Inspired by the story of Adam and Eve who were tempted by the snake to eat from the apple of the Tree of knowledge of good and evil, thus condemning mankind to become sinful.
The snake is represented by the leaf and Adam and Eve by the bite they took from the apple.
Since Splinter and Shredder are both ninjutsu masters, it seemed perfect to depict a battle between these two sensei like it would have happened ages ago and someone found it on an ancient Japanese scroll.