After insects, the earliest evolution of hearing was in fish, mostly likely a boney fish (teleost). But not all fish hearing stems from a single ancestor: there are at least 20 cases of the independent evolution of hearing in fish species (Fay, Popper, and Webb, 3). Of over 30,000 species of fish, only a small number have been tested for hearing capacity (Fay, Popper, and Webb, 2). Fish hearing is a good example of how evolution repurposes structures that initially evolved for some other adaptation.

Uses and Advantages of Hearing in Fish:

Since the intensity of light varies at different depth of water, hearing can be the more reliable long-distance sense for fish (Stebbins, 1983). Fish use their hearing as a way to avoid predation from other marine organisms and, for fish that swim close to the surface, from birds of prey detection. Hearing also aids predatory fish in detecting their prey (Gans, 7-9). Some species use hearing for social purposes such as mating and for the social-locomotion movement within a school (Putland, 2018).

Evolutionary History:

The structures described above are common to all Gnathostomes, the jawed vertebrates that emerged in the Devonian, around 419 million years ago (Fay & Edds-Watson, 2008). However, the semi-circular canals initially evolved as a vestibular system -- a component of proprioception. The octavolateralis system evolved first, as a mechano- and electro-receptive system (Gans 6). The structure of hair cells synapsing with nerves to detect motion is a function of this system. Then mechanoreceptive cells evolved to detect sound waves, a form of mechanical energy, as distinct from other mechanical features of the environment (Gans 11). The swim bladder, with its air filled sacs, did not initially evolve for hearing either, but in some fish species, it became adapted to amplifying a sound signal. The Weberian ossicles were initially vertebral bones, later evolving to their role in the fish middle ear. Thus the entire structure demonstrates how evolution repurposes what it already has to work with.

That theme in fish hearing is helpful for thinking about hearing in reptiles, birds, and mammals. The fish structures are analogues of those found in terrestrial vertebrates, but analogous structures are also common. Among fish there is insufficient data to distinguish analogies from homologies, but at least 20 cases of independent evolution of hearing in fish have been found (Ladich & Popper, 2004). The combination of repurposing adaptation and the evolutionary advantage of hearing is something we see in the other categories of organisms covered.