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Bonobos—our closest living relatives—create complex and meaningful combinations of calls resembling the word combinations of humans.

A study, conducted by researchers at the University of Zurich and Harvard University, challenges long-held assumptions about what makes human communication unique and suggests that key aspects of language are evolutionarily ancient. The research is in the journal Science.

The study has investigated the vocal behavior of wild bonobos in the Kokolopori Community Reserve (Democratic Republic of Congo). Researchers at the University of Zurich and Harvard University used novel methods borrowed from linguistics to demonstrate for the first time that, similarly to human language, bonobo vocal communication relies extensively on compositionality.

Compositionality is the capacity to combine meaningful words into phrases whose meaning is related to the meaning of the words and the way they are combined. In more trivial compositionality, the meaning of the combination is the addition of its parts: for example, "blond dancer" refers to a person who is both blond and a dancer.

However, in more complex, nontrivial compositionality, one part of the combination modifies the other. For example, "bad dancer" does not refer to a bad person who is also a dancer: "bad" in this case does not have an independent meaning but complements "dancer."

A bonobo dictionary

In a first step, the researchers applied a method developed by linguists to quantify the meaning of human words. "This allowed us to create a bonobo dictionary of sorts—a complete list of bonobo calls and their meaning," says Mélissa Berthet, a postdoctoral researcher at the Department of Evolutionary Anthropology of UZH and lead researcher of the study.

"This represents an important step towards understanding the communication of other species, as it is the first time that we have determined the meaning of calls across the whole vocal repertoire of an animal."

Compositionality is not unique to humans

After determining the meaning of single bonobo vocalizations, the researchers then moved on to investigating call combinations, using another approach borrowed from linguistics.

"With our approach, we were able to quantify how the meaning of bonobo single calls and call combinations relate to each other," says Simon Townsend, UZH Professor and senior author of the study. The researchers found numerous call combinations whose meaning was related to the meaning of their single parts, a key hallmark of compositionality.

Furthermore, some of the call combinations bore a striking resemblance to the more complex nontrivial compositional structures in human language.

"This suggests that the capacity to combine call types in complex ways is not as unique to humans as we once thought," says Berthet.

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Older than previously thought

An important implication of this research is the potential light it sheds on the evolutionary roots of language's compositional nature.

"Since humans and bonobos had a common ancestor approximately 7 to 13 million years ago, they share many traits by descent, and it appears that compositionality is likely one of them," says Harvard Professor Martin Surbeck, co-author of the study.

"Our study therefore suggests that our ancestors already extensively used compositionality at least 7 million years ago, if not more," adds Simon Townsend.

The findings also indicate that the ability to construct complex meanings from smaller vocal units existed long before human language emerged, and that bonobo vocal communication shares more similarities with human language than previously thought.