Co-author Sonja Koski of the University of Zurich told Discovery News that the ancestors learned most survival skills and knowledge socially, which gave rise to the possibility of cultural differences in behaviour between populations.
She said that the groups had many males and females, and individuals started forming co-operative friendships.
Koski and co-author Jorg Massen of the University of Vienna found that chimpanzee buddies are similar to human friends in the sense that both need emotionally rewarding, positive interactions with friends, and these connections result in durable and mutually beneficial bonds.
The authors also said that chimp friendships, like human ones, can last for a very long time.
They first investigated the personality similarity of pals among 38 chimpanzees in captivity and concluded that chimps were likelier to be pals if they possessed similar tendencies for sociability and boldness.
A basic form of bonding may go beyond common ancestor of chimps and humans, about as far back as 36 million years ago when the earliest socially living primates evolved.