Adaptive origins of primates revisited Christophe Soligo a, * , Robert D. Martin b a Human Origins Group, Department of Palaeontology, The Natural History Museum, London SW7 5BD, UK b Academic Affairs, The Field Museum, 1400 S. Lake Shore Drive, Chicago, Illinois 60605-2496, USA Received 12 August 2004; accepted 1 November 2005 Abstract Interpretation of the adaptive profile of ancestral primates is controversial and has been constrained for decades by general acceptance of the premise that the first primates were very small. Here we show that neither the fossil record nor modern species provide evidence that the last common ancestor of living primates was small. Instead, comparative weight distributions of arboreal mammals and a phylogenetic reconstruc- tion of ancestral primate body mass indicate that the reduction of functional claws to nails e a primate characteristic that had up until now eluded satisfactory explanation e resulted from an increase in body mass to around 1000 g or more in the primate stem lineage. The associated shift to a largely vegetarian diet coincided with increased angiosperm diversity and the evolution of larger fruit size during the Late Cretaceous. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Body mass; Cope’s Rule; Nails and claws; Arboreal mammals; Angiosperms; Cretaceous Introduction There is currently a broad consensus regarding a range of morphological features that characterised the last common ancestor of living primates. Inferred adaptive features include grasping hands and feet with nails rather than claws, enhanced stereoscopic vision, and an enlarged brain (Martin, 1990; Cartmill, 1992; Fig. 1). Small body size and habitual foraging in a small branch environment complete the current consensus view of the last common ancestor of modern primates (Martin, 1990; Cartmill, 1992). 1 In the two main competing hypotheses, the traits listed above have been interpreted either as evidence that the ances- tral primate was a visual predator, adapted for stalking and grasping insects and other prey (Cartmill, 1972, 1974a,b), or as indicating that ancestral primates evolved in parallel with the angiosperms, exploiting their fruits, flowers and nectar (Sussman and Raven, 1978; Sussman, 1991). In terms of body mass, ancestral primates are often referred to as ‘‘mouse-sized’’ (Larson et al., 2000); i.e., by inference, no more than 200 g and probably substantially less (Nowak, 1999). Ancestral primates have also been compared to the marsupials Marmosa and Cercartetus (Cartmill, 1974b), which typically weigh around 100 g or less (Nowak, 1999), or to shrews, which weigh even less (Gebo, 2004). At most, the first primates have been estimated to have weighed in the region of 500 g or less (Dagosto, 1988; Martin, 1990; Hamrick, 1999). The inferred diminutive size of the last common ancestor of living primates is critical to the interpretation of early primate adaptations because of the pervasive influence that body size exerts on the biology of a species (Peters, 1983; Martin, 1990). It is important to note, for example, that all current estimates of primate ancestral body mass fall on or below what has become informally known as ‘Kay’s Threshold’: a body mass of around 500 g, representing the upper size limit for primarily insectivorous and the lower size limit for primarily folivorous primates (Kay, 1984; Fleagle, 1999). * Corresponding author. Tel.: þ44 20 7942 5669; fax: þ44 20 7942 5546. E-mail addresses: c.soligo@nhm.ac.uk (C. Soligo), rdmartin@fieldmuseum. org (R.D. Martin). 1 Our definition of Primates is restricted to ‘euprimates’ or ‘primates of modern aspect’, excluding both the tree-shrews, now commonly assigned to the separate order Scandentia, and the Plesiadapiformes, a predominantly Palaeocene (65-55 mya) group of fossil mammals, whose connection with primate evolution remains controversial (Cartmill, 1974a; Martin, 1990). 0047-2484/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jhevol.2005.11.001 Journal of Human Evolution 50 (2006) 414e430