Since the turn of the 20th century, most anthropologists agreed on one fundamental notion: the origin and evolution of the order Primates was closely tied with life in the trees. This view is founded on the obvious observation that the vast majority of extant primates live in the trees and have colonized many different arboreal habitats. Smith (1912) and Jones (1916) were among the first to relate some of the unique anatomical and behavioral characteristics of primates with arboreal life. Their views were promoted by LeGros Clark (1959), but later challenged and refined by Cartmill (1972, 1974a,b) who suggested that the forward-facing eyes and grasping extremities of primates can be interpreted as adaptations to cautious foraging for insect prey on thin, flexible branches. At the same time, Jenkins (1974: 112) suggested that "The adaptive innovation of ancestral primates was therefore not the invasion of the arboreal habitat, but their successful restriction to it." However, there are several extant mammal species other than primates that are restricted to an arboreal environment, particularly in which thin and flexible branches abound. As Cartmill (1972, 1974a,b) and Ramussen (1990) stressed, those nonprimate mammals offer great potential in addressing the problem of primate origins. The views of Jenkins and Cartmill had a profound influence on the adaptive explanations of the postcranial and locomotor features that define primates as a group. Several primate postcranial and locomotor characteristics, rare in other mammals, are now being interpreted as evidence of an invasion and restriction to a fine-branch, arboreal niche by the earliest primates. For example, primates have prehensile hands and feet that bear nails instead of sharp claws (Cartmill, 1970, 1972, 1974a,b, 1985; Jones, 1916, 1929; LeGros Clark, 1959; Lemelin, 1996; Martin, 1968, 1986, 1990; Mivart, 1873; Napier, 1961, 1993; Napier and Napier, 1967; Szalay and Dagosto, 1988; Szalay et al., 1987) and relatively long limbs (Alexander et al., 1979; Polk et al., 2000) with more mobile joints, particularly in the forelimbs (Reynolds, 1985b). In addition to these postcranial features, most primates share three locomotor characteristics that are unusual or unique compared to other mammals (Larson, 1998). During quadrupedal walking, primates are characterized by: (a) an almost exclusive use of diagonal-sequence (DS) walking gaits (i.e., each hind footfall is followed by the contralateral fore footfall) (Cartmill et al., 2002; Hildebrand, 1967, 1985; Rollinson and Martin, 1981; Vilensky and Larson, 1989); (b) a protracted arm position at forelimb touchdown (i.e., arm greater than 90 relative to horizontal body axis) (Larson, 1998; Larson et al., 2000, 2001); (c) relatively lower peak vertical substrate reaction forces (Vpk) on the forelimbs compared to the hindlimbs (Demes et al., 1994; Kimura et al., 1979; Reynolds, 1985b); and (d) forelimb compliance (Larney and Larson 2004; Schmitt, 1998, 1999, 2003a,b; Schmitt and Hanna, 2004). What has been lacking is a clear demonstration that mammals restricted to a fine-branch environment possess similar postcranial and locomotor characteristics that are functionally linked to moving and foraging on thin arboreal supports. In this chapter, we present the results of comparative and experimental studies that test the relationship between the presence of primate-like features and fine-branch arborealism using ecological convergence between didelphid marsupials and prosimian primates. Following a review of various models of primates, we present morphometric and behavioral data for opossums and primates that test specifically the functional link between the presence of more grasping, primate-like cheiridia and movement on thin branches. In the second part, we report experimental results that specifically test for the presence of three gait characteristics typical of most primates in a fine-branch arborealist, the woolly opossum (Caluromys philander). In the last part of this chapter, we discuss how these data accord with current theories of primate origins and assess the relevance of an opossum model in inferring the locomotor profile and ecological niche of the earliest primates. © Springer Science+Business Media, LLC 2007.