Scientists have proposed several theories in relation to primate origin and evolution, and the following three theories have gained recognition and acceptance – arboreality, predation, and ecology (Lehman, 2009).
Fred Szalay (1972) proposed the arboreal theory of primate origins and emphasized that primate origins represent an adaptive radiation of new species from early arboreal ancestral mammals. A change in dietary pattern from insectivory to herbivory was the impulse for this evolutionary shift. In order to ease out locomotion and movement in the complex web of flexible tree branches, grasping hands and feet also evolved in these early mammals. Although the theory throws light upon modification in dentition and limb morphology, it could not explain certain aspects of the primate visual system.
Mart Cartmill (1992) developed an alternative idea on primate origins, which is known as the visual predation theory. Cartmill emphasized that primate origins can be traced back to visual adaptations and modifications for hunting prey in arboreal dwellings. He further opined that arboreal hunting lead to evolution of grasping hands and nails which could facilitate primates to capture and hold prey in small and terminal branches. But the detailed ecological examination of insectivorous primates showed significant flaws in Cartmill’s theory. Wright et al. (2003) suggested that nocturnal insectivores are more likely to hunt using smell and sound rather than relying upon visual system and Garber (1980) suggested that tamarins, characterized with claw-like nails, exhibit adaptation in both hunting for insects in terminal branches and clinging to tree trunks to feed on exudates.
These observations led, in part, to Robert Sussman’s (1991) more ecologically based idea on primate origins, called the angiosperm co-evolution theory. He suggested that the prime impulse behind primate origins and adaptations was the roughly coeval evolution of angiosperm plants. The evolution of angiosperm plants formulated a new set of ecological niche for animals. The former lured animals to disperse their seeds and in turn provided tasty and nutritious fruit to them. Primates were one of the principle taxonomic groups that evolved to take advantage of this scenario. But it was emphasized that the field studies of living mammals similar to those that led to the first primates did not provide crucial evidence in support of either Sussman’s or Cartmill’s theories singly but provided support for aspects of both theories.