The body in the world: tools and somato-centric maps in the primate brain

Throughout their evolutionary process, primates acquired biological traits suited for object and tool manipulation. The vertical rotation of the body axis, first in sitting and later in standing and walking, promoted forelimb specialization for precise manual tasks. This verticalization was accompanied by a dissociation of body, hand, and head axes that complexified spatial processing. Tool usage by early hominids coincided with a massive increase in brain size and the emergence of neural structures dedicated to sensorimotor and cognitive processing of complex visually guided actions. Actual tool use triggers specific mechanisms by which the brain undergoes structural and functional changes, leading to the incorporation of the tool in the body representation. This process modifies the representation of the reachable/peripersonal space, expanding it beyond the limits of the body. As tools became more elaborate, social skills became essential for cultural transmission of tool manufacture and use. Observing oneself and others manipulating a tool enabled a convergence of egocentric and allocentric representations and paved the way for further mechanisms allowing the agent to situate itself in a social world.