A general problem concerns the lack of homogeneity in the definition of brain asymmetries and of the methods used to quantify them. For example, one of the most studied brain asymmetries on brain endocasts concern the petalias. LeMay
[35][36] initially considered the antero-posterior projection of the frontal and occipital lobes, respectively. By contrast, later studies generalized the term ‘petalias’ to a wide range of anatomical traits. Some studies indeed referred to bilateral differences in the lateral extension of the posterior area of the frontal lobes
[37], to other anatomical areas of the brain, and even to volumetric variations between hemispheres
[38][39][40]. It is therefore difficult to compare data obtained on petalias if studies do not consider the same brain features. Nevertheless, it was largely accepted that this pattern of asymmetries appeared with early Homo
[36][41] and is more common in right-handed individuals
[35][36][42][43][44]. Based on an original methodology applied to the largest samples ever used, we demonstrated a shared specific pattern of protrusions of the frontal and occipital across all hominids, including extant African great apes, modern humans, and hominin fossils
[31][45]. These asymmetries are a topic of debate in non-human primate brain studies
[35][38][39][46][47][48] and paleoanthropology
[49][37][50][51][52] because of their relationship with handedness and other specific aspects of human cognition. Similar results were obtained recently by an independent team
[7].
H. sapiens appear to have more asymmetrical petalias than other extant great apes, but a shared pattern is observed, suggesting that a globally asymmetric brain is the ancestral condition. A recent study questioned this observation
[53]. However, this is a good example of differences in the definition of the anatomical traits that are analyzed. These authors measured the bilateral variation in lateral extension of slices of the brain. This trait is not directly comparable to our analyses of the 3D position of the occipital poles
[54] or to the 3D displacement between the left and right corresponding anatomical area. Another good illustration of the problem is Broca’s area, whose extension is defined differently according to authors
[55]. This functional area is impossible to characterise on brain endocasts. However, we conducted a comparative study on the size, shape, and position of the third frontal convolution in great apes,
H. sapiens, and hominin fossils
[54]. The neuroanatomical asymmetries as quantified in our work show a pattern that is different from what was previously accepted based on qualitative data. Our main finding was a shared pattern of asymmetry in Broca’s area in all hominins and Pan paniscus, as well as an increase in the size of this area during human evolution. We also identified that Pan troglodytes and Pan paniscus have differences in their asymmetry patterns in the third frontal convolution. This topic is of great interest for future research. More generally, brain and endocranial studies have to rely on a clear definition of the anatomical features that are analyzed and an effort to use similar protocols will certainly enhance the reproducibility.