Overview: It makes up 16% of your body weight, is the largest organ in the human body, allows birds to fly, mammals to nurse their young, and provides a lifelong defence against predators and parasites alike. What is it? Skin. From the delicate membranes that encased the earliest animals to the leathery hides that protected the dinosaurs, this episode looks at how skin has changed and adapted to virtually any challenge it has faced throughout history.