Fig. 1
From: The Concept of Co-option: Why Evolution Often Looks Miraculous
The evolution of the proteinaceous cocoon in a hypothetical group of leeches. The cocoon originated in ancestor X, where it protected the eggs from predators. The new function (anti-desiccation) in ancestor Y represents the co-option of the plesiomorphic trait, which allowed the leeches to successfully move from an aquatic to a terrestrial environment