So I read some interesting stuff on this recently, (ignoring that brain size isn’t as important as brain compelxity for intelligence) a lot of creatures that have big brains including our ancestors and elephants had/have most of the extra mass in regions related to memory. The theory goes that simply remembering where everything is and picking the most likely solution (e.g. the neares watering hole that you saw water at this time last year) is generally more effective than traits like creativity and imagination… right up until you hit a break point where you start making tools and seriously modifying your environment. As we developed agriculture we had less of a need to remember every little thing so while we didn’t get less intelligent we did end up with worse memories, possibly gaining an even greater degree of creativity in return as those parts of the brain became more valuable in the new self created environment.

The plants domesticated us!

It’s also rare that we lived in caves.