Stephen Porges, PhD, Professor of Psychiatry at the University of Illinois, suggests that there are three circuits which drive one of three possible responses, depending on how we sense the relative safety, danger, or threat to life in our body.
These sensations happen in the body and are first dealt with in the lower, non-thinking, unconscious brain. From your heart, stomach, and gut, the sensations zoom up your spinal cord and enter the lowest part of your brain , where they are immediately assessed by that fidgety alarm button, your amygdala, and other deep-in-the-brain limbic players, including your insula -- all well below your conscious awareness, before you can even think about it.
Your limbic brain is what I like to call The Determinator. The Determinator makes one of three calls in the face of your body's signals of potential danger:
If the Determinator judges the incoming information as life-threatening danger, one of the three parts of the circuit (the dorsal vagus, which runs between the stomach and the brainstem) leaps into action, and the body immobilizes - shuts or slows way down, basically "playing dead" to protect itself.
If the Determinator determines that there is danger that isn't life-threatening, a different part of the circuit (our old friend the sympathetic branch, a.k.a. "the accelerator") gets the body into mobilization in response to the threat - the all-too-familiar "fight-or-flight" response - which is also a way that the body tries to protect itself.
Finally - here's the really cool part - if the Determinator's assessment is that the incoming information indicates that things are safe, a third part of the circuit (the ventral vagus) essentially "turns off" the fight-flight response, and social engagement can happen - a calm state that supports being connected with others. Being in this state allows for better health, growth, and communication. This could be thought of as a third, more "advanced" method of self-preservation, and it would make sense that this would be hard-wired into our bodies just like the first two, if only for survival and evolutionary purposes.