![]() Early authors relied exclusively on post mortem brains, used terminology that is now outdated, failed to recognize important sulci such as the middle frontal ( fm) sulcus and Affenspalte (lunate sulcus, L), and illustrated specimens with photographs or line drawings from limited perspectives (typically right or left lateral views). The literature on chimpanzee brains prior to and during the first half of the 20th century focused mostly on individual brains, although a few studies included relatively large samples of chimpanzees. ![]() It is important to note, however, that the only information about sulci that can be gleaned from primate, including hominin, endocasts depends on superficial traces, which are often fragmentary.īecause chimpanzees are the closest living phylogenetic relatives of humans, descriptions of their brains have traditionally been important for identifying sulci on australopithecine endocasts, and comparison of chimpanzee and human brains remains essential for addressing brain evolution in hominins since the two groups split from a common ancestor. Although these discussions about the mechanisms of how sulci form during development may seem somewhat tangential to the present paper, the shape of the developing brain (endocast) modulates the mechanical factors that influence the placement and orientations of sulci so that, in real brains, the “primary convolutions are consistently reproducible in their location”. A recent study of developing cortex in rhesus monkey fetuses suggests that maturation of dendrites within the cortex may be linked to the formation of sulci, which may be more consistent with the mechanical folding model. The mechanical folding model, on the other hand, suggests that tangential expansion of the cerebral cortex generates compression that leads to the development of cortical sulci. The axon tension hypothesis posits that species-specific sulcal patterns are caused by tension along axons in subcortical white matter. There are currently two prevailing hypotheses about the mechanisms that govern the formation of sulci during fetal development. ![]() It is our hope that these labeled in vivo chimpanzee brains will assist future researchers in identifying sulci on hominin endocasts, which is a necessary first step in the quest to learn how and when the external morphology of the human cerebral cortex evolved from apelike precursors. The sulcal patterns in these new images also suggest that changes in two gyri that bridge between the parietal and occipital lobes may have contributed to cortical reorganization in early hominins. Our findings, even in a small sample like the present one, overturn published claims that australopithecine endocasts reproduce derived configurations of certain sulci in their frontal lobes that never appear on chimpanzee brains. These images and sulcal identifications exceed the quantity and quality of previously published illustrations of chimpanzee brains with comprehensively labeled sulci and, thus, provide a larger number of examples for identifying sulci on hominin endocasts than hitherto available. Sulci on the exposed surfaces of the frontal, parietal, temporal, and occipital lobes are identified on the images based on their locations, positions relative to each other, and homologies known from comparative studies of cytoarchitecture in primates. Here, we provide images of lateral and dorsal surfaces of 16 hemispheres from 4 male and 4 female adult chimpanzee brains that were obtained using in vivo magnetic resonance imaging. ![]() The shortage of adequate descriptions of chimpanzee sulcal patterns partly explains why the identities of certain sulci on australopithecine endocasts (e.g., the inferior frontal and middle frontal sulci) have been controversial. However, the few comprehensive descriptions of cortical sulci published for chimpanzees usually relied on post mortem brains, (now) antiquated terminology for some sulci, and photographs or line drawings from limited perspectives (typically right or left lateral views). Paleoneurologists rely heavily on published descriptions of sulci on brains of great apes, especially chimpanzees (humans’ phylogenetically closest living relatives), to guide their identifications of sulci on ape-sized hominin endocasts. Surface traces of sulci that separate the brain’s convolutions (gyri) are reproduced sporadically on early hominin endocasts. The only direct source of information about hominin brain evolution comes from the fossil record of endocranial casts (endocasts) that reproduce details of the external morphology of the brain imprinted on the walls of the braincase during life.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |