Humans seem to be instatiable in their need to improve their tools and their lives. From a strictly biological point of view, we are the most successful species.

Stonehenge was created, at considerable effort, by people, for *some* purpose, who were not goofing off. It was an investment in something, e.g., more efficient agriculture or after-life favors from higher being(s).

I just don't see the planet supporting a hunter/gatherer lifestyle for 7+ billion "wise tool makers."