Intraspecific variation in M1 enamel development in modern humans: implications for human evolution

Abstract

The timing and sequence of enamel development, as well as enamel thickness, was documented for individual cusps (protoconid, hypoconid, metaconid, entoconid) in 15 unworn permanent lower first molars (M 1 s) from a sample of modern human juveniles. These data were compared with previously published data for modern and fossil species reported in the literature.Crown formation in all teeth was initiated in the protoconid and completed in the hypoconid. These cusps had significantly longer formation times (2.91 and 2.96 yrs, respectively) than the metaconid and entoconid (2.52 and 2.38 yrs, respectively), as well as thicker enamel, and each represented between 92e95% of the total crown formation time. Rates of enamel secretion in all cusps increased significantly from 2.97 mmin the inner enamel to 4.47 mm in the outer enamel. Two cusps of one individual were studied in more detail and did not follow this typical trajectory. Rather, there was a sharp decrease in the middle of enamel formation and then a slow recovery of secretion rates from the midto outer enamel. This anomalous trajectory of enamel formation is discussed in the context of other nondental tissue responses to illness. Neither secretion rates nor periodicity differed significantly when compared between the cusps of each molar.Differences in cusp formation times, initiation, and completion suggest a relationship between the rates of enamel formation and enamel thickness. This fits with expectations about the mechanics of the chewing cycle and general lower molar morphology. A comparison with similar data for some nonhuman primates and fossil hominoids suggests this relationship may hold true across several primate taxa. Other aspects of enamel growth differed between this human sample and certain fossil species. The lower molars formed slowly over a longer period of time, which may reflect the extended growth period of modern humans. The methodological approach adopted in this study is discussed in the context of that used in other studies.

Two-dimensional patterns of human enamel thickness on deciduous (dm1, dm2) and permanent first (M1) mandibular molars

“…Often, these sections are produced for studies of dental development to examine growth rates and formation times, and are then measured opportunistically for studies of enamel thickness. 29,45 Non-destructive microtomographic imaging techniques produce 2D planes of section 46 and 3D reconstructions of the whole tooth crown. 16 Generally, these different techniques have identified similar broad trends in enamel thickness when several taxa are examined (e.g., gorillas have thin enamel compared to thick enamel of humans).…”

“…This provided a large sample size, from which the dental sample was chosen. Therefore values for enamel thickness from both the deciduous and permanent molars did not include measurements based upon reconstruction due to wear ( internal signs of hypoplastic defects (see Mahoney 45 his Fig. 1) were removed from the study.…”

“…Fifteen of the M1 sections already existed, and dental development from microstructure was previously reported in Mahoney. 45 The enamel thickness measures are reported here. Histological sections of the deciduous molars were produced to study enamel development (enamel secretion rates; variation in cusp initiation) which is scarcely reported in the literature.…”

“…Human cusp tip enamel thickness is positively correlated with appositional formation time. 45,59 Mandibular M1 buccal cusp enamel in Pan is, on average, thicker and takes longer to form compared to the lingual cusp. 60 Similar indications of this association have been reported along the tooth row.…”

Retrieving chronological age from dental remains of early fossil hominins to reconstruct human growth in the past

“…Mean values for modern human (protoconid) formation times (Reid & Dean 2005) are greater than these: M1 (3.1 years), M2 (3.2 years) and M3 (3.27 years) but there is overlap in the ranges (for example, see Dean 2005 andMahoney 2008) such that an individual molar tooth could not always be attributed to Pan or Homo on the basis of molar crown formation time alone.…”