How Asymmetries Evolved: Hearts, Brains, and Molecules

Abstract

Humans belong to the vast clade of species known as the bilateria, with a bilaterally symmetrical body plan. Over the course of evolution, exceptions to symmetry have arisen. Among chordates, the internal organs have been arranged asymmetrically in order to create more efficient functioning and packaging. The brain has also assumed asymmetries, although these generally trade off against the pressure toward symmetry, itself a reflection of the symmetry of limbs and sense organs. In humans, at least, brain asymmetries occur in independent networks, including those involved in language and manual manipulation biased to the left hemisphere, and emotion and face perception biased to the right. Similar asymmetries occur in other species, notably the great apes. A number of asymmetries are correlated with conditions such as dyslexia, autism, and schizophrenia, and have largely independent genetic associations. The origin of asymmetry itself, though, appears to be unitary, and in the case of the internal organs, at least, may depend ultimately on asymmetry at the molecular level.

Cognitive and Neurophysiological Models of Brain Asymmetry

“…Intriguingly, this idea is very much in line with the "from hand to mouth" theory about the origins of language, proposed by Mike Corballis 20 years ago [30]. Mike Corballis also contributed a theoretical paper to this Special Issue [31]. Entitled "Asymmetry research in human subjects and in non-human species-How Asymmetries Evolved: Hearts, Brains, and Molecules", this article gives a cross-species overview of the evolution of asymmetries in the body and brain, and their potential molecular basis.…”

Brain Asymmetry: Towards an Asymmetrical Neurovisceral Integration

“…Although no one yet knows for sure how life on earth acquired its asymmetric character or what the biological advantage of asymmetry over symmetry would be [10], some authors suggest that the asymmetry could have a molecular origin from which life would evolve [11][12][13]. If we go back to pure physical concepts, Joe Rosen suggests that symmetry underlies nature [14], but the same author indicated that symmetry implies asymmetry, and that the universe cannot have perfect symmetry [15].…”

Language and reading impairments are associated with increased prevalence of non‐right‐handedness

“…A higher rate of non‐right‐handedness (NRH), which includes left‐ and mixed‐handedness, has been reported for neuropsychiatric and neurodevelopmental conditions, such as schizophrenia, autism, and intellectual disability (Hirnstein & Hugdahl, 2014 ; Markou et al, 2017 ; Papadatou‐Pastou & Tomprou, 2015 ). The underlying hypothesis is that the genetic pathways required for establishing left–right brain asymmetries might also contribute to handedness and be involved in neurodevelopmental conditions (Corballis, 2021 ). It is important to emphasize that the majority of left‐handers are not affected by these conditions and therefore left‐handedness should not be equated to a pathological status.…”