Imitation and Memory: An evolutionary scenario for the uniqueness of human cognition
Tetsuro Matsuzawa
Primate Research Institute, Kyoto University
This study aims to illuminate an evolutionarily plausible scenario for the emergence of unique aspects of human cognition, especially on the imitation (Myowa-Yamakoshi et al, 2004) and memory (Inoue and Matsuzawa, 2007). My scenario highlights the importance of stable supine posture rather than bipedal locomotion as a unique human feature (Matsuzawa, 2007). Let us look at the evolutionary stages of mother-infant relationships. Among mammals, primates developed a unique mother-infant relationship, clinging and embracing. Primates have potentially four limbs to grasp objects. In contrast, the physical separation of mother-infant soon after birth is unique to humans. The mother chimpanzee has to raise offspring on her own and in succession, one at a time. Humans differ from this system of rearing common to hominoids. Human mothers evolved to have shorter inter-birth intervals and therefore gave births successively to offspring who still needed to be taken care of post-weaning thus requiring the help of the spouse, grandmother, helpers, etc. Because of the necessity for care of multiple youngsters, human mothers had to put their infant aside on the ground. Human infants can lie on the back, adopting a stable supine posture but chimpanzee infants cannot – when placed in this position they slowly move their contra-lateral limbs and try to cling. Human mother-infant separation and the stable supine posture resulted in the facilitation of vocal exchange, and provided freedom of limb movement for object manipulation and gesture whereas chimpanzee infants were almost exclusively clung closely to their mothers. Through these changes, humans developed a unique way of communicating, incorporating multiple sources of signals such as facial expressions, manual gestures, and vocalization. Thus, the stable supine posture gave the sound basis for the imitation, tool use (Inoue-Nakamura & Matsuzawa, 1997), and the human-like memory process sensitive to the temporal and syntactic structure of actions.
Inoue-Nakamura, N. & Matsuzawa, T. (1997) Development of stone tool-use by wild chimpanzees (/Pan troglodytes/). /Journal of Comparative Psychology/, 111(2), 159-173.
Inoue, S. and Matsuzawa, T. (2007) Working memory of numerals in chimpanzees./Current Biology/ 17: R1004-R1005.
Matsuzawa, T (2007) Comparative cognitive development. /Develpmental Science/, 10, 97-103.
Myowa-Yamakoshi, M, Tomonaga, M., Tanaka, M., Matsuzawa, T. (2004) Imitation in neonatal chimpanzees (/Pan troglodytes/). /Developmental Science/, 7, 437-442.