1. Multicellularity: Complex animals are composed of many cells that work together to perform different functions. This allows for a higher level of organization and specialization than is possible in single-celled organisms.
2. Differentiation: The cells of complex animals are differentiated, meaning that they have specialized structures and functions. This allows for a greater division of labor and efficiency than is possible in single-celled organisms.
3. Tissues: Complex animals have tissues, which are groups of cells that work together to perform a specific function. This allows for a higher level of organization and complexity than is possible in single-celled organisms.
4. Organs: Complex animals have organs, which are structures composed of different tissues that work together to perform a specific function. This allows for a higher level of organization and complexity than is possible in single-celled organisms.
5. Body plans: Complex animals have body plans, which are the overall organization of their bodies. This allows for a higher level of organization and complexity than is possible in single-celled organisms.