Building on Skinner's theory of verbal behavior, research over the last few decades confirmed verbal speaker operants, added the role of the listener, added the identification of speaker and listener interaction between and within individuals, and identified verbal behavior developmental cusps. Meanwhile, comparative biology focused on how and why language evolved in Findings about differences in behavior that neurotypical children demonstrated in their verbal development, and even more so in research that identified and established missing verbal behavior cusps, suggested changes analogous to metamorphosis. These striking changes in stimulus control found in the onset of cusps from the preverbal to the fully verbal child led us to an expansion of the concept of metamorphosis from morphology to the domain of behavior. The major findings of this comparative perspective are presented here as they have led us from experimental analyses of verbal development to metamorphosis as complex verbal behavior transformation and finally to a novel hypothesis about the evolution of language based on the concepts and research described here. To our knowledge, this is the first formulation of verbal development as behavioral metamorphosis in the context of evolutionary developmental biology.
