I'm talking about evolutionary fitness: how well we fit our environment.

Climactic change in Africa forced our ancestors to fend for themselves in the savannah, where they couldn't just climb a tree to escape a predator. They were pushed to the very edge of extinction until they compensated by increasing their brain size. It was a huge gamble. For a billion years no other species was successful in harnessing intelligence as a survival strategy. We just didn't have any other choice and we got extremely lucky to slip through a very narrow bottleneck.