Why we fear?

Fear it is a natural mechanism of the body to protect itself. But I am not told that mechanism that why fears that seem to be able to feel from within consciousness. Or not only the fear but also the emotions, whether the body evolved to represent the basic information of the sense or simplicity of the body that creates it and the sense of learning to survive deep inside and is the source of information for the Evolution
. 
Essentially I am quite confused about this problem.