| Axial twist | |
|---|---|
 Schema of the development of the axial twist. Developmental phases are (from top to bottom): (1) the embryo turns on its left side; (2) the anterior head grows in the same direction, but the rest of the body grows oppositely into a twist. So that ultimately (3) external bilateral symmetry is regained. Note, that there is no evolutionary pressure for internal symmetry so the heart (and other organs) remain asymmetric. | |
| Details | |
| System | vertebrate body plan |
| Anatomical terminology | |
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.
The axial twist theory competes with a number of other proposals that focus on more limited, specific aspects (see section #Relation to other theories and hypotheses), most of which explaining contralateral forebrain organization, the phenomenon that the left side of the brain mainly controls the right side of the body and vice versa.[3] None of the proposed theories explaining this phenomenon, including axial twist theory, have gained general recognition.[4] The genetic basis underlying the developmental twist is not yet understood.
The axial twist theory explains various phenomena, and addresses how and when the twist between the end of the head and the rest of the body develops. It also addresses the possible evolutionary history. One prediction of the theory was the aurofacial asymmetry, which was then found empirically,[5] albeit by the author who had also co-authored the original theory.
Phenomena the theory can explain include:
- Contralateral organization of the brain
- Left-sided orientation of the heart
- Asymmetric position of the gastrointestinal tract, the liver, and the pancreas
- Optic chiasm
- chiasm of the trochlear nerve
- Non-crossed olfactory tract
- Aurofacial asymmetry
- Yakovlevian torque
- Asymmetry of the thoracal vertebra
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.
- ^ de Lussanet, M. H. E.; Osse, J. W. M. (2012). "An ancestral axial twist explains the contralateral forebrain and the optic chiasm in vertebrates". Animal Biology. 62 (2): 193–216. arXiv:1003.1872. doi:10.1163/157075611X617102. S2CID 7399128.
- ^ Kinsbourne, M. (2013). "Somatic twist: a model for the evolution of decussation". Neuropsychology. 27 (5): 511–515. doi:10.1037/a0033662. PMID 24040928. S2CID 11646580.
- ^ Viggiano, D.; Pirolo, L. (2002). "Testing the model of optic chiasm formation in human beings". Brain Res. Bull. 59 (2): 111–115. doi:10.1016/S0361-9230(02)00846-8. PMID 12379441. S2CID 2687785.
- ^ Mora, Carla; Velásquez, Carlos; Martino, Juan (2019-09-01). "The neural pathway midline crossing theory: a historical analysis of Santiago Rámon y Cajal's contribution on cerebral localization and on contralateral forebrain organization". Neurosurgical Focus. 47 (3): E10. doi:10.3171/2019.6.FOCUS19341. ISSN 1092-0684. PMID 31473669.
- ^ de Lussanet, M. H. E. (2019). "Opposite asymmetries of face and trunk and of kissing and hugging, as predicted by the axial twist hypothesis". PeerJ. 7: e7096. doi:10.7717/peerj.7096. PMC 6557252. PMID 31211022.
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