A shrew-sized origin for primates

The origin of primates has had a long history of discussion and debate, with few ever considering the impact of the original body weight on subsequent primate adaptive radiations. Here, I attempt to reconstruct early primate evolution by considering the initial size of primates as well as the critical functional‐adaptive events that had to occur prior to the early Eocene. Microcebus is often viewed as a living model, and thus 40–65 g might represent a practical ancestral weight for the origin of primates. I consider a smaller original body weight, likely 10–15 g in actual size, and I address the biological implications for shrew‐sized primates by comparing the behavioral ecology of mouse lemurs, our smallest living primates, to another tiny‐sized mammalian group, the shrews (Family Soricidae). Several behavioral and ecological characteristics are shared by shrews and mouse lemurs, and several mammalian trends are evident with decreased size. I suggest that a shrew‐sized ancestral primate would have had high metabolic, reproductive, and predation rates, relatively low population densities, and a dispersed and solitary existence with a promiscuous mating system. Although small mammals like shrews provide insights concerning the ancestral size of primates, primate origins have always been tied to arboreality. I assess other potential arboreal models such as Ptilocercus and Caluromys. By combining all of this information, I try to sequence the events in a functional‐adaptive series that had to occur before the early Eocene primate radiations. I suggest that all of these important adaptive events had to occur at a small body size below 50 g. Yrbk Phys Anthropol 47:40–62, 2004. © 2004 Wiley‐Liss, Inc.