Why primates evolved as they did and how they filled and exploited the range of ecological niches they now fill are questions that have not yet been adequately addressed. Over the last century, various hypotheses have been raised to account for the evolution of primates and their unusual anatomical characteristics. These theories include the arboreal theory, the visual predation hypothesis, and the angiosperm theory. The arboreal theory proposes that primates evolved the traits they did as an adaptation to life in the trees. Specifically, primates evolved thumbs and big toes that are perpendicular to the other digits to help them grasp onto branches. Matt Cartmill, a professor of anthropology at Boston University who spent his career trying to understand why primates evolved the way they did, has complicated this theory. Cartmill recognized that forward-facing eyes are characteristic not only of primates but also of predators such as cats and owls that prey on small animals. Thus, forward-facing eyes, grasping hands and feet, and the presence of nails instead of claws may not have arisen as adaptations to an arboreal environment. Rather, they may be adaptations that helped early primates succeed as predators. According to the visual predation hypothesis, primate features are adaptations for hunting insects and other small prey in the shrubby forest undergrowth and the lowest tiers of the forest canopy. The angiosperm theory states that the basic primate traits developed in coevolution with the rise of flowering plants, also known as angiosperms. Flowering plants provide numerous resources, including nectar, seeds, and fruits, and their appearance and diversification were accompanied by the appearance of ancestral forms of major groups of modern birds and mammals. Some argue that visual predation is not common among modern primates and that forward-facing eyes and grasping extremities may have arisen in response to the need for fine visual and tactile discrimination in order to feed on small food items, such as fruits, berries, and seeds, found among the branches and stems of flowering plants.

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