Dental ontogeny in pliocene and early pleistocene hominins.

Tanya M Smith,Jacopo Moggi-Cecchi,Alexandra Houssaye,Fredrick Manthi,Anne Bonnin,Adeline Le Cabec,Paul Tafforeau,Joane Pouech,Colin G Menter,Masrour Makaremi,Carol Ward

doi:10.1371/journal.pone.0118118

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.

Highlights

Evolutionary biologists have debated the evolution of human ontogeny for nearly a century [1,2,3,4,5,6,7], which continues to be fueled by discoveries of important new australopith fossils such as the Dikika baby [8] and the Malapa juvenile [9]
Christopher Dean and colleagues’ initial studies of incremental tooth growth in Plio-Pleistocene juveniles suggested that the duration of early hominin dental development was more ape-like than human-like [14, 15], which was interpreted as evidence that these hominins had an abbreviated period of dental growth and a shorter childhood than extant humans [16,17,18]
We find a remarkably wide range of long-period line periodicity values in 22 Pliocene and early Pleistocene hominins, which range from at least 6–12 days, and possibly 5–13 days (Fig. 2, Table A in S1 File)

Summary

Introduction

Evolutionary biologists have debated the evolution of human ontogeny for nearly a century [1,2,3,4,5,6,7], which continues to be fueled by discoveries of important new australopith fossils such as the Dikika baby [8] and the Malapa juvenile [9]. Christopher Dean and colleagues’ initial studies of incremental tooth growth in Plio-Pleistocene juveniles suggested that the duration of early hominin dental development was more ape-like than human-like [14, 15], which was interpreted as evidence that these hominins had an abbreviated period of dental growth and a shorter childhood than extant humans [16,17,18]. First molar emergence ages in a small number of early hominins were estimated to be similar to living great apes [14, 15, 20], lending further support to the idea that early hominin life histories were more rapid than living humans. Other scholars emphasize a more mosaic-like or unique pattern of life history in australopiths [2, 4], while some suggest the possibility of a more rapid life history in early hominins than in the great apes [16, 29]

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