There exist three theories concerning the origin of primates. The first and the oldest being the arboreal theory, the second being the visual predation theory, and the last, a more recent theory, the angiosperm radiation theory. The arboreal theory was the predominant theory for most of this century until components of this theory were questioned, and from these objections new theories arose.
The arboreal theory says basically that because early primates evolved to meet the needs of living an arboreal life (Conroy, 1990). For example because these creatures lived in a three-dimensional world they evolved full stereoscopic vision to be better able to move through the environment. The orbits of these early primates under went orbital convergence to achieve stereoscopic vision (Conroy, 1990). Corresponding with this increase in reliance in vision occurred the reduction of olfaction and the shortening of the snout (Conroy, 1990). Also grasping hands and feet with nails instead of claws was thought to be adaptations to living in an arboreal world (Collins, 1921; cited in Conroy, 1990).
Now issues have been raised that are in opposition to the arboreal theory of the origin of primates. It has been noted that there are orders besides primates that have arboreal species and none of them shows any of the adaptations found in primates (Cartmill, 1972, 1974; cited in Conroy, 1990). Also animals with more squirrel-like hands and feet are better adapted to move through trees with sturdier supports (Cartmill, 1972; cited in Conroy, 1990). Claws are a better adaptation to climbing up and down vertical supports than are nails that are found in primates (Cartmill, 1972; cited in Conroy, 1990). Other arboreal animals also show an importance of using olfactory signals in communication, which goes against the arboreal theory (Cartmill, 1972; cited in Conroy, 1990). Also other acrobatic arboreal mammals do not show the orbital convergence that is found in primates which lead to improved stereoscopic vision; it should be noted the primate with the most orbital convergence, Loris tardigradus, is actually a slow-moving arboreal quadruped (Conroy, 1990).
Another theory that explains the origin of primates is the visual predation theory. This theory puts forth the notion that orbital convergence, grasping hands and feet, and reduced claws were an adaptation for the nocturnal foraging for fruit and insects on terminal branches in the shrub layer of the forest (Cartmill, 1974; cited in Conroy, 1990). Orbital convergence would assist in gauging the prey's distance without having to move the head much like seen in modern day owls (Cartmill, 1974; cited in Conroy, 1990). Also olfactory regression is explained by orbital convergence mediated by the coming together of the medial walls of the orbits (Conroy, 1990).
The last theory concerning the origin of primates is called the angiosperm radiation theory. This theory says that the adaptive radiation primates occurred with the radiation of angiosperms (flowering plants) that offered new opportunities and an unexplored niche (Conroy, 1990). The early primates were omnivores that were able to feed on objects such as fruits, flowers, gums, nectars, and insects that fed upon these plant parts (Conroy, 1990). The stereoscopic vision evolved to discriminate between food items at low levels of light and handling them would have necessitated better hand-eye coordination (Conroy, 1990). Problems with this theory come from that angiosperms first appear in the fossil record millions of years before the first primates and that orbital convergence and the correlated neurological specializations occurring with are not found in the early Paleocene primates (Conroy, 1990).
Based upon the evidence the visual predation theory is probably the best theory at this time. More research needs to be done to definitively provide an answer for the reason behind the origin of primates.
Cartmill, M. 1972. Arboreal Adaptations and the Origin of the Order Primates. In Functional and Evolutionary Biology of Primates. Ed. R.Tuttle. Aldine: Chicago.
Cartmill, M. 1974. Rethinking Primate Origins. Science, Vol. 184, 436-443.
Collins, E.T. 1921. Changes in the Visual Organs Associated with the Adoption of Arboreal Life with the Assumption of the Erect Posture. Trans. Ophthalmol. Soc. U. K., Vol. 41, 10-90.
Conroy, G.C. 1990. Primate Evolution. W.W. Norton and Co.: New York.