The origin of primates : a new synthesis of fossil record and mammalian genomic data

The question of the origin of primates has fascinated generations of evolutionary biologists and paleontologists, partly because primates differ from other mammals in fundamental ways, and partly because humans are an integral part of primates. Many of the anatomical features that distinguish primates from other mammals are linked to an arboreal lifestyle. This explains why primates have nails rather than claws on their fingers and toes; why they have large, opposable toes, similar to the human thumb; why their shoulders, elbows, hips and knees are highly mobile, enabling them to adopt a wide range of postures in a complex three-dimensional environment. Primates also differ from other mammals in the clear predominance of sight over olfaction. This predominance results in forward-facing orbits and widely overlapping visual fields of both eyes, enabling stereoscopic vision. Primates have larger brains than most mammals. Primates have relatively long life expectancies, and generally give birth to only one or two offspring, which are precocious in their development. Given the multiple and important differences between primates and other mammals, it is clear that an anatomical and ecological gulf separates primates from all other living beings. As a result, there is little consensus on the position of primates on the mammalian evolutionary tree, on the place of their emergence, or on the transitional forms present in the fossil record.

This conference integrates current data from the fossil record and mammalian comparative genomics, which are leading to major advances in our understanding of primate origins. Findings from different assemblages of plesiadapiforms, a now-extinct group of mammals of which Plesiadapis tricuspidens from the Paris Basin is the type fossil, indicate that these animals are not "archaic primates", contrary to what is often stated in textbooks. In fact, the specialization of their post-cranial skeleton reveals a close relationship with Southeast Asian dermopterans. Some plesiadapiforms appear to share a more recent common ancestor with dermopterans than with other plesiadapiforms, and paromomyid plesiadapiforms were probably able to glide like their modern-day cousins. Although plesiadapiforms must now be considered "archaic dermopterans", their anatomy helps to bridge the gap between dermopterans and modern primates. These two mammalian lineages, both highly arboreal, appear to be sister groups collectively included in the taxon Primatomorpha. The first primatomorphs differed from Scandentians and other mammals in a series of post-cranial adaptations for exploiting the rich food resources found on vertical trunks. Early primatomorphs had also developed an elaborate neural system linking the eyes to the midbrain, indicating early stereoscopic vision. Thus, the first primatomorphs were already fully tree-dwelling, and this adaptive tendency has remained marked in the vast majority of today's primates and dermopterans.

Paleontological and anatomical findings suggesting that dermopterans are the closest relatives of primates have been confirmed by advances in research into a group of rare genomic rearrangements (exonic indels) common to both groups. Since dermopterans are only known from Southeast Asia, it is more parsimonious to consider a primate origin on this continent rather than in Africa.