Hand movement action units in chimpanzees show temporal segmentation similar to but shorter than that in humans; median durations are 0.9 s in non-repetitive behaviour and 2.4 s in repetitive behaviour (vs. 2.0 s and 3.0 s in humans: Schleidt 1988; Feldhütter et al. 1990). In neither chimpanzees nor humans does one repetition of a movement within an action unit increase the duration of the action unit; each further repetition of movements in the action unit tends to prolong it by only 0.5–1 s. Thus, in both chimpanzees and humans, there appears to be some presyntactical planning in advance in repetitive hand movements. The quantitative differences in segment length may reflect uniquely human abilities of timing or sequencing muscle contractions. These findings fit well with current hypotheses that syntactical ability in language could have evolved from motor organisation.