The "unnecessarily intelligent" nature of a lot of humans was something I was thinking about too. For example, crows obviously have significantly smaller brains

The "unnecessarily intelligent" nature of a lot of humans was something I was thinking about too. For example, crows obviously have significantly smaller brains, yet demonstrate enough intelligence for at least basic tool use. So, in a biological sense it shouldn't be too surprising to find fire use at that point in evolution. But perhaps it's more interesting because of how that would relate to human level intelligence in general. If they were capable of manipulating fire and were capable of enough planning to use them to maintain a path within a deep cave, they would probably have had some ability to communicate as well. In which case, was our larger brain mass due to it simply not being enough of a detriment to survival or was there some big cognitive difference between these ancestors and modern humans that was also so much more advantageous to lead to our much larger brains? Yes, step functions don't really exist in evolution, but even for gradual changes it's worthwhile to understand why we specifically were successful in comparison to others which seem to meet the requirements we think might be important.