Sexual dimorphism in the human mandible: A developmental and evolutionary perspective

A true understanding of the nature of human sexual dimorphism and the many factors that affect its expression is fundamental to the study of human growth, development and evolution. Moreover, phylogenetic pathways and taxonomic assignments remain problematic unless sex differences can be distinguished from evolutionary changes in hominid fossils. Since metric assessment is highly population specific and necessitates a large database, the key to reliable diagnosis of sex is morphology. Therefore, the focus of this thesis is to elucidate the development and expression of mandibular morphology in terms of its association with sex. A multipopulational database consisting of 609 mandibles from African whites and blacks and American whites, blacks and Amerinds was analyzed to trace the development of and quantify the effectiveness of morphologic (and metric) traits traditionally associated with sex as well as present newly discovered indicators in both adult and immature mandibles.; Results indicate that: (1) Traditional morphologic traits are ineffective because they are either primarily size based (condyle configuration at 68%) or are genetic and/or only coincidentally associated with sex (gonial eversion: 49% and chin shape: 71%). Also, female chin shape is commonly shared by both the sexes in modern humans, while the male shape overlaps in prehistoric Amerinds. (2) Although better than traditional morphology, metric differentials (introduced here) in dimorphism between African (91.7%), and American blacks (85%) add to the evidence of population specificity and also reveal Africans are more dimorphic than any other group for whom there are standards from the mandible alone. (3) New standards introduced in this work for mandibles aged 7 mos. to 3.5 yrs. averaged 81% discrimination in blind tests by 3 judges. (4) No single skeletal metric or morphologic indicator approaches the consistently reliable accuracy of ramus flexure. Ranging from 91%-99% (m = 94.2%), this new trait discovered during this research is on a discriminatory par with a complete pelvis. In conclusion, the discovery of ramus flexure demonstrates that if a morphologic trait is not size-based, is primarily and developmentally sex linked, and is maintained by normal function, it can be a reliably accurate indicator of sex. Observations of fossil hominids reveal its presence in all forms from Australopithecines to anatomically modern humans.