Scientists are still pondering the advantages of bilateral symmetry for animals. Top among the possibilities is how this type of symmetry enables effective movement in purposeful and intentional directions for the animal, such as towards food sources or better environments, or away from danger.
A feature of vertebrates and some non-vertebrates, bilaterally symmetrical body structures might also favor the development of complex central nervous systems, such as the human spinal cord and its vast network of nerves funneling into the brain. Scientists also believe bilateralism might foster the development of improved sensory organs, such as eyes and ears, that help to support the intentional and focused movements of which these types of symmetrical bodies are capable.
The other main type of symmetry found in the animal kingdom is radial symmetry. Unlike a bilaterally symmetrical body, which has only one axis about which it can be equally divided, a radially symmetrical animal can be divided into equal halves along one of several axes, so long as that axis passes through the body's center. A starfish is a good example of this type.
Bilateral symmetry first appeared in the fossil record some 550 million years ago, exhibited by a creature called an acoel. Only a few animals have body structures that exhibit no symmetry, the main example being sea sponges.