Man's creativity and ingenuity is unbounded. There are numerous contemporary examples of what can be accomplished by one individual given time, energy and inclination:

Simon Rodia's Watts Towers

Nit Wit Ridge

Hole N' The Rock

The Damanhurian Temple

There most likely was a time in evolutionary history when the balance between humans and food source was such that living was relatively easy and afforded humans much time to express their interpretations of the natural world in creative ways. The above links demonstrate what one person is capable of making by using nothing but time, creativity and simple tools, it is not difficult to project what whole communities of people could produce from the same motivators.