Evolution of the Human Hearing
The human ear has two major functions: ensuring body balance and sound detection. The balancing mechanism seems to have evolved earlier than the hearing mechanism. Early developing vertebrates had balancing organs, but they have no cochlea. A good example is fish. The basic function of the human ear is to sense sound. Sound is a compression of a wave, which travels via a medium. For vertebrates, the medium perceived is air. Sound waves can travel through other mediums, such as water or ground.
Hearing is a result of vibration of the surrounding medium that resonates parts of the body. The resonance is then transformed into electrical signals such that they can be interpreted by the human ear. The way human beings recognize sound has undergone changes since the discovery of the ear.
According to paleontologists, our earliest ancestors breathed through their ears. According to (Gangestad, 2000) tubes that form the middle ear developed from gill-like structures that allowed sea creatures to breathe from the back of their heads.
Another study was done by ( Anthwal, 2012) which sought to examine 370-million-year old fossils, Panderichthyts. This is an immediate species between the amniotes and fish. The study reveals that Panderichthys had tiny bones in its skull that appeared similar to the early analogues of the gill system and the ear canals. The study suggests the canals are the ones which developed into true ears. This occurred after Panderichtys’s ancestors had become air breathers, freeing up their former gills structures for sensory functions.
Another study of a hominid that was discovered in South Africa confirms that the human ear has undergone several changes. The fossil dated 1.9 million years was found to have several bones which are found on the modern human ear. However, the bones were not exactly the same. Three ear bones were identified. The malleus appeared to be human-like while the stapes and the incus appeared to resemble those of Chimpanzees.
The study asserts that since the malleus of our early ancestors looks similar to ours, the changes of the bone must have occurred during our evolutionary history. The discovery is important in two ways. First, it suggests that ear ossicles are adult-sized and fully-formed at birth, but do not change in our lifetime. Second, the bones show that the hearing ability of ancient creatures was very different from that of modern humans. This is not necessarily, better or worse. But certainly, the hearing capability was different (Texas University, 2013).
Land vertebrates, birds, reptiles and mammals evolved primitively from amphibians, specifically the primitive fish. The formation of the inner ear started during the Devonian Period. There has been a debate whether the lateral line structure, a sensory system used to detect variations in pressure in amphibians, developed into the inner ear. Most scientists and researchers agree that the ear evolved from this lateral system. The lateral system is found beneath the skin of fish. It is a series of grooves and depressions with a group of hair cells, which lets the fish adapt to variations in eddies and currents.
According to (Duane, 1981) primitive fish possessed a simple sensory organ. The grooves evolved into the complex inner ear in vertebrates. The nerves cells in the human ear are adaptations of the earlier hair cells. In the course of evolution, fish become more amphibious. Finally, it evolved into a land animal, which required a new sensory system to detect differences in air pressure. This was a way of improving their survival rates, such as danger recognition. According to (Clark, 2003) the Eustachian tube and the middle ear evolved from the respiratory mechanism of the fish, while the inner ear evolved from jaws.
With time, the inner ear changed and developed. The part of the inner ear responsible for body balance evolved to the membrane of the oval window. The oval window is responsible for transmitting variations in air pressure. As this happened, the inner ear was growing bigger and bigger. In the fish, a tiny swell emerged in the vestibular of the ear.
As the evolution progressed, the bulge evolved into a spiral cochlea. This is what forms the hearing system of vertebrates. Gradually, fish evolved into amniotes, which are fully terrestrial vertebrates. Early amniotes did not have eardrums. According to (Gangestad, 2000) eardrums evolved six times in primitive amphibians, in anurans, in sysnapis, in diapsids (lizards, dinosaurs, and bids), in anapsids (turtles and its relatives), in reptiliomorpha and in temnospondyls.
The study plays a critical role in understanding the evolution of the human ear. Our ability to ear relies on structures, which started as a gill opening in a fish, the study reveals. Humans and all other vertebrates have special bones in the ear responsible for hearing. Ancient fish relied on the same structures to breath while in water.
The study further argues that the human ear evolved into a complex structure after animals established themselves on land. To draw valid conclusions, the study compared the fossil with its close cousin of the first land animals. In another fossil, Eustenopteron, a discovery of a small bone called hyomandibula was made. The bone later on developed a kink and blocked the gill opening.
Furthermore, in early land animals like tetrapods Acanthostega, the bone receded, forming a larger opening. This is now a part of the middle ear in all vertebrates, including humans. A close study of the Panderichthys fossil offers scientists a crucial missing link between the ears and fish gill openings. According to (Anthwal, 2102) the characteristics are much more like those of tetrapods; there is no longer kink but the spiracle is widened and opened up. He found out that the hyomandibula is shorter, but rod-like in Eusthenopteron.
Following the discussion presented above, it can be noted that the human ear underwent several stages of evolution. The studies presented above clearly shows that the hearing capability of ancient creatures was different from that of modern humans. In addition to this, it can be noted that the human ear evolved from a simple gill structure all through the jaw bones of a reptile. The following diagram summarizes our discussion.
Clark, A. G. (2003). Inferring nonneutral evolution from human-chimp-mouse orthologous gene trios. Science, 302(5652), 1960-1963.
Gangestad, S. W. (2000). The evolution of human mating: Trade-offs and strategic pluralism. Behavioral and brain sciences.
23(04), 573-587.Texas A&M University. (2013, May 13).
Prehistoric ear bones could lead to evolutionary answers. ScienceDaily. Retrieved March 22, 2014 from www.sciencedaily.com/releases/2013/05/130513174048.html.
Anthwal N. (2012). “Evolution of the mammalian middle ear and jaw: adaptations and novel structures”. Journal of Anatomy 221 (1): 1–96.
Diagram Adapted from: http://evolution.berkeley.edu/evolibrary/article/evograms_05References
Duane T. Gish, “The Mammal-like Reptiles,” Impact, no. 102, December 1981.
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