Perhaps This Explains Theism?

[Angrboda]

In order for a theory to convincingly explain why large brains evolved in hominid ancestors, it needs to offer an non-trivial explanation for why similar brain enlargement did not confer enough advantage to evolve in other animals.

Currently most theories aim largely at appearing to arrive at where it wants to go, that human evolved large brains, but give only casual, sometimes flippant, glance at the equally important validation - why large brain did not evolve more frequently elsewhere.

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As it happens, I think evidence point to indirect realation between brain size and behavioral complexity. Large brain is costly to maintain. If small brain can do the trick of conferring the needed cognative power and behavior sophistication, large brain is an evolutionary burden, not advantage.

Is small brain enough? Apparently yes. Homo flourensis exhibit evidence of equal sophistication in stone tools as homo erectus. Yet it's brain size would embarrass a chimpanzee. So a brain even aller than those of a chimp can support behavior and cognition of a high order. So why the profligacy of a big brain.

Just off the top of my head, one could argue that big brains, and specifically the skills that came with them, allowed hominids to exploit an ecological niche which was unavailable to other species for whatever reason (bipedalism, opposable thumbs [homo habilis?], lack of other skills versus competitors). This is all very post hoc, but I don't think it would be a stretch to imagine a just-so story that explains it, and there appears to be circumstantial evidence for it in the extinction of hominid species in Eurasia following the migration of homo sapiens into the area (presuming the late migration theory). Quite simply, large brains aren't as big a selection advantage for most animals, particularly highly capable predators, and herbivores.

Oh, and re: flourensis, the habitat of flourensis was considerably different from Africa and Eurasia, so even adjusted for size, it's unclear that they were competitively intelligent with a hominid that evolved in a less evolutionarily isolated environment like that on an Indonesian island, such as habilis or sapiens. Moreover, if they evolved down, as it were, there's a totally different selection process occurring.