Arboreal Hypothesis

Arboreal Theory: Elliot Smith, Wood Jones, Clark

A major paradigm, the arboreal theory of primate origins, defines primates by a complex of characters that adapted them to arboreal life. Indeed, we can find virtually all of the trends listed in the preceding section in the writings of the first exponents of the arboreal theory. It was first formulated by Grafton Elliot Smith and his assistant, Frederic Wood Jones, in the early decades of the 20th century by the study of comparative anatomy.

Elliot Smith, a neuroanatomist, was interested in explaining the distinguishing features of primate brains. In his address at Dundee to the Anthropological Section of the British Association for the Advancement of Science, he stated that it was the evolution of the brain and the ability to learn that led to the origin of the mammals, and that we should pay particular attention to the development of the cerebral cortex in primate evolution. Elliot Smith observed that orders of mammals became successful specialists for modes of life that depended on flight, fast running, or aquatic existence. They lost, however, their primitive simplicity and plasticity of structure. In contrast, the primates did not become narrowly specialized. Elliot Smith considered treeshrews to be insectivores, but he began a tradition in physical anthropology by using them in reconstructing the adaptations that gave rise to the primates. He believed that olfaction was the dominant sense in early mammals, which were essentially terrestrial. Natural selection would favor reduction of the olfactory apparatus and increased development of the neocortex (neopallium) of the cerebrum in arboreal mammals. The arboreal existence favors the development of vision, touch, hearing, agility, and quickness of movement while limiting the utility of smell. Small arboreal early primates could maintain the plasticity of a generalized structure while their brains developed. Elliot Smith pointed to tarsiers as manifesting significant reduction in the size of the olfactory parts of the brain and an increase in the visual cortex of the neo-cortex. He argued that the entire neocortex was affected by the emphasis on vision rather than smell. The sense of touch also became enhanced, and this [Page 230]assisted vision in the conscious appreciation of the environment and in the performance of agile locomotor behaviors. Hearing increased in importance, and the corresponding portion of the cortex expanded. These changes, while increasing the size of the brain and increasing agility, would not modify the primitive characters of the limbs and body. Elliot Smith stated that the interaction of tactile and kinesthetic senses with vision developed the cortex and stimulated the process of specialization of a mechanism for regulating the action of the cerebral cortex (an organ of attention to efficiently manage the nervous centers controlling muscles of the body). He ultimately derived the hominin prefrontal area from reliance on vision rather than smell.

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