If someone says you have a "dead fish face", it's probably not an insult...

From a broader perspective, the evolution of fish blowholes involves several important leaps in the evolution from fish to humans, and is also closely related to us humans.

Written by reporter Duan Ran Edited by Liu Zhao

Interview experts

Zhu Youan (Associate Researcher, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences)

Recently, after a detailed study of ancient fish fossils unearthed in my country that are more than 400 million years old, the ancient fish research team of the Chinese Academy of Sciences announced that they have found fossil evidence that the fish blowhole evolved from a gill structure with oxygen-taking function, thus confirming a hypothesis proposed by paleontologists for more than a hundred years: that the fish blowhole evolved from a typical gill opening or gill slit.

Judging from the conclusion alone, this seems to be just a small step in fish evolution. But from a broader perspective, the evolution of fish blowholes involves several important leaps in the evolution from fish to humans, and it also has a great relationship with us humans.

Many readers may have questions. Aren’t we studying the origin of fish blowholes? How is it related to human evolution? In fact, the middle ear, an important part of our human auditory system, evolved from the blowholes of fish. And if we expand our perspective to the important organs of our entire face, our eyes, nose, mouth and ears, we can actually find the source of evolution in ancient fish billions of years ago. I started from the forehead and touched the chin and even the neck with my hand. This important area that reflects human emotions and wisdom is actually the evolutionary legacy left to us by our distant fish ancestors.

▲Reconstruction of the dawn fish, which is about 5 cm long. The oval holes in front of its eyes are nostrils rather than mouths. Its mouth is located under the head like most jawless fish (drawing/B.Choo)

Fish gradually gaining a "face"

According to the view of modern mainstream biology, all terrestrial vertebrates, including humans, evolved from ancient fish. Many of our basic physiological structures can be traced back to ancient fish, such as the limbs that evolved from pectoral and pelvic fins; another example is our complex facial structure: the coordination of the five senses and facial muscles not only meets basic physiological needs such as breathing and eating, but also serves as a window for receiving and processing external information and outputting feedback, and is an important carrier for expressing emotions such as joy, anger, sorrow, and fear, as well as for identity recognition. This is a part that is highly adapted to human social life, but its basic structure had already been designed in ancient fish billions of years ago.

Since we are exploring the true origin of the human face along the evolutionary path of "from fish to man", we must first clarify what fish are. There are several different interpretations of the definition of fish. "One method is to start from the perspective of evolution. All vertebrates that are not quadrupeds and have not landed are considered fish," Zhu Youan, an associate researcher at the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, explained to reporters. "Of course, there are also some narrow definitions. For example, in the traditional Linnaean biological classification, the definition of fish only includes bony fish and cartilaginous fish."

No matter from what angle we define fish, it is still somewhat unbelievable to regard the face of a fish as the original state of a human face. Zhu Youan used a rather interesting metaphor to vividly explain the potential connection between human and fish faces: "Look at the fish monsters that appeared in Journey to the West: Babolben and Benbolba. Ordinary people only need to put on relatively simple makeup or use a headgear to show the characteristics of the fish monsters. But those shrimp soldiers and crab generals in the play are not so easy to show."

In addition, we often come into contact with anthropomorphic images of fish in real life, such as Nemo, the clownfish in "Finding Nemo" and the "salted fish emoticons" used for self-deprecation and spoofs on the Internet. Behind these, there are actually some inevitable evolutionary connections.

If we expand our vision from the simple distinction between fish and humans to all vertebrates: whether it is modern fish or birds and animals on land, all vertebrates have their own "face", and when talking about the face, we must mention the head, and the face of vertebrates is located at the front end of the head.

According to traditional biological classification, vertebrates are a subphylum of chordates, and other subphyla include urochordates, cephalochordates, etc. "Vertebrates are also called cephalanxes in evolution, because only vertebrates have heads in the chordate class," Zhu Youan told reporters.

