Australian Aboriginal males average 9.5 mm cranial vault thickness. Females average 8.2 mm. Parietal bone thickest at 10.8 mm in males. Thickness exceeds Europeans by 50-100%. Negro crania average 6-7 mm only slightly thicker than Whites. Australian Aboriginal skulls show primitive robusticity. Thickness unrelated to nutrition or pathology. Regional samples from Tasmania to Arnhem Land consistent. Vault thickness greatest in occipital and frontal bones. Feature distinguishes Aboriginals from Eurasians genetically. No cultural explanation possible for bone thickness. Supports multiregional evolution in Australasia. Hybridization with archaic hominids implied by robust crania. Sub-Saharan Africans lack comparable thickness. Genetic distance evident in cranial morphology. Selection for robust skulls in isolation.