The replacement of Neanderthals by modern humans began with the emergence of Homo sapiens about two hundred thousand years ago. In RNMH project (*), we examine the working hypothesis that the replacement event comes from the difference in learning ability between modern humans and Neanderthals (learning hypothesis) by combining evidence from the morphological analysis of fossil brains and functional mapping of modern human brain functions (Kochiyama et al., 2014; Tanabe et al., 2014, in Dynamics of Learning in Neanderthals and Modern Humans, Springer). To achieve this, we first constructed the method for extrapolating human brain to Neanderthal's one as follows: the endocast reconstructed from CT image of a fossil brain is spatially deformed to a modern human endocast estimated by the segmented MR images using LDDMM (Large Deformation Diffeomorphic Metric Mapping, e.g. DARTEL algorithm implemented in SPM) approach. Using resulting deformation field, the modern human cerebral cortex was inversely deformed to create the virtual Neanderthal's brain. We then evaluated the morphological differences between modern human's brain (NIPS young Japanese brain database N=614) and virtual Neanderthal's brain (N=3) using (1) deformation-based morphometry (DBM), which is a method for identifying macroscopic anatomical differences among the brains of different populations and (2) AAL brain percellation based morphometry, in which we compare voxel counts within AAL regions among different populations after deforming the AAL atlas to match an individual brain (or virtual brain). These quantitative morphological analyses revealed that a significant decrease of brain volume in the parietal cortex and cerebellum was observed in Neanderthals. The comparison of Neanderthals > modern humans showed a significant volume increase in the visual cortex and orbitofrontal cortex. These results provide important evidence for validate the learning hypothesis since these areas (e.g. cerebellum) play an essential role in some types of learning.
(*) Research Project on Replacement of Neanderthals by Modern Humans (RNMH): Testing Evolutionary Models of Learning