Context: When and why did human brains decrease in size 3,000 years ago? Ants may hold clues
If information processing and storage is offset to an environmental context, brain size becomes less important. I would probably ask the (corollary) question in this context as to the extent that cultural (as information processing and knowledge transmission) systems underwent significant changes around the same time. The degree to which we assert ourselves as central in any reflexively self-validating narrative of identity is quite equally mirrored in the extent to which that identity and all moving frames of ontological (as much as epistemological) reference are distributed properties of an adaptive, dynamical field of information and energy-processing (i.e. computation).
The kind of generalised, inductive logic and conceptual framework I am gesturing towards here is that in which we take quite seriously the foundational interdependence of systems and their environments. I doubt whether I could provide enough evidence to satisfy anyone of mechanistic ilk, but we see these kinds of complex, accelerating decompressive patterns with oscillating ubiquity across biology and cosmological history.
The benefit of such a systems perspective is self-evident, the “cost” is of a new kind of Self.