Your middle ear was once a fish gill? This evolutionary process is no joke

Human middle ear

It is an important organ of hearing

Exploring its evolutionary origins in nature

Always in the spotlight

At present, there is sufficient embryonic and fossil evidence to prove that the human middle ear evolved from the blowhole of fish. However, where the blowholes of fish came from has been a century-old problem that has plagued the academic community for a hundred years.

Recently, Chinese scientists led an in-depth study of ancient fish fossils dating back more than 400 million years ago collected from Changxing, Zhejiang and Qujing, Yunnan, confirming that the fish blowholes evolved from the first pair of gills. This is also the first time that the scientific community has revealed through fossil evidence that the middle ear used by humans for hearing originated from the gills used by fish for breathing .

Today we will study:

Where do fish blowholes come from?

How did fish gills evolve into blowholes?

How was the evolutionary sequence from gills to ears established?

Early theories

Gai Zhikun, a researcher at the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, said that fish have a hole on their backs behind their eyes and between the mandibular arch and the lingual arch. This is an important respiratory organ of fish - the blowhole . The blowhole is actually the water intake hole for fish to breathe. It is very different from the gills of fish. It only contains false gills and cannot exchange oxygen.

In terms of the evolution of fish blowholes, after the French anatomist Saint-Cyran published his classic work "The Philosophy of Anatomy" in 1822 and the German anatomist Karl Gengebauer proposed the theory of vertebrate head segmentation in 1872, the British paleontologist Watson further proposed the "free tongue arch theory" in 1937, which believed that the primitive state of early jawed fish was that they should have a complete, non-degenerate gill slit between the jaw arch and the tongue arch, rather than a blowhole .

This theory inspired paleontologists in the 20th century to search for an undegenerated gill cleft between the jaw arch and the lingual arch of early jawed fish. Unfortunately, after a century of searching, paleontologists from various countries have carefully examined the fossils of all armored fish, spiny fish, cartilaginous fish and bony fish in the living jawless fish and early jawed fish, but have not found any definite fossil evidence.

Practice verification

Since 2002, a team from the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences has been conducting field work in the early Silurian (about 438 million years ago) marine red strata in Changxing, Zhejiang, and found a most primitive true armored fish fossil, which was later named "Aurora fish".

Through in-depth research, it was found that the gill sac behind the eyes of the armored fish is the lingual-mandibular sac located between the maxillary arch and the lingual arch, which is no different from the five gill sacs behind it. It opens on the ventral side of the head carapace, and does not open on the back of the head carapace like the blowhole. Therefore, it is basically judged that the lingual-mandibular sac of the armored fish is still a non-degenerated gill sac .

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

Gai Zhikun said that the research team turned their attention to Qujing, Yunnan, which is known as the "kingdom of ancient fish" and the "birthplace of human ancestors". Through several years of field excavation in the Early Devonian strata of Qujing (about 419 million years ago), in 2017, they were lucky enough to collect the first new material of the broad turtle fossil with the gill filaments fully preserved in the first gill sac behind the eyes.

Through 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 the most conclusive anatomical and fossil evidence for the claim that "the blowholes of vertebrates such as fish originated from degenerate gills."

Building an evolutionary sequence

Gai Zhikun, a researcher at the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, said that based on the fossil evidence that solved a century-old scientific problem, the research team clearly proposed the evolutionary process of blowholes from jawless fish to tetrapods, thus establishing the evolutionary sequence of blowholes from the gills of jawless fish to the human middle ear.

With the splitting of the paired nasal sacs in armored fishes, an abnormally large dorsal aperture (single nostril) developed in front of the head armor to serve as the main respiratory organ for inhaling water. Therefore, the lingual maxillary sac between the maxillary arch and the lingual arch developed into a complete gill sac for the first time. This gill sac, like the 5 normal gill sacs behind it, has complete anterior and posterior half gills. The half gills have gill filaments and are the main place for oxygen exchange.

With the origin of jaws and double nostrils, jawed fish successfully evolved double nostrils, but the double nostrils are not connected to the mouth and have no respiratory function, only olfactory function. However, the respiratory needs of fish have not decreased, so the first gill sac behind the eyes (lingual jaw sac) was transformed into a blowhole, becoming the main respiratory organ. This has already occurred in the most primitive jawed armored fish, because the most primitive armored fish, the armored fish (grooved scale fish) and the most advanced armored fish, the unicorn fish, already have blowholes. Therefore, the blowhole is likely to have originated in jawed fish at the same time as the appearance of jaws and double nostrils .

In cartilaginous fish, the blowhole is mainly used to inhale water, while in early bony fish, it is mainly used to breathe air, thus providing a prerequisite for fish to come to land and breathe air . With the origin of the internal nostrils in lobe-finned fish (such as Kemp's fish), the passage between the nasal cavity and the oral cavity was successfully opened, and the nostrils became the main respiratory organ, thus further laying the foundation for fish to come to land and breathe with lungs.

The four-legged animals that came onto land faced a completely new environment and had to develop new sensory organs in order to survive better in the air. In this context, the blowhole that had lost its respiratory function was recycled and gradually evolved into the tympanic cavity of the human middle ear cavity , while the quadrate bone and articular bones of the lingual jaw and its joints gradually degenerated and became smaller, eventually entering the middle ear and evolving into three ossicles, which were renamed stapes, malleus and incus, responsible for transmitting sound to the brain, allowing humans to finally have sensitive hearing.

"If you pinch your nose, close your mouth and blow hard, you will feel the eardrum swell up. This shows that our ears and mouth are still connected, and what connects them is the Eustachian tube that originally led from the blowhole to the mouth. This is also one of the evolutionary relics left to us by fish," said Gai Zhikun.