Symmetry in nature is the balanced distribution of complementary parts of the natural world. This balanced distribution is exemplified in the bilateral symmetry of most vertebrae, whose left and right sides are mirror images of one another.
Over millions of years, the human brain has evolved a robust pattern recognition system that allows it to detect natural symmetry. There are far more ways to arrange a structure asymmetrically than there are symmetrically, leading scientists to postulate different reasons for the prevalence of this seemingly unlikely trait. The bilateral symmetry present in the most developed branches of the animal kingdom has been attributed to the ease of spatially coordinating the motor functions of such a body through a central nervous system. This ease of orientation could have provided early bilateral ancestors with an evolutionary advantage that allowed them to survive better than their asymmetrical counterparts.
Another theory has attributed bilateral symmetry to the sexual preference of opposite genders to members possessing the most symmetrical features. Studies have indicated that throughout the animal kingdom, birds, insects and mammals all tend to favor mates with the most symmetrical features, such as tail feathers or wing patterns. Even the perception of human facial beauty is strongly linked with the symmetry of facial features.