Our language-based complex communication system is one of the defining features that makes us unique among all species. Syntax -combining words together into phrases- makes language-based communication limitless in terms of the amount and type of information we can communicate about. This is due to the compositional nature our syntax mostly holds, where the meaning of the phrases we pronounce is directly related to the meaning of the words within the phrase (e.g. duck and cover). However, recent evidence demonstrated the presence of compositional syntax in other species such as monkeys and birds. These accumulated observations challenge the notion that compositionality is unique to humans and question the evolutionary origins of such capacity. 

 

Therefore, we decided to retrace the evolutionary history of compositionality to understand where language came from, and when it emerged exactly: is it truly unique to humans or is it evolutionary more ancient? To begin with, we investigated the combinatorial capacities of chimpanzees. Chimpanzees are our closest living relatives. We share 99% of our DNAs and our last common ancestor was extinct less than 7 million years ago. 

 

To investigate the compositional properties of chimpanzee communication, we observed chimpanzees in their natural habitat for more than two years, in the Budongo forest, Uganda. Previous work established that chimpanzees produce a call named “alarm-huu” when individuals are surprised or afraid (e.g. during an earthquake, or when seeing a dead monkey, or a snake…) and another call named “waa-bark” when recruiting other group members (e.g. during a hunt). Following our observations, we noticed chimpanzees combined the two calls in a larger sequence specifically when encountering a snake, and that the individuals in the vicinity that heard this “alarm-huu+waa-bark” combination immediately joined the chimpanzee calling. The call combination appeared to function as a recruitment call in the presence of a snake specifically, making it a very promising case for compositionality in chimpanzees. 

 

To verify the meaning of this structure, we presented chimpanzees in the Budongo forest with a dead python that we animated using a fishing line. In this specific context, chimpanzees produced this combination, and we further demonstrated that the production of the combination systematically led to the approach of individuals in the vicinity. We concluded the “alarm-huu+waa-bark” combination did indeed function as a recruitment signal in a dangerous situation specifically. 

 

To confirm the meaning of this structure is derived from the meaning of the calls composing it, we conducted playback experiments. We broadcasted the different calls to chimpanzees in Uganda and compared their reaction to the “alarm-huu” alone, the “waa-bark” alone, or the combination of the two vocalisations. We observed the strongest response to the playback of the combination. We also observed specific behaviours after playing the combination, such as displaying toward or approaching the loudspeaker, or climbing up on nearby trees. These are typical behaviours exhibited by chimpanzees in the presence of snake, suggesting that hearing a call combination (but not the “alarm-huu” nor the “waa-bark” produced alone) was sufficient for chimpanzees to understand a snake was around, and that they should approach to locate the source of the threat and/or potentially scare it away. 

 

These results indicate chimpanzees are able to combine two calls together into a larger sequence, and that the meaning of this sequence is related to the meaning of the calls composing it. This suggests rudimentary forms of compositional syntax are present in our closest living relatives, and that this capacity we considered unique to our species might be evolutionary more ancient and inherited form our last common ancestor with chimpanzees, almost 7 million years ago.