By comparing the DNA of living animals we can produce a family tree. A group of unicellular organisms called choanoflagellates come out as the closest living relatives to animals. They are simple organisms with only one cell and a long tail called a flagellum which is used to swim about; they look a bit like sperm! The first true animals which branch off the animal family tree are called the cnidaria and the sponges. Cnidaria include things like sea anemonies, corals and jellyfish. Sponges of course, include sponges.