Metrical methods for describing the variance in extant hominoids are applied to phenetic groupings of later Miocene hominoids in order to produce palaeospecies whose variance is compatible with that seen in living hominoids. Enamel thickness measurements are presented for samples of living and fossil hominoids. An index of relative enamel thickness scaled for size has been developed and this defined four categories of relative enamel thickness metrically: Thin enamel (mean values of relative enamel thickness between 8.90 and 11.30), intermediate/ thin enamel (mean values between 11.30 and 14.65), intermediate/ thick enamel (mean values between 14.65 and 17.25), and thick enamel (mean values between 17.70 and 26.20). Thin enamel has been found in Pan, Gorilla and Hylobates; intermediate/thick enamel is found in Pongo; and thick enamel is found in Homo. Thick enamel is also found in Sivapithecus (17.73 - 21.69). The distribution of enamel prism packing patterns at different depths in hominoid enamel show that Homo, Hylobates and Sivapithecus have almost entirely Pattern 3 enamel. Pongo has an outer thickness (less than 25%) of Pattern 1 enamel, and Pan and Gorilla have an outer (40%) thickness of Pattern 1 enamel overlying the Pattern 3 enamel. Pattern 3 enamel in hominoids is formed quickly (5-7μm per day) and has well marked Hunter-Schreger bands. Pattern 1 enamel is formed slowly (less than 2μm per day) and has no Hunter-Schreger bands. On the basis of these new data the commonancestral condition of hominoid enamel has been shown to be thin enamel which formed at the fast (Pattern 3) rate. The common ancestor of the great ape and human clade had thick enamel which formed at the fast (Pattern 3) rate, and this was primitively retained in the common ancestor of the African ape and man clade and in the exclusively hominid clade. The common ancestor of the African apes had thin enamel, a large proportion of which (40%) formed at the slow (Pattern 1) rate. Thick Pattern 3 enamel evolved non-adaptively through the relative increase in dental developmental period, and dietary factors were only of subsequent importance in the maintenance of thick enamel.