Not entirely speculative, there are simple logical tests that demonstrate that humans are far more likely to solve the same logical problem if stated in social terms rather than numerical or alphabetical forms. The research however on the gulf between primate social and logical intelligence is very well documented, it's not entirely speculative to suggest there were intermediate stages.

And there's the famous habit of interpretation to conform to the political ideology of the day rather than an objective documentation. Many have gone to find the paradise lost they wanted to find and ended up creating it where it didn't exist.
Still, there's hope that we might be getting an accurate insight into living cultures right now, though there's few untouched communities and these are unlikely to have stayed static all this time!

Funnily enough I've often thought it strange that the same basic brain building blocks are found across such diverse species, and the human brain is, anatomically at least, built layer on top of layer that appear to consist of ever more primitive components (that appear in primates and mammals) crowned by the absurdly large pre-frontal cortex. Phyiscal design seems to have less to do with how the mind is structured, us primates have very enlarged visual areas and social intelligence areas but all built on a seemingly generic template. Similarly, psychological disorders that have genetic components are not obvious in the structure of the brain but have a profound effect on how the resulting mind operates in the world.