A good friendship is priceless - but it appears to be almost timeless as well.
Researchers suggest that being close to others with similar personality traits goes back millions of years.
They found the phenomenon occurs not only in people but also in chimpanzees. This indicates its origins date to when humans and the apes had a similar evolutionary path before going their separate ways.
The last common ancestor of chimps and humans lived at least six million years ago, indicating the first 'best friends' walked the Earth all that time ago, said Austrian and Swiss researchers.
They studied colonies of chimpanzees in two Dutch zoos and firstly rated their personality traits.
Sociability was measured by physical proximity, for example how often individuals were within 6ft of another chimp.
The frequency of grooming, in which the apes remove dirt and parasites from each other, was also crucial as it builds bonds.
Then, to work out which chimps were friends, they examined how often one sat or lay in physical contact with another.
This could include sitting side by side, back to back or resting a whole leg or arm on another ape - all signs of friendship.
Analysis revealed the closest friends had similar characters.
The researchers said: "Chimps of a feather sit together."
Jorg Massen, of the University of Vienna, said: "The most sociable and bold individuals preferred the company of other highly sociable and bold individuals. The shy and less sociable ones spent time with other similarly aloof and shy chimpanzees."
In the US journal Evolution of Human Behavior, he added: "What draws both chimpanzee and human friends together is similarity in gregariousness and boldness, suggesting that preference for self-like friends dates back to our last common ancestor."
He added that teaming up with someone of similar character can help build trust, while friendships between daring chimps would help when trying to fight off predators.
- Daily Mail