A recent study published in the journal Scientific Reports has revealed surprising findings that challenge prevailing theories about the evolution of human speech. This work, conducted by Axel G. Ekström and collaborators, analyzes historical recordings of two enculturated chimpanzees who managed to produce the word mama. The results of this study suggest that chimpanzees possess the phonetic ability necessary for the production of syllables, a finding that could rewrite our understanding of language evolution.

The ability to learn new vocalizations, known as vocal learning, has been considered a crucial prerequisite for the development of spoken language in humans. However, until now, it was assumed that non-human primates were only capable of using pre-existing vocalizations in new contexts, without being able to modify or imitate them socially. This conclusion had been reached mainly through indirect reports and not by directly analyzing the original recordings of the so-called ape language projects.

In these projects, it was documented that some great apes, such as chimpanzees, learned human words, including cup and mama. However, these reports were dismissed due to a lack of rigorous analysis, perpetuating the belief that great apes lack the neural circuits necessary for voluntary motor control of voice and articulators.

The protagonists of the study are two chimpanzees, Johnny and Renata, who lived in different eras and continents. Johnny was recorded in a home video while living at the Suncoast Primate Sanctuary in Florida, USA, and Renata was featured in a newsreel from Universal Studios in 1962. Both chimpanzees were recorded saying mama, a word that the researchers analyzed phonetically.

Spectrograms of utterances by Johnny (top), Renata (middle), and /mama/, uttered by one of the authors (bottom). Sampling frequency of all files was 44,1 kHz. Frequency range 0–4 kHz. Window length 5 ms. Time scales 0–500 ms (top), 0–1000 ms (middle), and 0–1010 ms (bottom). Johnny’s utterances appear with formant transitions indicating incomplete lip closure. Renata’s utterance exhibits rapid formant transitions, consistent with /mama/, as uttered by one of the authors. Utterances by both chimpanzees show voicing across the utterances, consistent with consonant–vowel–consonant–vowel cycles, implying consistent and overlapping innervation of the larynx and jaw.
Spectrograms of utterances by Johnny (top), Renata (middle), and /mama/, uttered by one of the authors (bottom). Sampling frequency of all files was 44,1 kHz. Frequency range 0–4 kHz. Window length 5 ms. Time scales 0–500 ms (top), 0–1000 ms (middle), and 0–1010 ms (bottom). Johnny’s utterances appear with formant transitions indicating incomplete lip closure. Renata’s utterance exhibits rapid formant transitions, consistent with /mama/, as uttered by one of the authors. Utterances by both chimpanzees show voicing across the utterances, consistent with consonant–vowel–consonant–vowel cycles, implying consistent and overlapping innervation of the larynx and jaw. Credit: Axel Ekström et al.

Johnny was recorded responding to a request from his caretaker, who asked him to say mama. This context suggests that Johnny may have learned the word through imitation and that his vocalizations were maintained through positive reinforcement, such as rewards. Renata, on the other hand, was recorded in a similar situation where her caretaker seemed to use tactile signals as cues for the emission of the word.

The researchers conducted a detailed phonetic analysis of the recordings of Johnny and Renata. These analyses showed that the chimpanzees were capable of producing the syllabic structure necessary for the word mama, achieving consonant-vowel phonetic contrasts through the simultaneous use of voice, jaw, and lips.

To assess how humans perceive these vocalizations, the researchers designed an online experiment using the Qualtrics XM platform. The participants, who did not know they were listening to chimpanzees, had to transcribe the presented vocalizations orthographically. These chimpanzee vocalizations were mixed with recordings of speech from patients with Parkinson’s disease, characterized by imprecise articulations and dysphonic phonation, to mask the inhuman quality of the chimpanzee sounds.

The results of the experiment showed that participants consistently perceived the chimpanzee vocalizations as syllables similar to mama. This supports the phonetic analyses, suggesting that chimpanzees possess a vocal ability that has been underestimated until now. The human perception of these vocalizations as coherent words suggests that chimpanzees have the necessary neural foundations for speech.

The ability of chimpanzees to produce human-like vocalizations suggests that vocal learning and syllable production are not exclusive human abilities. This challenges the hypothesis that such capacities arose exclusively in the human evolutionary line after the divergence of the great apes.

The researchers argue that these vocal abilities could have been present in a common ancestor and that modern chimpanzees still possess them. This revelation could mean that the evolution of human language did not require the emergence of new brain structures, but rather the adaptation and refinement of existing capacities in primates.

The findings also call for a reevaluation of past ape language projects. Over the years, cases of great apes learning and using human words in specific contexts have been documented. However, the lack of detailed analyses and the prevalence of preconceived notions about the vocal limitations of primates led to the dismissal of these achievements.

The study by Ekström and collaborators suggests that chimpanzees and other great apes could be more useful models for studying the evolution of language than previously thought. The results underscore the importance of conducting direct and data-based analyses to fully understand the vocal capabilities of primates.

SOURCES

Ekström, A.G., Gannon, C., Edlund, J. et al., Chimpanzee utterances refute purported missing links for novel vocalizations and syllabic speech. Sci Rep 14, 17135 (2024). doi.org/10.1038/s41598-024-67005-w

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