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The cochlea is a recent evolutionary development. Mammalian ears, including our own, still also rely on features from our early vertebrate ancestors.
Most of us think of our senses as a group of five- vision, hearing, touch, smell, and taste-and can readily identify the parts of the body that let us process information about the world through each of these senses. Less easy to locate are the organs that underlie our sense of balance, although we depend on them at every moment. Hidden deep inside the ear, the organs of the vestibular system generally perform their role without requiring conscious attention. Yet an illness or injury that affects vestibular function can be devastating, as anyone with an innerear infection will testify. Sufferers may be almost entirely incapacitated, unable even to stand up, raise their head, or keep their eyes steady, without vomiting; as if this weren't enough, they often feel acute anxiety as well. The unusually broad range of symptoms that can erupt when our sense of balance is disturbed, affecting everything from vision and hearing to posture, gut functioning, emotions, and cognition, should tell us just how crucial Öie vestibular system is for maintaining a normal physiological state.
In mammals the vestibular apparatus shares close quarters with fire cochlea, the main organ of hearing, in the inner-ear structure known as the labyrinth. The two systems also have other features in common, such as the use of hair-cells as mechanoreceptors, to perceive sounds and head movements and encode them in the form of nerve signals. Even the sensory nerves which run from the cochlea and the vestibular apparatus are bundled together in their passage up to the brain. This close partnership bears witness to a shared history of sensing vibration and gravity that can be traced back in geological time at least 500 million years.
From an evolutionary perspective, the cochlea is a new kid in the neighborhood. The earliest vertebrates, including fish, do not possess a cochlea or any organ remotely analogous to it. One could conclude from this that fish must be deaf. How can they hear without a cochlea? Attempts to explain this seeming paradox began nearly 100 years ago with the publication of a paper by Canadian physiologist John Tait,...