Rg. Franciscus et E. Trinkaus, DETERMINANTS OF RETROMOLAR SPACE PRESENCE IN PLEISTOCENE HOMO MANDIBLES, Journal of Human Evolution, 28(6), 1995, pp. 577-595
The high frequency of mandibular retromolar spaces among the Neanderta
ls is often cited as a derived character relative to Early and Middle
Pleistocene members of the genus Homo. Four explanations have been put
forward relating variation in Neandertal dentofacial variables to the
high frequency of retromolar spaces: (1) an anterior migration of the
dental arcade (Coon, 1962; Howells, 1975; Wolpoff, 1980); (2) a poste
rior ''retreat'' of the zygomatic and anterior ramal regions relative
to a fixed molar position (Trinkaus, 1987); (3) a shortening of the de
ntal arcade due to mesiodistal molar diminution (Rak, 1986); and (4) a
shortening of the dental arcade due to a combination of anterior migr
ation of the postcanine dentition and posterior migration of the anter
ior dentition (Spencer & Demes, 1993). Analysis of the temporal and ge
ographical distributions of retromolar space frequencies for available
fossil mandibles (n=77) reinforces the interpretation of a high frequ
ency of retromolar spaces being derived in later Pleistocene Neanderta
l clade specimens relative to Early and Middle Pleistocene Homo. Yet,
the contrast between these late archaic humans and early modern humans
(especially from the Near East) is less pronounced, removing this fea
ture from the list of possible Neandertal autapomorphies. Logistic reg
ression analysis of a sample subset (n=22) for which mandibular length
, dental arcade length and ramus breadth could be measured demonstrate
s that retromolar spaces are best seen as a combined result of reduced
dental arcade lengths (i.e. mesiodistally reduced postcanine dimensio
ns) and reduced ramus breadths, in the context of little or no reducti
on in overall mandibular length. While the proximate spatial mechanism
s accounting for higher frequencies of retromolar spaces and related f
acial features among the Neandertals are straightforward, ultimate cau
sal mechanisms are more elusive. Posterior dental reduction and the as
sociated reduction in zygomatic and ramal dimensions can be explained,
at least in part, as an aspect of masticatory gracilization, probably
a result of technological shifts in food processing. The maintenance
of total facial prognathism, and hence mandibular length, has proven m
ore difficult to understand.