POSTERIOR DENTAL SIZE-REDUCTION IN HOMINIDS - THE ATAPUERCA EVIDENCE

In order to reassess previous hypotheses concerning dental size reduct ion of the posterior teeth during Pleistocene human evolution, current fossil dental evidence is examined. This evidence includes the large sample of hominid teeth found in recent excavations (1984-1993) in the Sima de los Huesos Middle Pleistocene cave site of the Sierra de Atap uerca (Burgos, Spain). The lower fourth premolars and molars of the At apuerca hominids, probably older than 300 Kyr, have dimensions similar to those of modern humans. Further, these hominids share the derived state of other features of the posterior teeth with modern humans, suc h as a similar relative molar size and frequent absence of the hypocon ulid, thus suggesting a possible case of parallelism. We believe that dietary changes allowed size reduction of the posterior teeth during t he Middle Pleistocene, and the present evidence suggests that the sele ctive pressures that operated on the size variability of these teeth w ere less restrictive than what is assumed by previous models of dental reduction. Thus, the causal relationship between tooth size decrease and changes in food-preparation techniques during the Pleistocene shou ld be reconsidered. Moreover, the present evidence indicates that the differential reduction of the molars cannot be explained in terms of r estriction of available growth space. The molar crown area measurement s of a modern human sample were also investigated. The results of this study, as well as previous similar analyses, suggest that a decrease of the rate of cell proliferation, which affected the later-forming cr own regions to a greater extent, may be the biological process respons ible for the general and differential dental size reduction that occur red during human evolution. (C) 1995 Wiley-Liss, Inc.