Parietal lobe variation in cercopithecid endocasts

Abstract

In extant primates, the posterior parietal cortex is involved in visuospatial integration, attention, and eye-hand coordination, which are crucial functions for foraging and feeding behaviors. Paleoneurology studies brain evolution through the analysis of endocasts, that is molds of the inner surface of the braincase. These may preserve imprints of cortical structures, such as sulci, which might be of interest for locating the boundaries of major cortical regions. Old World monkeys (Cercopithecidae) represent an interesting zoological group for evolutionary studies, because of their diverse ecologies and locomotor behaviors. In this study, we quantify parietal lobe variation within the cercopithecid family, in a sample of 30 endocasts including 11 genera and 17 species, by combining landmark-based and landmark-free geometric morphometric analyses. More specifically, we quantitatively assess variation of the parietal proportions based on landmarks placed on reliable anatomical references and of parietal lobe surface morphology through deformation-based methods. The main feature associated with the cercopithecid endocranial variation regards the inverse proportions of parietal and occipital lobes, with colobines, Theropithecus, and Papio displaying relatively larger parietal lobes and smaller occipital lobes compared with cercopithecins. The parietal surface is anteroposteriorly longer and mediolaterally flatter in colobines, while longitudinally shorter but laterally bulging in baboons. Large parietal lobes in colobines and baboons are likely to be independent evolutionary traits, and not necessarily associated with analogous functions or morphogenetic mechanisms.

Unfolding the evolution of human cognition

“…The size, shape, scale, ratio, or combinations therein of brain structures provide insights into the evolution of human cognition from the brain itself or from endocasts (e.g., 'brain cases' [1][2][3][4][5]). Endocasts either occur naturally during fossilization as the neurocranium is filled with sediment or can be artificially reconstructed.…”

“…Specifically, two genes (UBR4 and PHLPP1) are related to neurogenesis and myelination, further supporting that developmental mechanisms likely contribute to the shape of the human endocranium, which has implications for neural architecture and efficiency of cognition [3]. At the lobular level (lobular refers to primary sulci demarcating the frontal, parietal, temporal, and occipital lobes), recent research identifying primary sulci on endocasts and performing geometric morphometrics analyses across 11 genera and 17 species showed an inverse relationship between parietal and occipital lobes across species: the more evolutionarily recent, the larger the parietal lobe and the smaller the occipital lobe [5]. At the ratial level, considering an infant:adult ratio of endocranial volumes from a hominin species more than 3 million years ago indicates a protracted brain growth, which is likely critical of a long period of childhood learning in hominins [2].…”

“…In this Forum, we integrate findings from two parallel (anthropology and neuroanatomy) tracks that propose novel insights into the evolution of human cognition across millions of years. The first track reveals that the evolutionary trajectory of cortical expansion and sulcal morphology in the human brain is different than previously thought [1][2][3][4][5]. The second track shows that individual variation in cognition and functional brain networks is linked to variation in sulcal anatomy [6][7][8][9][10].…”