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, 7, e7096

Opposite Asymmetries of Face and Trunk and of Kissing and Hugging, as Predicted by the Axial Twist Hypothesis


Opposite Asymmetries of Face and Trunk and of Kissing and Hugging, as Predicted by the Axial Twist Hypothesis

Marc H E de Lussanet. PeerJ.


The contralateral organization of the forebrain and the crossing of the optic nerves in the optic chiasm represent a long-standing conundrum. According to the Axial Twist Hypothesis (ATH) the rostral head and the rest of the body are twisted with respect to each other to form a left-handed half turn. This twist is the result, mainly, of asymmetric, twisted growth in the early embryo. Evolutionary selection tends to restore bilateral symmetry. Since selective pressure will decrease as the organism approaches symmetry, we expected a small control error in the form of a small, residual right-handed twist. We found that the mouth-eyes-nose (rostral head) region shows a left-offset with respect to the ears (posterior head) by up to 0.8° (P < 0.01, Bonferroni-corrected). Moreover, this systematic aurofacial asymmetry was larger in young children (on average up to 3°) and reduced with age. Finally, we predicted and found a right-sided bias for hugging (78%) and a left-sided bias for kissing (69%). Thus, all predictions were confirmed by the data. These results are all in support of the ATH, whereas the pattern of results is not (or only partly) explained by existing alternative theories. As of the present results, the ATH is the first theory for the contralateral forebrain and the optic chiasm whose predictions have been tested empirically. We conclude that humans (and all other vertebrates) are fundamentally asymmetric, both in their anatomy and their behavior. This supports the thesis that the approximate bilateral symmetry of vertebrates is a secondary feature, despite their being bilaterians.

Keywords: Anatomy; Asymmetry; Brain; Developmental malformation; Ethology; Evo-devo; Fetal alcohol syndrome (FAS); Human; Optic chiasm; Scoliosis.

Conflict of interest statement

The author declares that there are no competing interests.


Figure 1
Figure 1. (A–B) Schema of the asymmetric development leading to the left-handed twist. (C) Schema of the predicted aurofacial asymmetry for the human face.
(A) The early embryo is at first positioned on its left side. The facial region, including the forebrain, grows asymmetrically (red arrow) to become symmetric. (B) The ears (gill region) with the rostral head and the soma reach symmetry by opposite asymmetric growth (blue arrow). (C) The eyes-nose-mouth region (red) belongs anatomically to the rostral head, whereas the ears (blue) are part of the posterior head. Since the asymmetric growth (red and blue arrows) is predicted to be incomplete, we hypothesize that the face is shifted to the left side (small arrows) with respect to the mid-plane between the tragi (vertical line). Green dots show paired and median facial landmarks; paired: tragus of the ear, frontotemporale, exo- and endocanthion of the eye; median from top: nasion, pronasale, subnasale, labiale superior, and menton. Small arrows show the asymmetry of the facial landmarks.
Figure 2
Figure 2. Result of the aurofacial asymmetry in adults.
(A) The average face from N = 200 (Blanz & Vetter, 1999; Troje & Bülthoff, 1996) with the tragus of the ears aligned with the horizontal axis of the image plane. Red-shaded area denotes significant aurofacial asymmetry to the left side. The asymmetry with respect to the symmetric reference face is displayed exaggerated by a factor five. (B) Magnitude of the aurofacial asymmetry (with respect to the symmetric reference face) along the three colored lines in (A). The asymmetry expresses the angular difference for each vortex between the average face and the symmetric reference face. Vertical lines in both panels are symmetrically arranged around the mid-plane between the tragi.
Figure 3
Figure 3. Age dependence of the aurofacial asymmetry, for the same groups and landmarks as in Tables 1 and 2.
Figure 4
Figure 4. Opposite rotational asymmetries as viewed from below (in accordance with medical conventions, so that right [R] and left [L] are mirrored).
The dorsal/occipital side is at the bottom. (A) The Yakovlevian torque of the cerebrum (exaggerated). (B) The angular torsion of a t5 vertebra of a typical healthy subject. Note the opposite direction of the rotational asymmetry. Redrawn from Kouwenhoven et al. (2006) and Toga & Thompson (2003). Source: M.H.E. de Lussanet, https://www.wikimedia, Creative Commons CC0 1.0.
Figure 5
Figure 5. Examples of left kissing (A) and right hugging (B).
The two schemas show a top view of the opposite behavioral asymmetries. Source: M.H.E. de Lussanet, www.wikimedia, Creative Commons CC0 1.0.

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The author received no funding for this work.

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