Axial twist theory
The axial twist theory (a.k.a. axial twist hypothesis) is a scientific theory put forward to explain a range of unusual aspects of the body plan of vertebrates (including humans).[1] It proposes that the rostral part of the head is "turned around" regarding the rest of the body.[2] This end-part consists of the face (eyes, nose, and mouth) as well as part of the brain (cerebrum and thalamus). According to the theory, the vertebrate body has a left-handed chirality.
Axial twist
The theory gives a phenomenal explanation and addresses how and when the twist between the end of the head and the rest of the body develops. It addresses the possible evolutionary history. The goal is to make testable predictions. For example, the theory predicted the aurofacial asymmetry, which was then found empirically.[3]
Explained phenomena include:
According to the axial twist developmental model, the anterior part of the head turns against the rest of the body, except for the inner organs. Due to this twist, the forebrain and face are turned around such that left and right, but also anterior and posterior are flipped in the adult vertebrate. There are some popular science videos and podcasts on the topic.[4][5]
History[edit]
The idea of a twist responded to severe inconsistencies in the prevailing scientific hypotheses to explain the contralateral organization of the forebrain (cerebrum and thalamus). Briefly, the visual map theory by Santiago Ramón y Cajal proposes that the optic chiasm restores the retinal image on the visual cortex,[6][7] but the loop of the optic radiation destroys this potential repair again, so there is no such consistency.[8] The parcellation theory proposes that an increasing brain size can conserve coincidental contralateral organization,[9] but does not explain the optic chiasm, nor that the pattern is conserved across all extant vertebrates regardless of brain size. Functional[10][11] or topological[12] explanations fail because same-side connections are just as important as crossing ones.[13] Moreover, these explanations leave open why the brain stem, including the cerebrum, has a same-side organization.
The idea of a twist was proposed two times independently. It does not suffer from these problems. The first presentation occurred at a conference in the 1970s by Marcel Kinsbourne,[14] but remained widely unknown. The first scientific publication in the form of a falsifiable scientific theory was made in 2012 by Marc de Lussanet and Jan Osse.[1] Kinsbourne subsequently also published his ideas in 2013.[2] In 2019, a novel kind of facial asymmetry, the aurofacial asymmetry, was predicted and confirmed on the basis of the axial twist theory.[3]
Kinsbourne developed his ideas from the dorsoventral inversion hypothesis by the early 19th-century naturalist Étienne Geoffroy Saint-Hilaire, proposing that the anterior head was the part of the head that was not inverted. He also stressed that the axial twist concerns all chordates. This proposal has the problem that such a turn does not occur during embryological development and has no evolutionary foundation.
De Lussanet and Osse used comparative functional morphology and embryonic development as a starting point to arrive at the axial twist hypothesis.
Evolution[edit]
The axial twist is thought to have evolved in a common ancestor of all vertebrates. Even the most distant clades of vertebrates — the agnathan lampreys and hagfish — possess an optic chiasm and contralateral brain organization,[25] as well as a left-sided heart and asymmetric bowels.[26] Also, every vertebrate has a contralateral organization of the forebrain.[25] Fossil skull impressions of early vertebrates from the Ordovician and later show the presence of an optic chiasm.[27]
Twisting and asymmetric development are well known from other deuterostomes — such as Hemichordata, Echinodermata, Cephalochordata, and Tunicata. Turning toward the side or upside-down also occurs frequently in these clades. For example, sea stars turn their mouth downwards, after the larva has briefly been attached to a substrate by a rudimentary stalk, with the mouth turned up. The adult lancelet buries itself with its back downward and their mouth reaching out. Some fish species tend to turn around when feeding from the water surface.
Thus, the axial twist evolved in an ancestor to all vertebrates, and possibly of all chordates or even in an early deuterostome.
Open questions[edit]
The axial twist theory is a novel scientific discipline and very few scientific papers have presently addressed it directly.[1][2][8][3] Although a considerable volume of research exists on the genetic and embryological mechanisms of asymmetric development, an open question is how the twist is initiated and how the inversion of the left-right and up-down axes in the anterior head region is established.
The embryology of the twisting has been addressed only rudimentary in the chick and the zebrafish.[1] The differences in timing and mechanisms across the vertebrate clades are completely unknown.
The evolution of the axial twist is an open question. The founders of the axial twist idea (de Lussanet & Osse, and Kinsbourne) agree that the axial twist is universal in vertebrates and probably is a feature of all chordates.[1][2] Although the asymmetric development of other chordates such as the Lancelet has been studied in detail, no study has analysed this development in the light of the axial twist theory. Moreover, even other deuterostomes, i.e. the echinoderms (sea stars, sea lilies, etc.) show a marked asymmetric development and even an axial twist[41] This twist has remarkable similarities to that in vertebrates, but no study has addressed this at present. Lastly, the asymmetric and twisted development is well known from gastropods and the relation to asymmetric development in vertebrates is an important question.
It has been proposed that problems in the axial twist development may play a central role in developmental malformations such as holoprosencephaly[1] and skoliosis[3] but these have not been looked into.