**1. Introduction**

Endocast, or brain endocast, is the cast made of the interior of the neurocranium of a skull [1]. Endocast is the only agen<sup>t</sup> to investigate how the human brain evolved physically in the process of evolution [2] regarding its volume [3–7], surface features [8–11], or size and shape [12–16].

Asymmetry of the brain, as one of the most debated questions, exhibits at different levels, such as the Broca's area [17,18], perisylvian region [19], central sulcus, cortical and subcortical regions, lobes, and hemispheres [20,21]. Brain asymmetry is often related to functional and evolutionary significance; for example, petalia and the Yakovlevian torque is a geometric distortion of the brain hemisphere, in which the left occipital lobe and the right frontal lobe are wider and longer than the opposite side [21]. While a combination of left occipital and right frontal petalia is common in modern humans and fossilized hominins and is regarded as evidence of right-handedness [22–24], such observations are less exaggerated or rarely consistent in grea<sup>t</sup> apes and other primates [15,25]. Also, capuchins display a leftward frontal petalia [26] while macaques show the rightward frontal petalia [27], and both were absent of a left occipital petalia.

Compared to that of the cerebrum, the cerebellum receives less attention and is still poorly understood. However, with the advances of neuroimaging technology and theoretical innovation, there is increasing study of the cerebellum regarding its morphology, function, and evolutionary changes. Moreover, evidence from neuroimaging uncovers the cerebellum as the "missing link" in many cognitive domains [28]. Sereno, et al. [29] revealed in a recent study that the cerebellar cortex covers almost 80% of the surface area

**Citation:** Zhang, Y.; Wu, X. Asymmetries of Cerebellar Lobe in the Genus *Homo*. *Symmetry* **2021**, *13*, 988. https://doi.org/10.3390/ sym13060988

Academic Editor: Antoine Balzeau

Received: 29 April 2021 Accepted: 31 May 2021 Published: 2 June 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

of the cerebral cortex, and the expansion of the human cerebellar surface area even exceeds the cerebral cortex when compared to that of monkeys. Also, neuroimaging evidence suggests that the function of the cerebellum is also highly involved in cognitive and social activities [30–33], which is also supported by clinical studies, such as developmental dyslexia [34,35].

The evolutionary changes of the cerebellum were also hotly debated. Studies sugges<sup>t</sup> that the cerebellum shows a similar asymmetric pattern as the cerebrum (the left-occipital, right-frontal petalia), with larger anterior lobules on the right side and larger posterior lobules on the left side [36,37]. Compared to that of humans, the cerebellar torque is opposite in chimpanzees [26], while absent in capuchins. MacLeod, Zilles, Schleicher, Rilling, and Gibson [31] found that hominins have a grea<sup>t</sup> increase in the lateral cerebellum compared to that of monkeys. It is also a popular idea that the cerebrum and cerebellum underwent several expansions and reorganizations in the process of human evolution [2]. Weaver [38] came up with a hypothesis that the evolution of human cerebellar/neocortical occurred in three stages; the first stage as early encephalization with an expansion of the neocortex during Early-to-Middle Pleistocene; the second stage as the dramatic encephalization primarily happened to the neocortex in Middle-to-Late Pleistocene humans, accompanied by a proliferation of cultural objects as well as an increase in complex behaviors, such as pyrotechnology and prepared core techniques, and the third stage happened in the late Late Pleistocene and Holocene, with an increase in cognitive efficiency as a result of expanded cerebellar capacity, which is the reason why modern humans can do more without an increase in net brain volume. Cerebellar specialization is thought to be an important component in the evolution of humanity's advanced technological capacities and languages [39,40].

The cerebellum is classically divided into three lobes, namely, the anterior lobe, the posterior lobe, and the flocculonodular lobe; it also has three major surfaces, with the superior surface toward the tentorium cerebelli, the posterior surface toward the internal occipital bone, and the anterior surface toward the petrous pyramid [41]. The cerebellar lobe reconstructed from the cerebellar fossa is surrounded by transverse sinus, sigmoid sinus, and occasionally occipital and marginal sinus. Accurate and homologous landmarks are important in the study of endocasts, such as in the case of Taung australopithecine endocast, in which a vague position of the lunate sulcus caused grea<sup>t</sup> debate [1,42–48]. Unlike the cerebrum, there are no gyrus or sulcus that exist on the surface of the cerebellum, and hence, it is difficult to identify landmarks to measure.

In this study, we tentatively define the major axis and four measurements on the cerebellum to achieve a more accurate between-group comparison. The main aim of this study is (1) to test whether certain asymmetric patterns existed within the genus *Homo*; (2) to find out possible factors that affected the asymmetry, and to (3) provide morphological evidence for cerebellar reorganization and cognitive increases in the genus *Homo*.

## **2. Materials and Methods**
