All living organism share a universal behavior set called self preservation. Self preservation is the set of behaviors that ensures the survival of an organism. Think of a deer in the woods that suddenly bolts away from you. Fear is an integral part of all organism’s survival mechanisms. Pain is just as important. Pain motivates one to withdraw from dangerous and deadly situations. Pain reminds us to protect our wounds until they heal. It encourages us to avoid similar situations in the future.
When the cause of pain is removed from a body and the body has healed, in most cases, the pain ceases. Sometimes the pain persists despite the removal of stimulus and the body’s healing. Sometimes pain occurs in absence of any detectable stimulus, injury, or disease. In terms of self preservations, fear can cause an organism to seek safety and even release adrenaline.
Adrenaline (or epinephrine) is a hormone which acts as a neurotransmitter involved in regulating things like respiration. Adrenaline plays a big part in fight or flight survival responses through pupil dilation, blood sugar level, output of heart, and increasing blood flow to muscles. Adrenaline is even found in some single celled organisms.
All organisms are under intense selective pressure. They must all act to avoid damaging environments. Many organisms have self preservation responses that are highly specialized for their environment. This means that when we enter environments for which we have not evolved to cope, our self preservation behaviors may not be specialized enough. Consider how helpless humans are at sea. Self preservation instincts are developed in tandem with reproductive fitness. Any organism’s fitness is qualified by its ability to pass on its genes. The genetic code is a language for specifying organisms. Genes get themselves replicated at the expense of their rivals. In order to survive long enough to reproduce, an organism must enact selfish behaviors that allow it to maximize its own chances for survival.