2007 Volume 35 Issue 4 Pages 116-122
Recent studies have revealed that both spontaneous smiling and neonatal imitation that have been considered unique human behaviors are also seen in macaques and chimpanzees. In humans, these neonatal facial expressions could contribute to the development of mother-infant interactions. A human mother enjoys interacting with her infant and would be encouraged to pay considerably more attention on him/her. In non-human primates, such dyadic mother-infant interactions have hardly been reported. Chimpanzees, however, exhibit mutual gaze and exchange play faces (social smiling); further, such face-to-face mother-infant interactions increase from the first month of birth to the second month. Dyadic mother-infant interactions, which develop from an early age, might play a role in promoting object manipulation in human infants, because a mother shows her infant objects to attract his/her attention or have him/her hold objects to play with. Such active sharing of attention by mothers with infants precedes the emergence of behaviors of combinatorial object manipulation in infants, such as inserting an object into a bowl or stacking a block on another one; this might cultivate infants’ behaviors of presenting their mothers with objects or giving them objects. Homologous action structures comprise behaviors of combinatorial object manipulation, presenting objects, pointing toward objects, and word utterances. I argue that these structures originally generated from mother-infant interactions through the heterochronic modification of ontogeny over the course of hominization.
