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All these findings, of course, closely track what we know about friendship benefits in humans. Studies have shown that people with close social networks have lower blood pressure, lower levels of stress hormones and more robust immune systems than those without. In 2010, scientists at Brigham Young University analyzed data gathered from more than 300,000 people. They found that having poor social connections can raise the risk of premature death as high as that from a smoking habit and even higher than that from obesity.
If humans came late to the idea that other animals have the same capacity to form friendships that we do and derive the same benefits, it may be that we weren't paying attention. Chimpanzees and baboons, which both form long-lasting friendships, share an ancestor with humans, one that lived 30 million years ago. Maybe that monkey-like progenitor formed friendships with its troopmates, and maybe it inherited the ability from a still more distant mammalian grandparent. Even as we all diverged into multiple species, pursuing our very different evolutionary arcs, all of us--Nicklo the dolphin and Hare the chimpanzee and Bob, the guy who's been your best friend since high school--may have retained the simple but powerful ability to find one another and care about one another.
Zimmer, a lecturer at Yale University, is the author of A Planet of Viruses
FOR PHOTOS OF REMARKABLE ANIMAL FRIENDSHIPS, GO TO time.com/animalpals