Bilaterally Symmetrical: To Be or Not to Be?

Michael C. Corballis

doi:10.3390/sym12030326

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.

Highlights

We belong to the vast clade of species known as the bilateria, whose body plan is fundamentally symmetrical with respect to left and right
The two sides of the body are to a high degree left-right mirror images, with limbs and sense organs are arranged in mirrored pairs on opposite sides of the body
The paper was widely distributed but was either ignored or dismissed by the French medical establishment, which firmly held to the principle of symmetry, until further evidence for cerebral asymmetry emerged in the 1860s [1]

Summary

Introduction

We belong to the vast clade of species known as the bilateria, whose body plan is fundamentally symmetrical with respect to left and right. A perfectly symmetrical brain would treat left and right as equivalent, and mirror-image patterns as though they were the same. A likely mechanism for this is homotopic transfer of learning between the two sides of the brain, so that learned patterns established on one side of the brain are reversed in transfer to the other side [4,5] This provides for what we termed mirror-image generalization. This is adaptive in the natural world. Children learning to read and write often suffer left-right confusions, treating mirror-image letters, such as b and d, or even words like was and saw, as though they were the same [6]. Women reported a higher incidence than did men, but were perhaps more honest

The Breaking of Symmetry

Differences between Species

Why Asymmetry?

Where did Asymmetry Come from?

Findings

The Evolutionary Tradeoff