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The question of why shovelling was particularly frequent only in Mongoloids made the present author begin this study. However, the purpose of this paper itself was to discover at least what the function or adaptive significance of shovelling was. The procedure chosen was statistical analysis of the interrelations between tooth crown characters based on their variability. The first of the three analyses performed here is on the relationships between shovelling and the component characters of the maxillary central incisor crown; the second the relationships between shovelling and other morphological tooth crown characters, including the mesiodistal crown diameters of the permanent teeth; and the third the associations between shovelling and some climatic factors, ways of life, etc. The first and second analyses were done by the prin cipal component analysis method based on two different Japanese samples. The third analysis was carried out by using the coefficients of association of four sorts based on data collected from the literature. Through the first analysis, it was confirmed that shovelling was a character almost equivalent to the depth of the lingual fossa in the incisor crown or to the mesial and distal marginal ridges. Additionally, it was clarified for the first time that the way in which the mesial and distal spines of the lingual tubercle contributed to shovelling was different in males and females. By reviewing the morphology of the shovel-shaped incisors of fossil hominids described by previous authors, the lingual tubercle was considered to be a structure supporting the wider crown of the incisor. Shovelling itself was inferred to be another structure resisting forces on the incisor crown inde pendent of the lingual tubercle. From the second analysis, it became clear that most of the variation in shovelling was independent of the variations in size of all the teeth, and, further, that shovelling could vary independent of the distal cusps of the molars. Based on these facts, it was concluded that shovelling might occur in response to a certain adaptive demand independent of the variations in the overall dentition or in the posterior teeth. In the final analysis, it was found that shovelling had relatively high positive or negative associations with the latitude, average temperature in the coldest month, lifeway that included milking, etc. By taking account of these and the results by previous authors, it was suggested that the development of shovelling was primarily associated with a facial structure resisting powerful biting forces, which was in turn a result from adopting a hunting lifeway with a heavy reliance on meat-eating. |