*2.1. Materials*

A total of 45 specimens were used in this study (as illustrated in Table 1), including *Homo erectus* (n = 11), *Homo neanderthalensis* (n = 4), and extant *Homo sapiens* (modern humans, n = 30). *H. erectus* specimens were mostly sourced from Asia and Indonesia to avoid significant regional differences. The 3D data of the fossil hominins were laser-scanned from endocast models housed in the American Museum of Natural History (AMNH) and the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) with a resolution of 0.5 mm or higher. Endocasts of extant *H. sapiens* were reconstructed from CT scans with a resolution of 160 μm. Frontal pole to occipital pole length was also measured in this study and compared to that of those from literature to ascertain whether no grea<sup>t</sup> discrepancy existed.


**Table 1.** List of specimens used in the study.
