Introduction

The known fossil record for undoubted primates of modern aspect (i.e., confined to Euprimates and excluding Plesiadapiformes) dates back to the beginning of the Eocene epoch, about 55 million years ago (MYA), and it is widely accepted among primate paleontologists that primates originated during the preceding Paleocene epoch, some 60-65 MYA. A parallel conclusion has been reached for most orders of placental mammals, and it is generally assumed that the origin and radiation of most if not all placental orders with extant representatives took place after the extinction of dinosaurs at the end of the Cretaceous. In common parlance, the Age of Mammals followed on from the Age of Dinosaurs. A comparable explanation has been given for the adaptive radiation of modern birds. All such interpretations depend on the

Christophe Soligo • Human Origins Programme, Department of Palaeontology, The Natural History Museum, London, UK Oliver Will • Statistics Department, University of Washington, Seattle, WA Simon Tavaré • Program in Molecular and Computational Biology, University of Southern California, Los Angeles, CA Charles R. Marshall • Department of Invertebrate Paleontology, Museum of Comparative Zoology, Harvard University, Oxford Street, Cambridge, MA Robert D. Martin • The Field Museum, Chicago, IL

common procedure of dating the origin of a group by the earliest known fossil representative, perhaps adding a safety margin of a few million years in tacit but conservative recognition of the fact that the earliest known fossil is unlikely to coincide exactly with the time of origin. Such direct dating from the fossil record faces two problems: (1) if the fossil record represents a very poor sample, the first known fossil representative of a given group is likely to be considerably more recent than the actual origin of that group, and (2) various kinds of bias in the fossil record may introduce further error. In this light, it has been suggested that a relatively low sampling level of the fossil record for primates has led to substantial underestimation of their time of origin (Martin 1986, 1990, 1993; Tavare et al., 2002).

Correct timing of the initial emergence of a group such as the primates is of great importance if the mechanisms that led to its evolution are to be understood, as both biotic and abiotic environmental conditions can be taken into account only if the origin of the group and the prevailing environmental conditions can be accurately correlated chronologically.

In this chapter, we review available paleontological and molecular evidence pertinent to the timing of the origin of the primates. We also present new analyses using a recently developed statistical method that estimates times of origin of clades based on their modern diversity, their known fossil record, diversification models, and estimates of relative sampling intensities.

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