Human adults appear different from other animals in their ability to form abstract mental representations that go beyond perceptual similarity. In short, they can conceptualize the world. This apparent uniqueness leads to an immediate puzzle: WHEN and HOW does this abstract system come into being? To answer this question we need to explore the origins of adult concepts, both developmentally and phylogenetically; When does the developing child acquire the ability to use abstract concepts?; does the transition occur around 2 years, with the onset of symbolic representation and language? Or, is...