Dental development and life history in living African and Asian apes

Abstract

Life-history inference is an important aim of paleoprimatology, but life histories cannot be discerned directly from the fossil record. Among extant primates, the timing of many life-history attributes is correlated with the age at emergence of the first permanent molar (M1), which can therefore serve as a means to directly compare the life histories of fossil and extant species. To date, M1 emergence ages exist for only a small fraction of extant primate species and consist primarily of data from captive individuals, which may show accelerated dental eruption compared with free-living individuals. Data on M1 emergence ages in wild great apes exist for only a single chimpanzee individual, with data for gorillas and orangutans being anecdotal. This paucity of information limits our ability to make life-history inferences using the M1 emergence ages of extinct ape and hominin species. Here we report reliable ages at M1 emergence for the orangutan, Pongo pygmaeus (4.6 y), and the gorilla, Gorilla gorilla (3.8 y), obtained from the dental histology of wild-shot individuals in museum collections. These ages and the one reported age at M1 emergence in a free-living chimpanzee of approximately 4.0 y are highly concordant with the comparative life histories of these great apes. They are also consistent with the average age at M1 emergence in relation to the timing of life-history events in modern humans, thus confirming the utility of M1 emergence ages for life-history inference and providing a basis for making reliable life-history inferences for extinct apes and hominins.

Fig. 1.

Polarized light micrograph of a section through the mesial cusps (protoconid on left) of the Gorilla gorilla M₁ (specimen ZSM 1913/1163). (A) Entire section and (B) magnified view of the protoconid cusp showing the neonatal line (red arrows) in the enamel. (C) Magnified view of the lateral enamel illustrates a series of successive striae of Retzius (white arrows) that appear as black lines running from the outer enamel surface (left) toward the dentine (lower right).

Fig. 2.

Close-ups of a section through the mesial cusps of the Pongo pygmaeus M¹ (UIC specimen). (A) Striae of Retzius in the lateral enamel (long white arrows) outcropping at the outer enamel surface (right) and daily cross-striations (short white arrows). (B) Close-up of a field of dentine tubules that run obliquely from the upper left to lower right; arrows indicate linear measurements across 10 successive daily von Ebner lines, which can be seen oriented perpendicular to the long axis of the tubules. In this region of dentine, the daily lines are spaced, on average, 2.99 μm apart.