**1. Introduction**

In addition to helping us understand the evolution of lateralization [1–3], asymmetries of the brain's surface seen in closely related species such as chimpanzees (*Pan troglodytes*) can also help us to understand the role development plays in brain evolution itself. As an example, a major shape difference in the brains of human (*Homo sapiens*) versus nonhuman primates is that in nonhuman primates the occipital lobe operculates part of the parietal lobe, including a buried annectant gyrus that connects these lobes, known as the 1st parieto-occipital "pli de passage" of Gratiolet or the parieto-occipital arcus [4–6]. The posterior portion or bridge of this gyrus is consistently seen on the brain's surface in humans but is only occasionally seen (often asymmetrically) in chimpanzees [4–8]. Relative reduction of the occipital operculation and expansion of the posterior parietal lobe is a major hallmark in human brain evolution, although debate on when this occurred has been contentious, and currently we have no model of what transitional states between the human ancestral and derived conditions may have looked like. Studying the presence or absence of a visible bridging gyrus in chimpanzees, who are our closest living relatives and who have brains very similar to that of the last common ancestor [7–10] allows us to understand its relationship to the size of the occipital lobe; when this trait is asymmetrical in chimpanzees (who unlike humans still show occasional asymmetry in this region) it allows us to understand this trait developmentally rather than genetically, as it occurs variably in different hemispheres of the same individual, while giving us a greater range of variation in which to build models of transitional states, and to study the evolution of asymmetries and symmetries, since it is asymmetrical in chimpanzees while it is symmetrical in humans. Such an understanding would also be very valuable for the interpretation of hominin endocranial casts, which have morphology that is difficult to interpret in this region due to

**Citation:** Hurst, S.; Holloway, R.; Pearson, A.; Bocko, G. The Significance of Chimpanzee Occipital Asymmetry to Hominin Evolution. *Symmetry* **2021**, *13*, 1862. https:// doi.org/10.3390/sym13101862

Academic Editor: Antoine Balzeau

Received: 18 August 2021 Accepted: 29 September 2021 Published: 3 October 2021

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our lack of transitional models, and so very valuable to the study of brain evolution. If this trait is only associated with occipital lobe height this would sugges<sup>t</sup> that the primary factor in the exposure of the bridging gyrus is posterior movement of the occipital operculation, which retracted inferio-posteriorly during human evolution revealing buried parietal gyri which then expanded; association with asymmetry and/or width in addition to height would sugges<sup>t</sup> a relative change in the size and shape of the entire occipital to the parietal lobe is a more important factor. Using preliminary data, we observed these relationships in a large sample of chimpanzees. The aim of this study is an exploratory assessment of whether the presence or absence of the occipital bridging gyrus is associated with left or right hemispheres, and how hemisphere siding is associated with occipital lobe width and height in the chimpanzee brain. Regression analysis examines the correlation between left and right hemispheres and occipital lobe width and height, where reliable predictions (±1 s.e.) determined if occipital lobe height or width was a more reliable predictor of hemisphere siding. Ultimately, we found that asymmetry, height, and width are all associated with a visible bridging gyrus, in increasing order.

## **2. Materials and Methods**

This study used three-dimensional surface models of a sample of 83 chimpanzee brains. These brains were reconstructed using MRIs from the National Chimpanzee Brain Resource (https://www.chimpanzeebrain.org (accessed on 1 September 2021)) using BrainVISA software (Pune, India) and measured using MeshLab [11–13]. Although the measurements were able to be collected on the entire sample, the original collectors [12] could not guarantee that the left or right hemisphere siding was correctly labelled. To accommodate this uncertainty, subsample (*n* = 15) was obtained by one of us to allow a comparison and analysis of 'known' and 'unknown' hemisphere siding'. Each brain was rotated such that the lowest points of the left occipital and left temporal lobes both lie on a plane at right angles to the longitudinal fissure. The width of each hemispherical occipital lobe was measured as the distance in millimeters from the longitudinal fissure to the lobe's most lateral extent. Height was measured as the greatest vertical extent between points on each hemispherical lobe, barring its most medial edge if a bridging gyrus was visible; the presence of a visible bridging gyrus between the superior-medial occipital lobe and the parietal-occipital arcus was scored as a Y, while a fully operculated and thus hidden bridging gyrus was scored as an N (see Figure 1).

**Figure 1.** Occipital Measurement Definitions. W = width, H = height. The right hemisphere has a bridging gyrus (BG) not fully operculated by the occipital lobe and was scored as a Y; in the left hemisphere this gyrus is fully operculated, so its condition was scored as an N.
