Sixteen Neanderthal mandibles together with those of five European middle Pleistocene Homo specimens, and over one hundred prehistoric Jomon crania from Japan were examined to clarify whether the presence of large retromolar space, one of the unique facial features of Neanderthals, is at least in part a consequence of forward migration of the teeth caused by heavy interproximal attrition. When the Neanderthal and the Jomon samples were divided according to age or magnitude of tooth attrition, the reduction of olar teeth length and the enlargement of retromolar space with the vance of age or tooth attrition were indicated in both samples. The results suggest that the retromolar space is a morphological complex containing heterogeneous factors, one of which is the secondarily acquired one caused by forward migration of the postcanine dentition.