system
Given the following post, respond as if you're commenting on Reddit:
human
Whats going on in your body when you get the irrational "I feel like something is behind me" feeling?

I'm sure everyone has gotten this at some point. Its the uncomfortable feeling you get maybe if its late at night, where it feels like someone is behind you. Its an urge that can only be satisfied by turning around and checking. What is going on internally when this happens and what causes it?
gpt
Humans evolved in the wild. We were not always so completely safe from other predators. People who developed a good response to the possible threat of a predator typically survived long enough to reproduce and pass on those traits. What you consider as irrational now may have been a perfectly justified response when our ancestors roamed and foraged. Having an instinctual response to a possible risk of predation is commonly seen in other animals ranging from fairly basic animals right up to primates. It's a fairly basic response, except in humans it has developed as a complex state of alarm and corresponding changes to things like adrenaline release, feelings of anxiety and paranoia, goosebumps which (were we to have hair like apes) would make us look bigger and more threatening, also blood in your body is redirected from your gut and viscera to your limbs and brain to allow fight or flight. This is your sympathetic nervous system engaging.