Consuetudinal tool use, evidenced in chimpanzees (Pan troglodytes), Sumatran orangutans (Pongo abelii), and humans, is probably a synapomorphy of the family Hominidae (see van Sheik et al. (1999), "The Conditions for Tool Use in Primates: Implications for the Evolution of Material Culture," Journal of Human Evolution, 36: 719–741; the article's hypothesis is that social tolerance is a condition for the development of material culture (i.e., consuetudinal tool use)). How strong is the evidence, then, that "an unprecedented level of behavioral developmental plasticity" is a hominini (Pan/Homo) synapomorphy (van Sheik (2004), "Book Review of Behavioural Diversity in Chimpanzees and Bonobos, Journal of Human Evolution, 46: 517–518)?
The possibility that consuetude represents a phyletic elaboration of plasticity doesn't preclude the tracing of our understanding of plasticity back to a primitive consuetutindal tribalism ("phylism"). We face a dilemma. The natural history of plasticity, like the natural history of reflexive consciousness, appears to leap out from the physis that precedes it. We can no more reasonably deny that an understanding of plasticity arises from human experience than we can deny phyletic origins of plasticity. A revolution has occurred at some point, to choose a metaphor, or, to choose another, the birth ("nature") of something new on earth, something rather paradoxical. However, vestiges of a parthenogenic fantasy may constrict our thinking here. Reciprocally, fantasial thoughts about plasticity appear to be constrained by the tribe, operating through a sphere of shared affects which steer the thinker back from frontiers of knowledge. We need to know how flexible plasticity is—and generally how plasticity works, what it does—before we can profitably determine that x or y degree of plasticity (supposing developmental plasticity admits of degrees, which there is some basis for if one looks at, for example, weaning times, or the period it takes to learn rudimentary locomotive skills, not to mention consuetudinal tool use) represents an unprecedented level of plasticity among Hominidae, Primates or, indeed, Animalia.