Perhaps ancient people had a lot of time on their hands...
Or maybe the men courted the women by dragging and carrying big stones to make them pretty pictures in the earth.
Or possibly conservative aliens came to the planet and put the lazy liberal natives to work before realizing that liberals were good for nothing and they left... but not before releasing a genetic modifier to inject the natives with a little bit of conservatism.
"Or maybe the men courted the women by dragging and carrying big stones to make them pretty pictures in the earth. "
that goes hand in hand with my theory that men invented civilaztion to impress chicks. :)
Or maybe it was the women: "Honey, don't you think this rock would look better over here?"