Without the ability to process faces, our ancestors' ability to recognize danger would have been less. They needed the ability to see danger before it was coming, or else run the risk of having their tribes infiltrated by strangers from other tribes who meant them harm.
Without women having the ability to recognize faces, she would not be able to fulfill her duties as a member of the tribe and also as a caretaker of the family. This is why face recognition and face processing would have been important.
The other thing that has to do with face recognition ability is neuropsychological ability as well as cognitive capacity. Obviously, the structure of one's brain and its ability to function plays a large role in whether or not someone will be able to correctly identify faces.
Basically, face recognition and face processing are very dependent upon one's ability to figure out whose face is whose. This is particularly important in evolving societies which are still not industrially advanced.
Neuropsychological and cognitive elements are included in the necessity to identify faces properly. People need to know who their family members are, as well as how to identify potential threats in a community.
Neuroimaging helps scientists determine what parts of the brain are ...