The human ilium plays an essential role in weight bearing, locomotion, and providing support for the body. The study of its ontogenesis offers valuable insights into both skeletal development and the evolutionary history of the human pelvis. Moreover, the comprehension of its developmental process, which is influenced by both genetic and epigenetic factors, is crucial for understanding the adult form and function. Among species, the ilium of Homo sapiens and Neanderthals show significant differences in their adult morphologies as well, reflecting plesiomorphic, autapomorphic, and functional modifications. The ontogenetic trajectories of these two species are thought to differ, with Neanderthal infants appearing to have reached skeletal maturity faster than humans. Here, we analyze the external morphology of the ilium using Geometric Morphometrics (GM) to analyze its developmental pattern and to assess the differences between recent Homo sapiens (RH) and Homo neanderthalensis (HN). Statistical analyses were performed in R. The sample is composed of 28 ilia from the modern collection of the University of Bologna [1], divided into three age classes (0-1 year; 1-5 years; 5-11 years). The fossil sample comprised the well-preserved juvenile ilia of Mezmayskaia 1 (MZ1) [2] and Le Moustier 2 (LM2) [3], and the restored ilium of Amud 7 [4]. Shape space Principal Component Analysis (PCA) shows that the first three PCs explain 55.9% of the total variance. Most of the variance is explained by PC1 (28.2%), which is strongly correlated with size (r=76%, p=&lt;0.001), hence representing an allometric model of ontogenetic morphological modifications of the modern human ilium. PC2 and PC3 explain 16.4% and 10.7%, respectively, of the variance. ANOVA and Tukey post-hoc tests reveal significant differences along PC1 (p=0.001) and PC3 (p=0.01), with significant differences between the Neanderthal and Sapiens individuals in each age class only along PC3 scores. No significant differences are present between contiguous age classes. Negative values along PC1 describe individuals whose ilia show a weakly developed anterior and posterior superior iliac spine, greater sciatic notch, and iliac crest curvature, i.e., younger individuals. PC1 positive values depict ilia with a more defined anterior and posterior superior iliac spine, greater sciatic notch, and a well-developed iliac crest curvature, i.e., older individuals. The acetabulum is more concave and defined in older individuals. PC2 scores describe ilia whose superior-inferior length is greater than the antero-posterior length (positive scores), while this difference is reduced in younger individuals (more negative values). PC3 scores describe differences linked to the development of the iliac crest curvature. Along PC1, the fossil individuals fall in the middle of the variability of modern humans, towards more positive values. When PC2 and PC3 are considered, the fossils plot outside modern human variation, with MZ1 distant from the modern human cluster. Morphological differences between modern humans and Neanderthal individuals are mainly related to the curvature of the iliac crest, and a slightly different orientation in the acetabular area, i.e., more postero-lateral oriented in HN. The more posterolateral orientation of the acetabulum has been described by several studies [5] in individuals of different ages. This particular morphology has been described also in adult Neanderthal individuals and has been interpreted as linked to differences in locomotor biomechanics [5].These preliminary results add knowledge to the growing corpus of literature on the development of the hominin pelvis. Future analyses will focus on increasing the fossil and comparison sample and including the other coxal bones.We would like to thank prof. Bruno Maureille and the Musée national de Préhistoire for Le Moustier 2, which is part of its collections. This work is funded by the European Union under the Horizon Europe research and innovation program – Marie Sklodowska-Curie Actions, HORIZON-TMA-MSCA-PF-GF n.101108385-RISEN granted to C.F. References: [1] Belcastro, M.G., Bonfiglioli, B., Pedrosi, M.E., Zuppello, M., Tanganelli, V., Mariotti, V., 2017. The History and Composition of the Identified Human Skeletal Collection of the Certosa Cemetery (Bologna, Italy, 19th–20th Century). International Journal of Osteoarchaeology. 27, 912–925. [2] Weaver, T.D., Coqueugniot, H., Golovanova, L.V., Doronichev, V.B., Maureille, B., Hublin, J.-J., 2016. Neonatal postcrania from Mezmaiskaya, Russia, and Le Moustier, France, and the development of Neandertal body form. Proceedings of the National Academy of Sciences. 113, 6472–6477. [3] Maureille, B., 2002. A lost Neanderthal neonate found. Nature. 419, 33–34. [4] Rak, Y., Kimbel, W.H., Hovers, E., 1994. A Neandertal infant from Amud Cave, Israel. Journal of Human Evolution. 26, 313–324. [5] Gicqueau, A., Schuh, A., Henrion, J., Viola, B., Partiot, C., Guillon, M., Golovanova, L., Doronichev, V., Gunz, P., Hublin, J.-J., Maureille, B., 2023. Anatomically modern human in the Châtelperronian hominin collection from the Grotte du Renne (Arcy-sur-Cure, Northeast France). Scientific Reports. 13, 12682.Read less <