Forward-facing eyes and a reduced sense of smell are characteristic features of all modern primates. It is hence likely that these features evolved in the lineage leading to the last common ancestor of modern primates. Based, on the one hand, on the observation that optic convergence is a feature characteristic of many predators, and, on the other hand, on the assumption that most recent strepsirhines are insectivorous, Cartmill (1972, 1974b, 1992) concluded that the first primates were visually oriented predators, which preyed primarily on insects. Diet, however, could not have promoted the presence of social networks in ancestral primates as suggested by Müller and Soligo (2002, 2005), if it consisted mainly of invertebrate prey. By contrast, Sussman (1991, 1999; Sussman and Raven, 1978) sought to explain the typical adaptive traits of primates in the context of the Tertiary radiation of flowering plants (angiosperms) and argued that there are only very few species of primates that are known to include more animal matter than plant material in their diet. In addition, primate gut morphology reflects adaptations for an omnivorous diet (Martin et al., 1985). Sussman (1991, 1999; Sussman and Raven, 1978), therefore, proposed that early primates had an omnivorous diet that included a significant part of plant material and that primates coevolved with the angiosperms. Indeed most of the species that are believed to approach the ancestral primate condition most closely, such as cheirogaleids and lorisiforms, have a frugi-omnivorous diet (Atsalis, 1999; Fietz and Ganzhorn, 1999; Harcourt and Nash, 1986b; Hladik et al., 1980; Nash et al., 1989) and only few species are insectivorous (some of the small bushbabies (Galagoidessp.), slender lorises (Loris tardigradus) and angwantibos (Arctocebus sp.)) (Charles-Dominique, 1977; Harcourt and Nash, 1986b; Nash et al., 1989; Nekaris and Rasmussen, 2003). The suggestion that the earliest primates weighed more than 800 g (Soligo, 2001) adds support to the hypothesis that the first primates lived on a frugi-omnivorous diet. According to Kay (1984), a primate weighing more than 350 g cannot be primarily insectivorous. At more than 800 g, the first primates would, therefore, have had to include a substantial amount of fruit in their diet.

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