A simple way to describe the function of a vertebrate might be: “Get the mouth parts to the food”. The primary reason for the development of self-motored movement is to acquire food (well, that and sex). There are organisms which root in food, there are those that float in a medium which occasionally delivers food, and there are those which have evolved motors allowing them to seek out food. All vertebrates have evolved around a spinal column with brain, eyes, ears and mouth at the leading end. When snakes move, it is the head that leads. Quadrupeds and bipeds also lead with the head in basic walking and running patterns. For example, when a lion smells or hears prey, it will orient its head in the direction of the prey to see it. (tonic neck reflexes) The substantial weight of its head is then oriented in the direction of possible movement. If the lion decides to move towards its prey, a release of the weight of the head in the intended direction begins the extension of the spine, which then allows the extension of the limbs. The animal puts itself off balance in the direction of intended movement to create an extension of the whole body towards an objective. This is easily seen in running quadrupeds – the spine lengthens in the direction of movement – the faster a horse wants to run, the more its head will be moved forwards from the body.
In bipeds, such as humans, extensor reflexes create our vertical state, working against (and with) the pull of gravity. Our neutral upright condition is not, as is often claimed, created by stacking one element on top of another as in bricklaying. If we were truly evolved in this way, with the head centered on the spine, with the spine centered over the hip joints, knee joint, ankle joints, then movement would require contraction to displace something to initiate motion. In fact, there are wonderful imbalances in our structures that allow for quick movement with a simple release of existing muscle tone. See The Spinal Engine