Alternating locomotion (interlocking reciprocal movement) occurs through a specific neural network known as the reticulospinal tract, a framework that directly carries over to land animals. During alternating movement, the anterior (front) and posterior (back) cooperate symmetrically to steer, rotate, or drive locomotion forward. A clear example of this is a running dog: its head and tail move in coordinated harmony to maintain direction and balance. This coordinated movement is, in essence, kinetic symmetry.

Up to the evolutionary stage of fish, the symmetric modules generated through natural history consist of four distinct types:

• Radial Symmetry (for maintaining form)

• Bilateral Symmetry (for driving locomotion)

• Vertical Symmetry (for counteracting gravity)

• Anterior-Posterior Symmetry (for directional movement)

Every single one of these symmetric modules inherently includes the element of balance against gravity. Crucially, these modules represent the entirety of the structural baseline. As these four symmetric modules stack upon one another, they form a neurologically dense and consolidated pattern, which constitutes the internal structure. 

The single longitudinal axis completed in fish is an internal structure that encapsulates all of these modules, and its outward manifestation as an external structure is the vertebral column (spine). Consequently, in vertebrates, the internal structure of this single axis is projected throughout the entire body.

Conquering Land: Evolution Under Gravitational Stress

As life evolved from water to land, a massive transformation occurred in animal anatomy, driven entirely by gravity. Within water, the impact of gravity is relatively negligible due to buoyancy; however, on land, organisms must withstand its absolute weight. Before an animal can even initiate movement in a specific direction, it must first maintain its physical form and establish equilibrium under the relentless downward pull of gravity.

If you place a fish on dry land, it collapses onto its side. When we define the spine as the primary axis (Axis 1), this longitudinal line running from front to back is fundamentally insufficient for maintaining balance on land. The lateral axes must be heavily reinforced. This explains why amphibians, unlike fish, developed heads and torsos that are flattened and widened horizontally. 

If we define this widened horizontal axis as Axis 2, Axis 1 and Axis 2 intersect to form a cross-shaped (+) matrix, which prevents the animal from tipping over to the side. Structurally speaking, this cross-shaped configuration can be viewed as a form of radial symmetry. To sustain this structural equilibrium, the internal structure of land animals developed advanced neural networks: the vestibulospinal tract and the tectospinal tract.

The broad head and torso, and six-sided bilateral skeletal structure of the first amphibian, Ichthyostega

The Shift from Cross (+) to Diagonal (X) Symmetrical Matrices

While a cross-shaped dual-axis structure is perfectly optimized for maintaining static balance, it cannot facilitate forward locomotion. To move forward, the organism must engage in alternating lateral locomotion. For alternating locomotion to propel the body forward, the left and right sides of both the anterior and posterior sections must move reciprocally in opposing phases.

Therefore, the left-right axis does not remain a single horizontal line; instead, it splits into two diagonal axes forming an X-shaped matrix—one running from the front-left to the back-right, and the other from the front-right to the back-left. Naturally, the left-right and front-back structures also engage in anterior-posterior alternating movement, resulting in a highly complex, compound reciprocal locomotion. However, viewed through the lens of radial symmetry, this complex movement can be elegantly simplified into an X-shaped diagonal symmetrical movement.

The four muscular fins located at the front and back of sarcopterygian (lobe-finned) fish gradually evolved into the four limbs of amphibians, forming the physical foundation of this X-shaped axis. When a land animal walks, the front-left limb operates in tandem with the back-right limb, while the front-right limb syncs with the back-left limb. This alternating reciprocal movement is the tangible expression of the internal structure of the second and third axes (the X-shaped matrix).

This exact blueprint remains perfectly intact within human biomechanics: when we walk, we naturally swing our right arm with our left leg, and our left arm with our right leg in a continuous, alternating pattern.

Through this evolutionary sequence, the planar internal structure shared by all four-legged land vertebrates—the six-directional bilateral matrix—was finalized. The absolute geometric center of this symmetry is the umbilicus (belly button). This alternating quadrupedal locomotion on land directly drove the development and refinement of the rubrospinal tract within the central nervous system.

The Hexagonal Geometry of Functional Optimization

This six-sided (hexagonal) structure is an incredibly common motif found throughout the natural world. When circles—the most perfect form of radial symmetry—are packed together to fill a limited space with absolute efficiency, they naturally form hexagons. We see this geometry everywhere: in the structure of falling snowflakes, the compound eyes of insects, the architecture of honeycombs, and even in the structured hexagonal water inside the human body. The hexagon is the geometric shorthand that nature consistently selects for functional optimization.

Ultimately, the neural pathways discussed here—the reticulospinal tract (lateral tail movement/reciprocal drive), the vestibulospinal tract (equilibrium), the tectospinal tract (visual balance), and the rubrospinal tract (limb coordination)—are all motor output networks. They are intimately wired to the unconscious proprioception of the spinocerebellar tract, which constantly senses and processes the internal structure of form and movement under the eternal presence of gravity.