Dental Ontogeny in Pliocene and Early Pleistocene Hominins

Abstract

Until recently, our understanding of the evolution of human growth and development derived from studies of fossil juveniles that employed extant populations for both age determination and comparison. This circular approach has led to considerable debate about the human-like and ape-like affinities of fossil hominins. Teeth are invaluable for understanding maturation as age at death can be directly assessed from dental microstructure, and dental development has been shown to correlate with life history across primates broadly. We employ non-destructive synchrotron imaging to characterize incremental development, molar emergence, and age at death in more than 20 Australopithecus anamensis, Australopithecus africanus, Paranthropus robustus and South African early Homo juveniles. Long-period line periodicities range from at least 6–12 days (possibly 5–13 days), and do not support the hypothesis that australopiths have lower mean values than extant or fossil Homo. Crown formation times of australopith and early Homo postcanine teeth fall below or at the low end of extant human values; Paranthropus robustus dentitions have the shortest formation times. Pliocene and early Pleistocene hominins show remarkable variation, and previous reports of age at death that employ a narrow range of estimated long-period line periodicities, cuspal enamel thicknesses, or initiation ages are likely to be in error. New chronological ages for SK 62 and StW 151 are several months younger than previous histological estimates, while Sts 24 is more than one year older. Extant human standards overestimate age at death in hominins predating Homo sapiens, and should not be applied to other fossil taxa. We urge caution when inferring life history as aspects of dental development in Pliocene and early Pleistocene fossils are distinct from modern humans and African apes, and recent work has challenged the predictive power of primate-wide associations between hominoid first molar emergence and certain life history variables.

Age estimation in fossil hominins: comparing dental development in earlyHomowith modern humans

“…Cuspal enamel thickness in this upper canine is 1175 mm and Smith et al (2015) estimated average rates of cuspal enamel formation, assuming that these would be in the same range as enamel formation rates observed in molars of Australopithecus africanus, and early Homo (5.53 or 6.06 mm per day). On this basis, cuspal formation time in StW 151 would have taken either 194 or 212 days to complete.…”

“…Neither in histological studies when isolated teeth are used can the development of one tooth be linked to the development of other teeth developing in the same individual. Exceptions to this are when whole dentitions are available for histological study and when a common time marker exists (such as for birth or death or an accentuated marking or label within a tooth) that can be identified in more than one tooth and used to cross-match developing teeth in the same individual (Beynon et al, 1991;Dean et al, 1993;Dean & Smith, 2009;Le Cabec et al, 2015;Reid et al, 1998;Smith et al, 2015).…”

“…In a fuller analysis of this fossil, using synchrotron imaging techniques, Smith et al (2015, Table E, file S1) reported that the upper left permanent canine crown of StW 151 had an initiation age of 108-155 days and so less than the 0.8 years (292 days) estimated by Moggi-Cecchi et al (1998). Cuspal enamel thickness in this upper canine is 1175 mm and Smith et al (2015) estimated average rates of cuspal enamel formation, assuming that these would be in the same range as enamel formation rates observed in molars of Australopithecus africanus, and early Homo (5.53 or 6.06 mm per day).…”

“…Some of these specimens have been attributed to the earliest members of the genus Homo (Dean, 2009;Dean et al, 2001;Dean & Smith, 2009;Moggi-Cecchi et al, 1998;Smith et al, 2014Smith et al, , 2015. These new estimates for age at death are based (a) on external observations of enamel increments on tooth surfaces, (b) upon measurements and counts of regular incremental markings within enamel and dentine made from histological sections of fossil hominin teeth and/or (c) on counts of daily enamel increments made from 3D synchrotron images of fossil hominin tooth crowns.…”

Measures of maturation in early fossil hominins: events at the first transition from australopiths to earlyHomo

“…Superimposed onto this summary data for living great apes and modern human M1 eruption are published data for six fossil hominins [34,35,63]. The general impression is that M1 eruption ages in fossil hominins fall within the range reported for living great apes but some also overlap with the most advanced of the modern humans.…”