The evolution of shovel shaping: Regional and temporal variation in human incisor morphology.

Abstract

The present study examines regional variation and evolution of human maxillary incisor morphology. For a century shovel-shaped incisors have been assumed to distribute regionally and have been used as evidence of genetic relationships between populations. Unfortunately, in some cases, the phylogenetic relationships claimed to be supported by evidence of shovel shaping have been contradictory due to a definition that simplified the shape. I propose a new definition for the set of morphological features usually called shoveled. Shovel shaping is the occurrence of a basin on the lingual surface of the incisor caused by the development of any of three features of the tooth: marginal ridges, basal tubercles, and curvature, either alone or in combination. In order to resolve the significance of shovel shaping in testing hypotheses of phylogenetic relationship, it is necessary to review the variation in this morphology, both fossil and modern. Specific questions include: (1) How does shoveling vary in modern populations and does this variation distribute regionally? (2) What variation exists in shoveling in the human fossil record? and (3) Can shovel shapes be used to test theories of modern human origins, serving as evidence of continuity or discontinuity between populations? The present study quantifies each of the contributing features of shovel shaping separately, and examines the worldwide distribution of each character. Each of eight broadly defined world regions shows a significantly different distribution of the features of shoveling. Regional differences indicate a clinal distribution of each morphology that contributes to shovel shaping. Regions can be identified by a particular distribution of features of incisor morphology and by degree of development of shovel shapes. With regionality of shoveling established, incisors from the human fossil record are evaluated in relation to models of modern human origins. Regional variants can be identified in the human fossil record, but regions are not as clearly differentiated as they are in recent populations. Investigation of change within each region reveals no evidence of interruption of pattern at the time of appearance of modern humans. Evidence of continuity of pattern from Paleolithic to modern people may not be refuted in any region and is clearest in one area of the world, South and Central Europe. However, the data on fossil incisor shoveling is scant, and the current utility of shovel shapes in testing either theory of human origins has been overstated.