Bilaterally Symmetrical: To Be or Not to Be?

Abstract

We belong to a clade of species known as the bilateria, with a body plan that is essentially symmetrical with respect to left and right, an adaptation to the indifference of the natural world to mirror-reflection. Limbs and sense organs are in bilaterally symmetrical pairs, dictating a high degree of symmetry in the brain itself. Bilateral symmetry can be maladaptive, though, especially in the human world where it is important to distinguish between left and right sides, and between left-right mirror images, as in reading directional scripts. The brains of many animals have evolved asymmetries, often but not exclusively in functions not dependent on sensory input or immediate reaction to the environment. Brain asymmetries in humans have led to exaggerate notions of a duality between the sides of the brain. The tradeoff between symmetry and asymmetry results in individual differences in brain asymmetries and handedness, contributing to a diversity of aptitude and divisions of labor. Asymmetries may have their origin in fundamental molecular asymmetries going far back in biological evolution.

Symmetry Transformations in Metazoan Evolution and Development

“…The majority of Metazoa are bilaterally symmetric and fall in a group called the Bilateria, which are characterized as having a primary body axis (head to tail), a dorsalventral axis (top to bottom), and a left-right axis; Bilateria possess a central nervous system with a brain and sense organs concentrated at the anterior end of the body (cephalization), three germ layers, true muscle, and a through gut [25,36,66,76]. Bilaterian animals are patterned along two major body axes perpendicular to each other: the primary anteriorposterior axis and the secondary dorsal-ventral axis [9,33].…”

“…In early Bilateria, their "ecological engineering" [66] (p. 7) included also an epithelial through gut with two openings, ingesting food through a mouth and passing food unidirectionally before ejection of waste through an anus [36,66]. In evolutionary terms, bilateral symmetry is highly adaptive [25]. Locomotion in three-dimensional macro-world space is itself sufficient to explain the maintenance of bilateral symmetry in animal evolution, and bilaterality is the only type of symmetry that can maximize this force; thus, an actively moving bilateral body can have the maximal maneuverability as compared to other symmetry types, providing obvious selective advantages in the bilateral animal [25,60].…”

“…Body axes form the basis of animal body plans; polarity establishment requires a symmetry-breaking event, so body polarity is a breaking of symmetry [8,24,25]. Symmetry breaking inevitably arising during early development plays a key role in biological morphogenesis.…”

“…Symmetry breaking inevitably arising during early development plays a key role in biological morphogenesis. Symmetry breaking is one of the fundamental processes of development [25,26]. Symmetry breaking and "symmetry propagation" [27] are inevitable in evolution and development [3,4,20,25,27,28].…”

“…Symmetry breaking is one of the fundamental processes of development [25,26]. Symmetry breaking and "symmetry propagation" [27] are inevitable in evolution and development [3,4,20,25,27,28]. Biological morphogenesis has a dynamical character involving qualitative discontinuities as topological bifurcations tightly coupled with symmetry breaking, so developmental and evolutionary transformations of symmetry are discrete steps in biological morphogenesis inevitably disrupting a preexisting pattern of symmetry [17][18][19][20]29,30].…”