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). It proposes that the rostral part of the head is "turned around" regarding the rest of the body. 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 | |
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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 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.
Explained phenomena 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. There are some popular science videos and podcasts on the topic.