Identify a region in the brain that underlies fear and aggression and is primarily involved in automatic responding.

The region of the brain is Temporal Lobe.
To be more precise, the part of the brain that underlies fear and aggression is the Amygdala.
This part of the brain played the most crucial role during the fight-or-flight situation which is very important in our survival mechanism.

The amygdala is the region of the brain that identifies fear and aggression, and is therefore involved in creating automatic responses to fearful situations.