Bilaterians are a large group of animals characterized by having bilateral symmetry, meaning their bodies can be divided into two mirror-image halves. This group includes a wide variety of organisms, such as humans, insects, and mollusks. Bilaterians typically have a distinct head and tail, as well as a front and back, which allows for more complex movement and organization. The evolutionary significance of bilaterians is profound, as they represent a major branch in the animal kingdom. They are believed to have evolved around 600 million years ago, leading to the diversification of many animal forms. This group is essential for understanding the development and evolution of complex life on Earth.