The question of whether suprainiac depressions observed on Neandertals and in other human samples are homologous is widely discussed. Recently (Balzeau and Rougier, 2010), we ascertained the autapomorphic status of the Neandertal suprainiac fossa as a depression showing specific external bone features together with a thinning of the diploic layer with no substantial remodeling nor variation in the external table thickness. A suprainiac fossa with these characteristics is systematically present on Neandertals from the earliest developmental stages on, and since the beginning of the differentiation of the Neandertal lineage. Here, we present a detailed analysis of the micro-CT dataset (resolution of 50 μm) of the occipital bone of the La Ferrassie 8 Neandertal child, whose proposed age-at-death is around 2 years, and we compare it to the adult condition as represented by La Chapelle-aux-Saints 1 (resolution of 122 μm). We describe and quantify the boundaries between the different structural layers of the occipital bone, namely the external and internal tables and the diploic layer. We also describe very fine details of the diploic layer structure that had never before been observed on fossil hominins. This study illustrates for the first time that the internal particularities that make the suprainiac fossa a Neandertal autapomorphy are evident early during growth and development. Moreover, we demonstrate that the developmental pattern and causes of expression for the features observed in modern humans and Neandertals are certainly different, indicating that these features are not homologous traits from evolutionary and functional perspectives. Consequently, we confirm the autapomorphic status of the Neandertal suprainiac fossa. Am J Phys Anthropol 151:38–48, 2013. © 2013 Wiley Periodicals, Inc.