The Evolution of the Ear
Studies on the Human Ear

The Evolution of the Ear

Some eighteen days into the development of a human embryo, even before the brain has become a complete organ, a group of surface cells on each side of the head begins to dimple. Each one of these groups forms a hollow sphere of cells in the shape of a bubble as it moves into the substance of the head. The cells then squirm and contort to create the various parts of the ear.

The human ear is a sensory organ both of hearing and balance. The balance apparatus appears to have evolved prior to the hearing mechanism. Early developing vertebrates such as fish have organs of balance, but no cochlea.

Embedded under the skin of a fish, along the length of its head and body, is a series of depressions or grooves known as the 'lateral-line'. Groups of hair cells just beneath the grooves detect differences in water pressure, which allows the fish to adjust to variations in currents and eddies, and to warn against the proximity of other fish, including predators. At the beginning of life in the oceans, even the most primitive fish possessed this simple sense organ.

Gradually, the grooves in the head evolved into the structure of the inner ear found in all vertebrates, including humans. It is easy to imagine that nerve cells in the inner ear are adaptations of earlier hair cells sensitive to the motions of liquid.

During the course of evolution, as fish became more amphibious, and finally developed into pure land animals, they required a new kind of sense organ which could detect slight differences in air pressure as a means of increasing their survival advantages, such as recognizing food, danger, friends, and enemies.

It is likely that the middle ear and the Eustachian tube evolved from the respiratory apparatus of the fish, while various inner ear structures were developed from parts of the fish jaw.

Eventually, the inner ear began to change and develop, in combination with new environmental pressures. It is probable that a small region of the inner ear partially responsible for balance evolved into the membrane of the oval window, which was flexible enough to transmit changes in air pressure to the fluid in the inner ear. At the same time, the inner ear was increasing in size and complexity. In amphibians, a small bulge appeared in the vestibular region of the ear, and as evolution proceeded, the bulge eventually developed into the spiraled cochlea which today forms the hearing mechanism of the inner ear of all vertebrates.

The range of frequencies which the ear is able to detect and analyze is likely the result of evolutionary pressure to decode complex speech sounds. Similarly, the amplitude range probably evolved in response to the loudest sounds in the natural environment. This would include the cracks and booms of a thunderstorm at close range, as well as the loud roar of predatory animals. These sounds tend to rise slowly rather than abruptly. And this may explain why the ear has no defense against extremely loud sounds which occur suddenly, without warning.

The modern cochlea, with its power to recognize the separate vibrations of each sound, has an obvious survival advantage. Since any sound which has been analyzed and transmitted to the brain can be remembered, those sounds which are associated with danger or with a promise of fulfillment can be acted upon immediately when heard again.