ABSTRACT
Our bodies and brains are largely bilaterally symmetrical, an arrangement that goes back at least to the Bilateria, a lineage dating from some 600 million years ago (Chen et al., 2004) which includes over 1.5 million present-day animal species, including humans. Its emergence marks the transition from stationary or drifting planktonic animals to active swimmers and burrowers. Bilateral asymmetry was probably an adaptation to directional movement, which defines an anterior-posterior axis in addition to the dorsal-ventral one. The third axis, orthogonal to these two, is the left-right axis, and organs of movement are then arranged symmetrically to either side of this axis, presumably to ensure linear movement. With very few exceptions, legs, wings, swimming muscles (in fish), and flippers are bilaterally symmetrical.