Many chordates other than vertebrates do not have concentrated ganglia and sensory organs at the front of their bodies, which means they have no brains. For example, the lancelet, a chordate, is a small creature without a head or brain. Vertebrates are different. They integrate complex nervous systems such as the brain at the front of their bodies to form a head structure.

With a head, many sensory organs also began to be "deployed" around the head. These sensory organs used to receive external stimulus signals have gradually evolved into three types of organs according to different divisions of labor: one is the optical sensory organ, that is, our eyes. One is the chemical sensory organ, such as the nose and tongue we use to sense smell and taste. "Another type is the mechanical sensory organ. One of the manifestations of fish is the lateral line structure, which can sense changes and vibrations from water pressure." Zhu Youan said. Our auditory organ, the ear, is a mechanical sensory organ. The evolution of the ear is inextricably linked to the lateral line of fish, which we will focus on below.

The emergence of the jaw: a breakthrough leap in facial evolution

With the center responsible for thinking and these three sensory organs, the facial features of fish gradually became clearer, and they became creatures with "heads and faces". But at this time, primitive fish still lacked a crucial part of the face - the jaw, which is the bones and muscle tissues above and below the mouth.

Many primitive fish do not have upper and lower jaws, and therefore it is impossible for them to have the ability to open and close their mouths like modern fish or even humans. For example, the Kunming fish and Haikou fish that lived in the Cambrian period 500 million years ago, and the armored fish that flourished in the Ordovician and Silurian periods, these primitive fish that do not even have chins still have unclear facial contours.

Until about 430 million years ago, the gills used by primitive fish for breathing began to degenerate, and part of the gill arches gradually evolved into the structure of the jaws - the first full-jawed fish was born. The appearance of the jaws laid the basic prototype of our mouth structure today, achieving a leap in species evolution.

▲Details of the initial full jaw fish fossil specimen showing the lower jaw (Image source/Nature)

"With jaws, species can eat more actively. They can bite food, and the efficiency of hunting and eating is greatly improved." Zhu Youan pointed out. Of course, our mouth is not only used for eating. When we exercise vigorously, our mouth will breathe heavily, and when we dive, we will install a snorkel in our mouth. This points to another important function of the mouth - breathing.

Like the nose, the mouth is also an important respiratory organ, and this is also true for fish. Although fish breathe air through gills, the mouth is the main pathway for directing oxygen-rich water to the gills for breathing. Zhu Youan said: "With the jaw, the fish's mouth can open quickly, so it can better absorb water and then flow through its gill cavity, improving the efficiency of breathing, so that the fish's ability to absorb oxygen is also enhanced."

The enhancement of oxygen uptake capacity brings another more far-reaching impact: more oxygen means higher metabolic efficiency, so the size of vertebrates can become very large, and they have the conditions to occupy more ecological niches and even stand at the top of the food chain.

Under the challenge of jawed vertebrates, those ancient invertebrates with very large sizes, such as giant nautilus and sea scorpions, were quickly driven down from the position of large predators.

▲The largest predatory fish in the Silurian period - Macrognathus blunt-toothed, its body length may be about 1 meter (Photo source/Sci-News)

The appearance and continuous improvement of the jaw finally laid the basic outline of today's vertebrate face. If we look closely at these ancient fish ancestors, we will find that the initial form of facial features has appeared, and there are traces of eyes, mouth and nostrils, but there is still a considerable gap in function from the true facial features. This gap needs the appearance of internal nostrils and middle ear structures to make up for it.

From nose to ears: challenging the terrestrial environment!

First, the internal nostrils. Whether in the fish market or in the aquarium, if you pay close attention, you will find that fish also have nostrils above their mouths. However, their nostrils are very different from human noses. Since fish mainly rely on gills to breathe, and the mouth is used as an auxiliary, the nose naturally has no breathing function, but only exists as an olfactory organ, and the nostrils are not connected to the mouth.

