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Twitter evolution: Birdsong, speech and language

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A major stumbling block for the comparative analysis of language evolution is that, so far, there is no evidence for human-like language syntax in any non-human species. There is no a priori reason why a version of such a combinatorial computational system could not have evolved in non-human animals, either through common descent (e.g., apes) or convergent evolution (e.g., songbirds). Although the auditory-vocal domain is just one possible external interface for language (with signing being another), it could be argued that the strongest animal candidates for human-like syntax are songbirds and parrots. This is because they exhibit vocal imitation learning, a trait that is shared with certain marine mammals and hummingbirds, but that is absent in our closest relatives, the great apes. There are striking behavioural similarities between auditory-vocal learning in human infants and in songbirds. In both cases, auditory learning takes place during a sensitive period early in development, and there is a transitional period of early vocalisation which is called ‘babbling’ in humans and ‘subsong’ in birds. At the neural level, songbirds have ‘Broca-like’ brain regions involved in the production of song as well as in sensorimotor learning, and ‘Wernicke-like’ regions involved in auditory perception and memory. Furthermore, these regions exhibit patterns of hemispheric lateralisation that are very similar to those in human speech- and language related regions. Finally, there are interesting parallels regarding certain genes that are involved in vocalisation. Although birdsong can be quite complex, contrary to recent suggestions, to date there is no evidence to suggest that birdsong patterns exhibit the hierarchical syntactic structure that characterizes human language. Considering the evidence, an evolutionary scenario emerges where three factors are important. First, there is neural homology, where similar brain regions are involved in auditory learning and vocal production, not only in songbirds and humans, but also in other mammals. Second, there is evolutionary convergence with regard to the mechanisms of auditory-vocal learning, which proceeds in essentially the same way in songbirds and human infants, but not in non-human primates. Third, as yet there is no evidence to suggest that non-human animals possess the combinatorial complexity of human language syntax. It may be that the neural mechanisms that evolved from a common ancestor, combined with the auditory-vocal learning ability that evolved in both humans and songbirds, contributed to the emergence of language uniquely in the human lineage.

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