What is the natural history of the straight edge?

Driving home from work I got to thinking about rulers and other straight-edge tools and wondered what exists in nature that ancient man (and woman) might have used to generate straight lines. I thought of Leonardo DaVinci and the square box he drew to show arm length generally related to height and wondered what people before his time used to create straight lines in drawings, ciphers, etc.

There had to be something that occurred naturally that allowed early people to etch out straight lines, but I was at a loss to think what it might have been.