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 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 explain 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:
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.
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