Variation in modern human deciduous molar enamel formation time

Objectives

Histologically derived deciduous molar enamel formation times hold great potential for accessing information about the prenatal and postnatal ontogeny of juvenile fossil hominins. Yet our understanding of these formation times in modern humans is limited which inhibits comparative analyses. Here we utilise histology to investigate geographic and temporal variation in prenatal, postnatal, and total enamel cusp formation times amongst modern humans. We examine some of the processes whereby differences in formation time can arise by considering the underlying rate at which enamel forms and the average amount of enamel that is produced.

Samples and Methods

Longitudinal thin sections of n=356 deciduous first and second deciduous molars were selected from eight populations. Present-day samples were from the United Kingdom, North America, and Aotearoa New Zealand. Archaeological samples represented the Medieval, Roman, and Iron Age periods. Formation times were calculated from prism lengths and daily cross striations.

Results

Total cusp formation times for present-day populations were similar except for Pacific Island peoples whose molars formed over a relatively short period. Enamel cusps of the archaeological periods were complete on average 2.5 to three months ahead of those from present-day populations. Longer formation times were due to slower growth rates and an extended period of enamel formation after birth. Enamel thickness varied only slightly between all populations.

Discussion

Our results reveal few differences in formation times between present-day populations. Enamel formation was complete relatively early in the archaeological samples, which we explore through shifts in the pace of somatic growth.