Each fossil of a H. erectus child is a snapshot of stages of growth and development. Some, like KNM-ER 820, a mandible of a young H. erectus, show a series of simultaneous stages of tooth formation (Dean, 1987a). In other cases, linear hypoplastic bands mark growth interruptions, defining concurrent stages of tooth growth across the dentition at a particular chronological age. KNM-WT 15000 and another H. erectus individual, S7-37 from Sangiran, Java, are examples of where such evidence has been useful in reconstructing dental development in H. erectus. Histological analysis of S7-37 allowed Dean et al. (2001) to cross-match the internal microstructure of the developing M1 with the developing Pm4, even though the specimen was adult. Combining information from all these specimens is gradually allowing us to build a chronology of tooth formation and eruption for the species. A preliminary chart, based on many underlying comparisons, appears in Fig. 10.9. Molar and premolar
Fig. 10.9 Estimates for the age of some events in the dental developmental of three Homo erectus specimens made from microanatomy/ histology. Sangiran S7-37 lived to become an adult, whereas KNM-ER 820 and KNM-WT 15000 died as juveniles. The period of crown formation is indicated by the thick solid horizontal lines and of root formation by the thick dotted horizontal lines. B = birth, I = initiation of mineralization of the crown, C = crown completion, E = estimate of age of gingival eruption for some teeth. LEH = linear hypoplastic lines that cross-match the time of events in different tooth types of the same individual (shown as thin vertical lines). Note that the estimated age of gingival emergence for M1 in Sangiran S7-37 is close to the age estimated for canine crown completion in KNM-WT 15000. Canine crown completion in KNM-WT 15000 may then also have occurred close to the time of M1 eruption in this specimen. M2 crown completion in KNM-WT 15000 was calculated from perikymata counts on the upper canine and M2 to be 4.9 years using a 10 day periodicity. However, M2 initiation and crown formation time in KNM-WT 15000 are estimated here to be the same as in S7-37. KNM-WT 15000 died before completing M2 development at ca. 8.3-8.8 years (see Table 10.4); KNM-ER 820 died at closer to 6 years (assuming some detailed similarities with KNM-WT 15000). Each new fossil examined may extend the chart to new teeth or ages until a chart can be built of the entire dentition and growth period.
crown formation times in early hominins were less than those in modern humans (Beynon and Wood, 1987; Beynon and Dean, 1987, 1988; Ramirez Rozzi, 1993, 1995; Reid and Dean, 2006; Lacruz and Bromage, 2006; Lacruz et al., 2006; Lacruz, 2007; Mahoney, 2008). However, they are unlikely to have been less than 2.5 years in H. erectus and there is little evidence of any substantial overlap in the crown formation periods of the M1 and M2 in H. erectus (Dean et al., 2001). These facts suggest the end of M2 crown completion would have been closer to 5 years of age than to 4 years, which lends support for seriously considering a 10 day periodicity in this analysis. As this chart is refined, it can be used to age other juveniles, and as the basis for more technical studies of the evolution of tooth formation in Hominidae, and to more fully describe the early life of Homo erectus. In the Nariokotome youth, we can bracket the age of the two growth disturbances that occurred just as the upper canine crown completed: For an 8-day periodicity these occurred at an estimated 1,220 and 1,390 days (3.3 and 3.8 years), and for a 10-day periodicity, at 1,340 and 1,540 days (3.7 and 4.2 years). Growth disturbances at this age may well indicate adjustment to a post-weaning diet, or at least, periods of risk in infant health (see Katzenberg et al., 1996; Guatelli-Steinberg et al., 2004). Learning more about the infancy of Homo erectus will eventually add knowledge of the kind that may allow us to reconstruct key demographic variables for these early hominins.
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