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(a) Infant banging a cube, green dots indicate placement of motion tracking markers. Motion tracking trajectories of five strikes for a (b) 214-day-old infant and a (c) 408-day-year-old infant. Trajectories become straighter and less variable with age. Adapted from Kahrs et al., 2012, Experimental Brain Research. Copyright 2012 by Springer Verlag (d) Long-tailed macaque using a hammerstone as a pounding tool. Photo by A. W. Y. Tan.
