This theory is proposed by Wood Jones (1916).
The arboreal theory claims that primate

Primate

A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...

s evolved from their ancestors by adapting to arboreal life.

Primates are thought to have developed several of their traits

Trait (biology)

A trait is a distinct variant of a phenotypic character of an organism that may be inherited, environmentally determined or be a combination of the two...

 and habits

Habituation

Habituation can be defined as a process or as a procedure. As a process it is defined as a decrease in an elicited behavior resulting from the repeated presentation of an eliciting stimulus...

 initially while living in trees. One key component to this argument is that primates relied on sight over smell

Olfaction

Olfaction is the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates...

. They were able to develop a keen sense of depth perception

Depth perception

Depth perception is the visual ability to perceive the world in three dimensions and the distance of an object. Depth sensation is the ability to move accurately, or to respond consistently, based on the distances of objects in an environment....

, perhaps because of the constant leaping that was necessary to move about the trees. Primates also developed hand

Hand

A hand is a prehensile, multi-fingered extremity located at the end of an arm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs...

s and feet that were capable of grasping. This was also a result of arboreal life, which required a great deal of crawling along branches, and reaching out for fruit and other food. These early primates were likely to have eaten

Diet (nutrition)

In nutrition, diet is the sum of food consumed by a person or other organism. Dietary habits are the habitual decisions an individual or culture makes when choosing what foods to eat. With the word diet, it is often implied the use of specific intake of nutrition for health or weight-management...

 foods found in trees, such as flowers, fruits, berries, gums, leaves, and insects. They are thought to have shifted their diets towards insects in the early Cenozoic era

Cenozoic

The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...

, when insects became more numerous.