In the present study a unifying framework is proposed for the development of forms of locomotion in vertebrates. Four kinematic processes are identified that illustrate the divergence of modes of locomotion, from an ancestral undulatory lateral movement to the different modes seen in recent vertebrates: (a) Lateral movements of the segments of the trunk are incorporated, and then removed, in a cephalocaudal order; (b) a proximodistal transition is then evident, in which appendages (fins or legs), rather than the segments of the trunk, take over the task of active propulsion; (c) synchronized action of fore- and/or hind legs follows the primeval alternate action of legs; and (d) movements of the segments of the trunk in the vertical domain are incorporated to facilitate locomotion. These processes can be illustrated in the ontogeny of both amphibians and rodents: Lateral undulatory swimming in tadpoles is replaced during metamorphosis by synchronized hind leg strokes in toads and frogs; and pivoting around the hind legs, which is the primary quadrupedal activity in rats, is followed by alternate, and then simultaneous, stepping of the fore- and hind legs. The same processes may be applied to the development of locomotion in the different vertebrate classes, from the numerous species of fish that swim with lateral movements, to the widespread use of vertical movements in facilitating walking and swimming in mammals. When functional reasoning is ignored and only the form of locomotion is considered, a regular order in development is apparent.
- Lateral movement