Unlike the peacock’s tail and many other fitness-signaling phenotypical characteristics, intelligence is not merely an adornment. Here’s why:

We are a social primate species and therefore our most persistent competition comes not from predators or animals consuming the same resources we wish to consume; it comes from other members of our species. What’s the best way to compete at relatively low risk of injury? By trickery. Once we have a rudimentary “theory of mind” (e.g. the ability to infer intention in others) we can use that to deceive those we’re competing with.

And we humans do this every day, par excellence. Sometimes the trickery is visual, with calf-pads and shoulder-pads and cod pieces and padded bras and makeup; sometimes it’s more subtle, like claiming one has wealth or skill one does not truly possess. And so we enter an arms race scenario: in order to detect trickery we need to become smarter, and as we become smarter the tricksters must try harder. Thus a feed-forward loop is created.

It is almost certainly this mechanism that has led to our current brain size and mental characteristics. We know from psychological studies that we’re actually terrible at reasoning, especially in the abstract. But we’re rather good at dealing with cues that suggest intentionality. Indeed, we overdo this to such an extent that we invent gods and ghouls and goblins to “explain” phenomenon that in reality have no intention at all. The fact that in a very tiny percentage of cases the human brain is also capable of more abstract functioning is merely a side-effect, a statistical artifact, and in no way central to the phenomenon of human intelligence.