[ZenGum]

Very interesting. Even practical structures like houses and defensive works are impressive, but this is really something. It involved a huge investment of effort for ... something very intangible. Why? What motivated this apparently impractical effort? What were they thinking?

If you have time, try The Origin of Consciousness in the Breakdown of the Bicameral Mind by Julian Jaynes. He attempts archeo-psychology, speculating about what (and how) people were thinking 3,000 to 2,000 years ago. Not absolutely convincing, but utterly fascinating.