If they did have significant additional brain mass, it's possible it was was used to give them really good instincts instead of the more general purpose circuits we have.
This is a quote from Wikipedia supposedly paraphrasing Jordan, P. (2001) Neanderthal: Neanderthal Man and the Story of Human Origins. "Since the Neanderthals evidently never used watercraft, but prior and/or arguably more primitive editions of humanity did, there is argument that Neanderthals represent a highly specialized side branch of the human tree, relying more on physiological adaptation than psychological adaptation in daily life than "moderns". Specialization has been seen before in other hominims, such as Paranthropus boisei which evidently was adapted to eat rough vegetation."
If they did have significant additional brain mass, it's possible it was was used to give them really good instincts instead of the more general purpose circuits we have.
It's also possible that it did any of a hundred other things. Or that it didn't strictly do anything itself, but was genetically tied to some other positively selected mutation. Or it was sexually selected. Or it arose without genetic change, from environmental factors, and there wasn't enough time or pressure for natural selection to remove it again.
Why privilege this hypothesis? Othe...
We have a sample of one modern human civilization, but there are some hints on how likely it was to happen.
Major types of hints are:
Data for:
Data against:
To me it looks like life, animals with nervous systems, Upper Paleolithic-style Homo, language, and behavioral modernity were all extremely unlikely events (notice how far ago they are - vaguely ~3.5bln, ~600mln, ~3mln, ~200k or ~600k, ~50k years ago) - except perhaps language and behavioral modernity might have been linked with each other, if language was relatively late (Homo sapiens only) and behavioral modernity more gradual (and its apparent suddenness is an artifact). Once we have behavioral modernity, modern civilization seems almost inevitable. Your interpretation might vary of course, but at least now you have a lot of data to argue for your position, in convenient format.