Outline
For mammals, the ear is a sensor to detect the existence of nearby enemies. During the evolution process from monkeys to humans,when there was a need to walk through grasslands, in order to sense predators, like lions or leopards, laying hidden in it beforehand, hearing was a sensory apparatus necessary for survival.

After seeing the lion, it was already too late to run. Hence, as hundreds of thousands of years of natural selection in the grasslands went by, the function of human hearing evolved into its current state. For us, lions are no longer such a threat. However, hearing is still an important apparatus to assert personal safety.

The following thought experiment is an example. Suppose you are walking late at night listening to your walkman on a crowded street. From there, you enter a back alley where there's a sign saying "Watch out for snatchers". It is the first time you cross that alley, there are few people around and no street lamps, so it's dark. What would you do? You probably would stop listening to your walkman.


 That's because under that kind of circunstances it's necessary to let the auditory sense perform its original function to protect oneself. Alarm clocks make people shift from sleeping to awaken condition by emitting sounds. In that case, it is hard to substitute sound for light. For example, an alarm clock that would turn on the lights of a room would not work. While sleeping, our ears are not closed, but our eyes are. As our auditory sense is still functioning as a sensor while we sleep, it may trigger a change from sleeping state to awaken state for protecting our safety.

Our ear's function of asserting personal safety faces some obtacles in modern society. For our hearing, which can detect extremely feeble variations in the atmospheric pressure (thus sound pressure), high sound pressures some machines emit become acoustic noise and may cause harm to people. In some cases, the auditory organ may be damaged resulting in dysfunction. Besides, even if there's no damage to the auditory organ, psychological impacts arise and disorders related to stress may occur. The sound environment is important to people. For us to truly live as humans and for people to continue evolving, it's necessary to develop a better sound environment.
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