Research into Human Infant Vocalizations Offers Clues to First Steps in the Evolution of Language

Vocalizations by human infants during their first months reveal capabilities critical for language that are but absent, as far as is known, in any other primate at any age. Research published April 2 in PNAS shows that by three months of age, human infants’ sounds may reveal deep roots of human language, hinting at a very distant break from our ape relatives.

Animal signals tend to express a single type of emotional state. Threat growls for example, are inherently negative in emotion and cannot occur in positive expressions for such things as greeting or alliance formation. This is one reason animal calls have been termed “fixed signals” – because individual calls have not been seen to change from negative to neutral to positive on different occasions. The new PNAS paper shows that precisely this kind of shift from negative, to neutral, to positive does occur, however, with human infants’ squealing, cooing and growling, the earliest precursors of speech.

Lead author Kimbrough Oller pointed out that “an infant squeal can be expressed with a smile on one occasion, and shortly thereafter, the same squeal can be used to express distress,  and can be accompanied by frowning. The same sort of shift can occur with infant growls or coos, and all these sounds can be expressed with comfortable neutral faces as well. Language requires such freedom of expression all the time, and from this we conclude that human infant vocalization provides a window into the foundations that were required for language to originate.”

The new paper demonstrates a vast difference between infants’ crying and laughter, which have fixed emotional functions (negative and positive, respectively), which are similar to those often seen in nonhuman primates, as opposed to infants’ squealing, growling and cooing, the sounds that display flexibility of expression. The authors point out that language could not exist without the sort of freedom of vocal expression found in the very young human infant, because all aspects of language are dependent upon flexibility of the usage of vocalizations. The authors argue that, as the evolution of language began, it was necessary for it to begin in such a primitive, yet deceptively powerful, way, with simple sounds free of any particular emotion.  Vocal expressive flexibility is the key, and the human infant shows it long before words or sentences occur in the earliest of baby talk. 

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