This creates a hidden danger: if the oxygen content in the water is insufficient and breathing with gills can no longer meet the oxygen intake requirements, what should be done? In the summer, when it is about to rain, many fish will swim to the surface of the pond and stream and swallow air with their mouths. They are forced to do so by the low oxygen content in the water. This method of supplementing oxygen from the air is very clumsy and obviously not a long-term solution.

As fish continued to evolve and expand into shallow water environments at the interface between land and water, the original nostrils changed and expanded their functions to include breathing. The emergence of the internal nostril structure shared a large part of the breathing task of the original mouth. "In this way, fish can float on the water surface, expose their nostrils above the water surface, and use the internal nostrils to guide air into the gill cavity for breathing. Then its mouth can focus on other things, such as eating. This has a great advantage in evolution." Zhu Youan explained.

The structure of the internal nostrils has allowed the nose to truly evolve into a respiratory organ, and the improvement of the nose's functions is also preparing for fish to land from shallow water areas in the future and opening the prelude to the vertebrates' march onto land.

In addition to the nostrils, the ears have undergone a more radical change in function. The ears of humans and terrestrial mammals are both hearing organs. Like the eyes, mouth, and nostrils, the ears actually already have a rudimentary form in fish, and all vertebrates have inner ear structures. However, this so-called "inner ear" is not actually used to perform the function of "hearing". Fish already have other organs to perform the "hearing" task, which is the lateral line structure of fish mentioned above.

The essence of hearing is to perceive the vibrations generated by sound waves in propagation media such as air and water. Sound propagates faster in water than in air, and it is much easier to perceive sound waves in water than in air. Therefore, many fish can perceive the vibrations of sound waves in water by relying on the lateral line structures on both sides of their bodies.

So what is the "inner ear" used for? "The main function of the fish's inner ear in the early days was to sense balance," Zhu Youan explained. "The inner ear is a vesicle-like structure called the membranous labyrinth, which contains endolymph. There are also many ciliated cells on the membrane of the membranous labyrinth." When fish are exercising, these ciliated cells are used to sense the pressure generated by the relative shaking of the endolymph in the membranous labyrinth, so as to gain a sense of balance.

Therefore, the inner ear can be easily transformed into an auditory organ based on the principle of perception. Even in an aquatic environment, the inner ears of some fish have evolved hearing and can perceive sound waves propagating in the water, which greatly supplements the role of the lateral line organ. "Bone bladder fish such as crucian carp and catfish have also developed hearing organs secondarily, namely the swim bladder. This structure is rich in air, and the vibrations from the outside world will hit the swim bladder like a drum." Zhu Youan explained to reporters. The swim bladder itself is placed on several small bones called Weber's organs, which are used to transmit vibrations to the inner ear to complete the perception of sound vibrations. This is similar to the human ear.

When fish began to move onto land, they faced a more complicated situation: they had to sense sound vibrations from the air, but sound wave vibrations were greatly attenuated in the air, so the lateral line was completely useless, and the inner ear structure alone was obviously not up to the task. As a result, some organs of fish that were not adapted to the terrestrial environment began to evolve, such as the blowhole. "The blowhole was originally part of the gill arch of early fish. Although it is called a blowhole, it is actually mainly used to draw water into the gill cavity for breathing," Zhu Youan emphasized.

When the fish came to the land environment, the need to breathe in water disappeared, but nature would not let this organ go unused. So as the fish head structure continued to evolve, the blowhole gradually moved from the original central position of the fish head to the approximate position of the ear today, and evolved into the eardrum of the middle ear cavity today. The lingual and maxillary bones, which were originally used as "suspenders" for the jaw arch, also degenerated and became small bones for transmitting sound waves. Only then did the basic structure of the middle ear become established. In the long evolutionary process, the external ear structure gradually appeared, which formed our ears today.

Over billions of years, our faces today have already completed the construction of their basic structure in water. Although the evolution of the face is only a small part of the grand epic of species evolution, we can follow this path, trace back to the origins, and trace the clues of fish faces, so that we can fill in more puzzles for our own evolutionary path.