ABSTRACT
Movement, allowing passage to areas of plenty, was a major advance for single cells. Early multicellular animals, living on the sea bottom, had similar needs for motion. Fossilized trails a millimeter in width, 570 million years old, testify to creeping along mucus-lubricated paths. Flatworms adapted muscles, possibly evolved for spawning purposes, to swim using body undulations.
The story continues with tetrapods repurposing the fish tail as an anchor for muscles moving the rear legs. A step cycle evolved in which the limbs on one side moved forward, followed by the other side. Today, by inheritance, most mammals walk likewise.
Early primates, however, moved along fine branches, minimizing branch shaking by adopting a diagonal gait: Right hind - left front - left hind - right front. Today, humans crawl similarly. Like all apes, we lack the balancing tail used by smaller primates, probably because large bodies can't be shifted by tail movement.
Crossover is a notable feature of the vertebrate nervous system: One hemisphere of the brain mostly controls movement on the opposite side. Almost all the sensory systems, too, cross over. This arrangement likely traces to the image-reversing eye, resulting in the left side of space projecting to the right hemisphere, and vice versa. The other senses -- and much of the motor system -- may then have crossed over to bring inputs and outputs into register within the same hemisphere.
