Primitive animals don't scar. They heal their wounds by mitosis, by regenerating exact duplicates of their missing parts. Humans possess this power, but in a limited capacity. Cells in the liver can regenerate after mild exposure to toxins. Some nerves regenerate.
Skin regenerates to patch minor scratches and burns.
But serious injuries overwhelm this capacity. Wounded humans scar. It is our primary method of repair. As the wound matures, collagen forms the primary ingredient in scar tissue. Under normal conditions, scar tissue remains within the boundaries of the wound and in time mimics the structure, even the color, of the tissues around it.