Bilateral symmetry, sometimes called reflection symmetry or mirror symmetry, means that an object or shape has two halves that are mirror images of each other. For example, the capital letter T and an isosceles triangle are bilaterally symmetrical. The term is most commonly used in biology, where it refers to organisms that have two symmetrical halves when divided along only one plane. This is called called the saggital plane, an imaginary vertical plane that goes through the center of the organism's body from front to back, splitting it into left and right sides. Most animal species have bilateral symmetry, including humans, and plants often have particular structures that are bilaterally symmetrical.
When the term "bilateral symmetry" is used in reference to living beings rather than geometric forms, the symmetry is only approximate, because the process of biological growth is not perfectly precise, and no organism is perfectly symmetrical. The placement and structure of internal organs also does not need to be perfectly symmetrical for an organism to be considered bilaterally symmetrical. The human liver is located on the right side of the abdominal cavity, for instance, and it is perfectly normal for a person to have asymmetries such as one arm or leg that is slightly longer than the other.
Bilateral symmetry is distinguished from radial symmetry, which refers to forms that can be divided into roughly symmetrical halves along more than one plane. Starfish and jellyfish are examples of radially symmetrical animals. Bilaterally symmetrical organisms, because they are symmetrical on only one plane, have distinct fronts and backs, but radially symmetric organisms do not. Many plants have parts that are bilaterally symmetrical, such as the leaves of trees.
Animal species that have bilateral symmetry are collectively referred to as bilateria. This is a huge category that includes the majority of animal phyla and encompasses an enormously diverse array of organisms, ranging from flatworms to human beings. The oldest confirmed fossil of an organism in this group is believed to be more than 500 million years old.
Bilateral symmetry provides a number of advantages for organisms. It allows an organism to be more streamlined and thus faster-moving. Indeed, it should be noted that animals species that do not have bilateral symmetry are either very slow-moving, as is the case with starfish, or completely sessile, as in the case of organisms such as sponges and see anemones.
A species that has a body specialized to move on a particular axis, rather than equally in all directions, can also concentrate the majority of its sensory organs and associated nervous tissue in the front of its body instead of needing to spread them in all directions. This specialization of the foremost part of the organism's body, called cephalization, can lead to the evolution of more complex behavior and greater intelligence as nervous tissue becomes centralized. All organisms with more than a very rudimentary nervous system are bilateria. The complex brains of humans and other highly intelligent animals have their evolutionary origins in this process.