Evolution is characterized by two attributes: one is to find sustenance for life and the other, the need to multiply. Any or all changes in shape, size, cerebration and adaptations are engendered by need to meet these primary requirements and the need to acquire higher efficiency in finding mate or manna. Towards achieving these ends, locomotion as an adjunct aid has evolved as a powerful tool.

One of more peculiar observations I have made in the process of evolution vis a vis propulsive mechanisms is the progressive and gradual caudal shift of appendages that engineer locomotion. The more primitive the form of life, the more cranial is the appendage that initiates movement. Invertebrates kick-start mobility using their mouth parts as anchoring mechanisms that help drag themselves forwards.

Classically, the earthworm and leech are examples where moth plays a very crucial role in movement. Fish use branchial apparatuses like gills or fins – early reptiles use rudimentary pectoral accruements. Observe the metamorphosis of the aquatic tadpole into a quadruped amphibian to confirm this hypothesis.

The cranio-caudal displacement of primary mechanisms that help mobility during the various stages of tadpole to frog development will show that when an early tadpole the amphibian uses its mouth to anchor itself to a blade or stalk to anchor itself against a flowing current (a paradoxical propulsion) – it graduates to using pectoral fin like appendages – thence to forelimbs, then it develops hind limbs as additions. The adult frog primarily depends on catapulting itself onwards relying mainly on the strength and power of its hind limbs. Aves depend heavily on the fore limb power to move fly), the hind limbs are merely adjuncts that aid the forelimb (modified into wings) to augment and refine aerial movement.

Almost all mammals use all four limbs so frequently and effectively that most are classed as quadrupeds. Among the mammalians, man alone, has developed into dependence on highly evolved hind limbs (legs) for movement. Even in man, the most extreme end of his foot, the great toe is the primary generator of bipedal gait.

What is seen from the examples cited is that from the lowest forms of life, as one goes up the evolutionary tree – the apparatuses and appendages that help it move – show a distinct tendency to shift from head to tail end, a cranio-caudal displacement of primary and principal locomotor mechanisms.

It is a scientific curio worth dwelling further on. That one can even place a life form into its slot in the evolutionary scale tree merely by noting the position of its propulsive mechanical apparatuses: The more head-ward propulsive appendages are, the more primitive the life form is, and vice versa.