Vocal learning, the substrate for human language, is a rare trait found to date in only three distantly related groups of birds (parrots, hummingbirds, and songbirds) and four distantly related groups of mammals (humans, bats, cetaceans, and elephants). Brain pathways for vocal learning have been studied in the three bird groups and in humans, and they have a number of similarities. In addition, use of learned vocalizations is also similar among vocal learners. Together this suggests common selection pressures. Here I present hypotheses on what could have selected for or against vocal learning and associated brain pathways in birds and mammals. The brain pathways I suggest were selected from a pre-existing motor pathway. Selection for vocal learning behavior I suggest occurred by two factors: mating preference for varied vocalizations and a need for rapid adaptation to propagate sound in different environments. Selection against vocal learning I suggest occurred by predation, where varied vocalizations makes an animal an easier target for predators. Once predator selection pressure is overcome, then, I suggest, learned vocalizations can be used for other functions, such as abstract communication.

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