Enamel thickness has assumed unique importance in the debate about the hominid status of Ramapithecus, despite the fact that there is little agreement about the meaning of 'thick enamel' or the significance of enamel thickness for hominoid taxonomy. My aim here is to evaluate the usefulness of enamel thickness and microstructure as characteristics for determining the relationships of the later Miocene hominoids, based both on a quantitative study of enamel thickness in extant hominoids and four species of later Miocene Sivapithecus (including 'Ramapithecus') and on scanning electron microscope analysis of enamel microstructure. Four categories of enamel thickness are defined metrically here and have been related to enamel microstructure and developmental rates. Thin fast-formed (pattern 3) enamel represents the ancestral condition in hominoids; it increased developmentally to thick pattern 3 enamel in the great ape and human clade and that condition is primitively retained in Homo and in the fossil hominoid Sivapithecus (including 'Ramapithecus'). Enamel thickness has been secondarily reduced in the African apes and also, although at a different rate and extent, in the orang-utan. Thick enamel, previously the most important characteristic in arguments about the earliest hominid, does not therefore identify a hominid.

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Afropithecus turkanensis, a 17-17.5 million year old large-bodied hominoid from Kenya, has previously been reported to be the oldest known thick-enamelled Miocene ape. Most investigations of enamel thickness in Miocene apes have been limited to opportunistic or destructive studies of small samples. Recently, more comprehensive studies of enamel thickness and microstructure in Proconsul, Lufengpithecus, and Dryopithecus, as well as extant apes and fossil humans, have provided information on rates and patterns of dental development, including crown formation time, and have begun to provide a comparative context for interpretation of the evolution of these characters throughout the past 20 million years of hominoid evolution. In this study, enamel thickness and aspects of the enamel microstructure in two A. turkanensis second molars were quantified and provide insight into rates of enamel apposition, numbers of cells actively secreting enamel, and the time required to form regions of the crown. The average value for relative enamel thickness in the two molars is 21.4, which is a lower value than a previous analysis of this species, but which is still relatively thick compared to extant apes. This value is similar to those of several Miocene hominoids, a fossil hominid, and modern humans. Certain aspects of the enamel microstructure are similar to Proconsul nyanzae, Dryopithecus laietanus, Lufengpithecus lufengensis, Graecopithecus freybergi and Pongo pygmaeus, while other features differ from extant and fossil hominoids. Crown formation times for the two teeth are 2.4-2.6 years and 2.9-3.1 years respectively. These times are similar to a number of extant and fossil hominoids, some of which appear to show additional developmental similarities, including thick enamel. Although thick enamel may be formed through several developmental pathways, most Miocene hominoids and fossil hominids with relatively thick enamel are characterized by a relatively long period of cuspal enamel formation and a rapid rate of enamel secretion throughout the whole cusp, but a shorter total crown formation time than thinner-enamelled extant apes.