Evolution of the Fight or Flight Response

The goal of any individual living creature is to ensure the survival of its species into future generations. It is why individuals reproduce. The whole purpose is to make sure the species continues long after that individual has passed away. If that individual's particular genes also can be passed on and survive into future generations, that is even better for that individual. That being said, it makes sense that, over time, species have evolved different mechanisms that help make sure that individual will survive long enough to reproduce and pass down its genes to some offspring that will help make sure that the species continues on for years to come.

Survival of the Fittest

The most basic survival instincts have a very long evolutionary history and many are conserved between species. One such instinct is what is referred to as "fight or flight." This mechanism evolved as a way for animals to become aware of any immediate danger and to act in a way that will most likely ensure their survival. Basically, the body is at a peak performance level with sharper than usual senses and an extreme alertness. There are also changes that happen within the body's metabolism that allow the animal to be ready to either stay and "fight" the danger or run away in "flight" from the threat.

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So what, biologically, is actually happening within the animal's body when the "fight or flight" response has been activated? It is a part of the autonomic nervous system called the sympathetic division that controls this response. The autonomic nervous system is the part of the nervous system that controls all unconscious processes within the body. This would include everything from digesting your food to keeping your blood flowing, to regulating hormones that move from your glands, to various target cells throughout your body.

There are three main divisions of the autonomic nervous system. The parasympathetic division takes care of the "rest and digest" responses that happen when you are relaxing. The enteric division of the autonomic nervous system controls many of your reflexes. The sympathetic division is what kicks in when major stresses, like an immediate threat of danger, are present in your environment.

Adrenaline's Purpose

The hormone called adrenaline is the main one involved in the "fight or flight" response. Adrenaline is secreted from glands on top of your kidneys called the adrenal glands. Some things adrenaline does in the human body include making heart rate and respiration faster, sharpening senses like sight and hearing, and even sometimes stimulating sweat glands. This prepares the animal for whichever response—either staying and fighting the danger or fleeing away quickly—is the appropriate one in the situation it finds itself in.

Evolutionary biologists believe that the "fight or flight" response was crucial for the survival of many species throughout Geologic Time. The most ancient organisms were thought to have this type of response, even when they lacked the complex brains that many species have today. Many wild animals still use this instinct on a daily basis to make it through their lives. Humans, on the other hand, have evolved beyond that need and use this instinct in a much different way on a daily basis.

How Daily Stress Factors Into Fight or Flight

Stress, for most humans, has taken on a different definition in modern times than what it means for an animal trying to survive in the wild. Stress for us is related to our jobs, relationships, and health (or lack thereof). We still use our "fight or flight" response, just in a different way. For instance, if you have a big presentation to give at work, most likely you will become nervous. The sympathetic division of your autonomic nervous system has kicked in and you may have sweaty palms, a faster heart rate, and more shallow breathing. Hopefully, in that case, you would stay to "fight" and not turn and run out of the room in fear.

Once in awhile, you may hear a news story about how a mother lifted a large, heavy object—like a car—off her child. This is also an example of the "fight or flight" response. Soldiers in a war would also have a more primitive use of their "fight or flight" response as they try to survive in such horrific circumstances.