The human body is complex, our diverse functions made possible by sophisticated, interacting organ systems: from muscle pairs attached to skeletons that allow us to run, jump, and dance to lungs that exchange gases with the atmosphere and a brain and nervous system that enable us to sense our environment and respond to it. None of these organ systems is well developed in early-branching animal groups. How then do sponges, cnidarians, ctenophores, and placozoans function, and what are the consequences of their anatomical organization for diversity and ecology? On the other hand, these animals have survived a half billion years of Earth history. What can we learn from the diversity and continuity of their remarkable adaptations?