Brown capuchin monkeys therefore seem to be capable of spiteful behavior and are sensitive to inequity, traits that together were formerly only associated with humans.
"As a result, it appears as though the evolutionary roots of some human-like punitive tendencies may extend even deeper than previously thought," the researchers believe.
If both monkeys and humans have inherited these tendencies, then they could have been present in the last common ancestor of both of these primates. While the precise identity of that ancestor remains unknown, scientists generally agree that the species lived around 35 million years ago, was small-brained and lemur-like.
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It is unclear why some primate species seem to be more cooperative (and, on the flip side, more spiteful) than others, but another new study sheds light on how such behavior can emerge.
Joshua Plotkin, a professor at the University of Pennsylvania's Department of Biology, and colleague Alexander Stewart determined that cooperation emerges when groups are small and memories are long. The findings were published a few weeks ago in the journal Scientific Reports.
"I think a fascinating takeaway from our study," Stewart said, "is that you can get a set of circumstances where there is a kind of runaway feedback loop. Longer memories promote more cooperation and more cooperation promotes longer memories. That kind of situation, where you go from a simpler system to one that is more complex, is a great example of what evolution does, it leads to more and more complexity."
Can cooperation and better long-term memory continue to evolve, however, without the emergence of spiteful behaviors? So long as inequality exists, that appears doubtful.