EVOLUTIONARY HISTORY OF BIOLOGICAL ASYMMETRY

Asymmetry is a fundamental characteristic of biological organisms. Biological systems seem to be symmetry-broken at all organizational levels, although whether there exists a series of direct connections between the chirality of amino acids, proteins, DNA, and whole organisms is not clearly established. Mirror-image pairs of organic molecules and structures may be present in nature, but evolution has evidently tended to favor one of the pair over the other. Biological asymmetry can be expressed in morphology (either conspicuously or subtly) and in behavior. Examples of morphological asymmetry include spiral morphology and left-right differences superimposed on otherwise bilateral body plans. Developmental studies suggest that genetic factors, sometimes combined with epigenetic factors such as imprinting through learning, contribute to asymmetric expression (behavioral expression, reinforced morphologically) in animals. In some organisms, the pattern of asymmetry can be reversed through ontogeny or phylogeny.

Morphological asymmetry has a deep evolutionary history. It evidently dates to the time of the earliest prokaryotic life on Earth (Archean Eon, >3 Ga), and is evident both in the earliest multicellular eukaryotes (Proterozoic Eon, ~2 Ga) and the earliest multicellular animals (Ediacaran Period, ~600 Ma). Behavioral asymmetry in animals is recorded in trace fossils dating to the Cambrian Period (~520 Ma), although it can be assumed to have a much deeper evolutionary history if morphological asymmetry and behavioral asymmetry were linked in the earliest metazoans. Whether biological asymmetry has one phylogenetic origin or whether superficially similar patterns of asymmetry developed at different times in evolutionary history is uncertain.