If you were part of a group there was always the chance that the predator would take the next guy, not you. It was a cold-blooded lottery that paid o… - Stephen Baxter

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If you were part of a group there was always the chance that the predator would take the next guy, not you. It was a cold-blooded lottery that paid off often enough to be worthwhile adapting for.
But there were disadvantages to group living: mainly, if there were large numbers of you, there was increased competition for food. As that competition resolved itself, the inevitable result was social complexity—and the size of the adapids’ brains had increased so that they were capable of handling that complexity. Then, of course, they were forced to become even more efficient at searching for food to fuel those big brains.
It was the way of the future. As primate societies became ever more complex, a kind of cognitive arms race would continue, increasing smartness fueled by increasing social complications.