There are a wide variety of brains across the animal kingdom. Mars suggests that foraging plays an essential role in brain structure. Complex decisions like "where should I look next?" and "is the food over there good to eat?" require some centralized processing. Warm-blooded animals have higher metabolisms and therefore need to eat more often across a broad geographic range. Brains can be refunctioned. Enhanced sense of smell, sight, socialization, domestication, culture, language, and the search for meaning can lead to brain growth. The structure of the human brain is very similar to that of other apes, though the temporal cortex has more pathways connecting different parts of the brain to each other.
Some small-brained birds are quite clever. Octopus intelligence is easy to observe. Bee behavior suggests some form of thought. The laws of physics restrict unbounded possibilities, but nature certainly has a lot. We did not evolve to read... Yet here we are.