Some species have properties that seem way beyond what would be necessary for evolutionary success. For example, an inland taipan has enough venom in a single bite to kill 100 people. There is no imaginable situation where a taipan is going to need to bite an entire village and take them all out. Why on Earth did it evolve such insanely strong venom?
When you dig into it, these outliers are often the result of an evolutionary arms race [1]. In teh case of the inland taipan, they are often prey for mulga snake and perenties, who have evolved immunity to their venom. So you've got a feedback loop where the taipan keeps evolving stronger venom to fight back against predators, who continue evolving stronger immunity to the same venom. Run that loop a million years and you get a snake who can kill a busload of people.
Human intelligence is another such outlier. I know it's popular to talk about how animal intelligence is underestimated but even so, human intelligence is just astronimcally greater than any other species. Sure a squirrel can find a bunch of nuts it buried. Humans have built machines and landed them on other planets. Our intelligence is orders of magnitude greater than any other species.
My pet hypothesis for years is that this must be the result of an evolutionary arms race within the species. Humans are a profoundly social species. We are mostly too fragile to survive in the wild on our own. The functioning survival unit of humans is a hunter-gatherer group. We are sort of like eusocial animals like ants.
But unlike ants who can mostly rely on simple chemical signals to tell which other ants are part of their anthill, we have to rely on social cues. There is a very strong incentive to be able to deceive humans in other groups and infiltrate or sabotage their group. If you're smart enough to sneak in, you can steal a lot or do a lot of damage. Likewise, there's an equally strong incentive to be able to suss those bad actors out to prevent them from doing that. The smarter you are, and the better you're able to remember people and describe them to others, the harder it is to get taken advantage of.
Turn that evolutionary crank a few hundred thousand years, and you get a species so smart that the only other animals that can possibly hope to compete with them in terms of intelligence are other Homo sapiens.
If we weren't so deeply social, I don't think we'd be so smart. We have these huge brains in order to navigate the fantastically complex social world which we have in turn created by having these huge brains.
[1]: https://en.wikipedia.org/wiki/Evolutionary_arms_race