner fish: A journey into the 3.5-billion-year history of the human body. Pantheon Books, New York.

[4]Zhang, Y., Li, X., Shan,

The relevant research of this article was published online in the international academic journal Historical Biology, titled: The first galeaspid fish (stem-gnathostomata) from the Silurian Xiushan formation of Hunan Province, China. The authors are Zhang Yumeng and Li Xutong, undergraduate students of Jiangxi Normal University, who were selected for the 2022 "Science and Technology Innovation Program" for college students of the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences. It was completed under the guidance of Researcher Gai Zhikun.

Produced by: Science Popularization China

Author: Zhang Yumeng (Jiangxi Normal University)

Zhikun Gai (Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences)

Producer: China Science Expo

This article only represents the author's views and does not represent the position of China Science Expo

This article was first published in China Science Expo (kepubolan)

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