That's a bit like debating what came first: the large beer or the voluminous glass?

Anyway, here's an interesting datapoint: there's something called "domestication syndrome", basically a set of traits that is common to diverse animals that have adapted to humans, from goldfish to cattle. One of those traits is reduced brain size. And the loss in brain size is more pronounced in social animals (dogs), compared to psychopathic loner species like cats. (No idea about goldfish. Really have to read up on goldfish sociology.)

The causality, per evolutionary theory, must obviously be "randomly larger brain" -> "better ability to cooperate" -> "increased survival". The other way around is Lamarckian Evolutions, which doesn't happen.

That's interesting, I've never heard of reduced brain size for domesticated animals.

Or perhaps navigating the complexities of social groups is a really hard task and requires a more developed brain.