Maybe we should gather a few more facts before wasting our time speculating. - Stephen Baxter

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.

Ussher’s intense and obsessive project seems very odd to the eyes of a modern scientist. After all he had deduced the age of the Earth without looking at a scrap of physical evidence—not a single rock.

For the genes it made sense, of course. Otherwise it would not have happened.