This 2013 primate summary study showed how nonsocial behaviors, neurology and neurochemicals were repurposed through evolution to serve social functions.

Oxytocin and vasopressin retained their:

• water regulation,
• reproduction, and
• anxiety relief

functionalities while they also evolved to become instrumental in:

• pair-bonding,
• parental care,
• selective aggression,
• social prominence,
• generosity, and
• trust.

http://www.pnas.org/content/110/Supplement_2/10387.full “Neuroethology of primate social behavior”