The Greek philosopher Aristotle classified living things as plants or animals. Scientists used these two kingdoms of life. The animal group contains creatures that move about and eat food. The plant group contains organisms that are rooted to the ground and make food. Both are divided into smaller groups based on how similar organisms are. For animals, the next division is the phylum, whose members share a main characteristic. The phylum Chordata contains animals with backbones. Members of this phylum include elephants, fishes, seals, dogs, and humans.