How Hearing Evolved from Water to Land

Lungfish and salamanders can hear, despite not having an outer ear or tympanic middle ear. Now, scientists are taking a closer look at exactly how hearing evolved about 300 million years ago.

The physical properties of air and tissue are very different. This means that up to 99.9 percent of sound energy is reflected when sound waves reach animals through the air. In humans and many other terrestrial vertebrates, the ear can be divided into three sections: the outer ear, the middle ear and the inner ear. The outer each catches the sounds waves and then directs them to the middle ear, where air pressure oscillations are transferred via the tympanic membrane and one of three small bones to fluid movements in the inner ear.

This configuration is important for hearing in present-day terrestrial animals. However, available fossils indicate that the tympanic middle ear evolved in the Triassic, which is 100 million years after the transition of the vertebrates from an aquatic to a terrestrial habitat during the Early Carboniferous.

In order to see how hearing evolved, the researchers studied the hearing of lungfish and salamanders, which have an ear structure that is comparable to different kinds of early terrestrial vertebrates. They measured auditory nerve signals and neural signals in the brainstem as a function of sound stimulation at different frequencies and different levels.

So what did they find? It turns out that the terrestrial adult salamanders and fully aquatic juvenile salamanders in addition to lungfish could detect airborne sound despite not having a tympanic middle ear. They sensed the vibrations induced by the sound waves.

The findings reveal a bit more about the evolution of hearing. More specifically, it shows that even vertebrates without outer and middle ears are capable of detecting airborne sound.

The findings are published in the journal Proceedings of the Royal Society B: Biological Sciences and The Journal of Experimental Biology.

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