With this sentence I’m a bit confused:
And when our ancestors finally did make their first cave
paintings, what were the first designs? A downward slash, lightning bolts through the bottom
and middle—behold, the Running Man.
Specifically, I can’t get it why the author used the word “behold” so I can’t imagine the drawing that he described.
The author is being somewhat dramatic. He starts with a plain, technical description of the individual markings on the cave wall. At “behold”, the sentence changes tone to be less literal and becomes his interpretation of what the markings as a set represent.