De-opercularization of the lunate sulcus in early Homo

Since Raymond Dart's first attempt to identify the lunate sulcus ("Affenspalte," simian sulcus) in a fossil hominin endocast-that of the Taung child (Australopithecus africanus)-paleoneurologists have debated this structure, which in the brains of monkeys and apes roughly coincides with the rostral boundary of the primary visual cortex. The classic view has been that the evolutionary expansion of the parietooccipital cortex "pushed" the lunate sulcus toward the back of the brain. However, there has been little consensus about how and when this might have happened during hominin evolution, as it has proven difficult to establish phylogenetic homology of potential lunate sulci in living humans with the lunate sulcus of great apes. Here we review the comparative neuroanatomical evidence and propose the hypothesis that the lunate sulcus underwent de-opercularization, that is, the structures buried within the sulcus expanded and became part of the external cortical surface. During this process, the lunate sulcus became shallow, fragmented, and eventually obliterated. Specifically, rather than migrating toward the occipital pole during brain evolution, the lunate sulcus was a hotspot for the evolutionary expansion of annectant gyri and their eventual emergence on the parietooccipital cortical surface. We test the de-opercularization hypothesis with an analysis of the parietooccipital endocranial region of early Homo from Dmanisi, Georgia, and conclude that in these fossils the lunate sulcus may have been in the evolutionary process of fragmentation as their brains became larger and their occipital lobes more caudally projected compared to earlier hominins.