Fig. 3.

Mirror neurons (MNs) in the tree of life. MNs can be categorized in relation to the effector with which executed actions are performed (hand, mouth or vocal tract) and the modality of the sensorial input in which the same actions are perceived (vision or hearing). Hand MNs (blue lines) are observed in humans, apes, macaques and marmosets. The violet line indicates the later exaptation of circuits devoted to hand MNs for tool use and the existence (after a sensorimotor training with tools; in the laboratory; Ferrari et al., 2005) of tool-responding MNs in macaques, humans and likely in apes. Mouth MNs (red lines) have been reported in humans and macaques, and are likely present also in chimpanzees and marmosets. We further propose that mouth and hand MNs may be present in prosimians (red or blue lines with question marks). Audio–vocal MNs (yellow line) have been found in humans and songbirds.