The earliest human societies, of about twenty to sixty members, were cooperative communities based on

The answer is - family bonds.

The earliest human communities were based on familiar bonds, and they were strongly connected and cooperative communities because of this. The fact that they were connected by blood made them very close to each other, supportive to one another, and protective towards every member of the group. So to put it in a simple way, this communities were literally one big family.

answer; limbic system;

answer; ///any two strangers are likely to share many coincidental similarities;