Australopithecus afarensis endocasts suggest ape-like brain organization

Abstract

Debates about evidence for brain reorganization in Australopithecus have often focused on the position of the lunate sulcus, ever since Raymond Dart’s seminal description of the Taung child’s natural endocranial imprint in 1925. In non-human apes a well-deܪned lunate sulcus approximates the anterior boundary of the primary visual cortex of the occipital lobes. Some have argued that an evolutionary reorganization of the parietotemporo-occipital association cortices displaced the lunate sulcus posteriorly on endocasts of australopiths, and eventually led to the disappearance of a clear endocranial impression in humans. Hypothetically, such brain reorganization in early hominins could have been linked to behaviors that were more complex than those of their great ape relatives (e.g. tool manufacture, mentalizing, vocal communication.) Conventional and synchrotron computed tomographic (CT) scans of original fossils from Dikika and Hadar (Ethiopia) yielded (i) an exceptionally preserved endocranium of the Dikika infant DIK-1-1, (ii) a precise age at-death based on virtual dental histology for DIK-1-1, (iii) new endocranial volume estimates of the best-preserved Australopithecus afarensis fossil crania, and (iv) previously undetected endocranial features on well-known Australopithecus fossils. An endocranial segmentation of DIK-1-1 reveals detailed sulcal impressions, including an unambiguous lunate sulcus in an anterior (ape-like) position. Moreover, the micro-CT data reveal a clear, previously undetected impression of an ape-like lunate sulcus on the partial cranium A.L. 162-28 from Hadar. Conventional and synchrotron computed tomographic (CT) scans of original fossils from Dikika and Hadar (Ethiopia) yielded (i) an exceptionally preserved endocranium of the Dikika infant DIK-1-1, (ii) a precise age at-death based on virtual dental histology for DIK-1-1, (iii) new endocranial volume estimates of the best-preserved Australopithecus afarensis fossil crania, and (iv) previously undetected endocranial features on well-known Australopithecus fossils. An endocranial segmentation of DIK-1-1 reveals detailed sulcal impressions, including an unambiguous lunate sulcus in an anterior (ape-like) position. Moreover, the micro-CT data reveal a clear, previously undetected impression of an ape-like lunate sulcus on the partial cranium A.L. 162-28 from Hadar.