I would argue that modern man has to remember far more than our ancient ancestors.

Consider our education; even disadvantaged nations have compulsory education until 12 or 16 where even the most basic of subjects would seem incredibly complex to prehistoric man, maths, reading and writing.

And even if the vast majority of what we know is comparatively mundane compared to the tools for survival they required, it doesn't detract from the fact of its complexity.

I'm no expert, but even their art was only as complex as our grade schoolers which to me is indicative of an ability to form and communicate abstract ideas.