Primate origins: evolutionary change in digital ray patterning and segmentation

This study presents evidence that the first primates share with extant lemu rs, tarsiers, and anthropoids hand proportions unlike those of their close relatives, the tree shrews (Scandentia), colugos (Dermoptera), and plesiada piforms. Specifically, early primates as well as modem strepsirhines and ha plorhines have relatively short metacarpals and long proximal phalanges giv ing them a grasping, prehensile hand. Limb development was studied in the p rimate Microcebus murinus and a comparative sample of rodents, artiodactyls , and marsupials to investigate the role of embryonic patterning in the mor phogenesis and evolution of primate hand proportions. Comparative analysis shows that the derived finger proportions of primates are generated during the early phases of digital ray patterning and segmentation, when the inter zone cells marking the presumptive metacarpo- and interphalangeal joints fi rst appear. Interspecific variation in relative digit and metapodial propor tions therefore has high developmental penetrance; that is, adult differenc es are observed at early ontogenetic stages. The paleontological, comparati ve, and developmental data are therefore consistent with the hypothesis tha t the early Cenozoic origin of primates involved an evolutionary change in digital ray pattern formation ultimately yielding a grasping, prehensile ha nd. (C) 2001 Academic Press.