You see a cat chasing a mouse. You probably don’t realize it, but as soon as you catch sight of this scene unfolding, your brain makes a key distinction between the cat and the mouse: It identifies who’s chasing, and who’s being chased. This capacity to distinguish between the “agent” (the entity performing an action) and the “patient” (the entity upon which that action is being performed) is called “event decomposition,” and it’s long been thought that it was unique to humans.
However, a new study published in PLOS Biology on November 26 suggests that this is not the case: great apes (specifically gorillas, chimpanzees, and orangutans) also seem to track events in the way that we do, distinguishing between agent and patient. This finding is notable because scientists believe event decomposition lies at the heart of something that is unique to humans. It’s no coincidence that the concepts of “agent” and “patient” bear a strong resemblance to the linguistic concepts of subject and object—scientists believe that the cognitive mechanism of event decomposition underlies the syntax and structure of human language.
Vanessa Wilson, the paper’s lead author, explains to Popular Science that her team set out to answer a key question about the relationship between event decomposition and language, one that recalls the classic conundrum about the chicken and the egg: is our capacity for language predicated on the ability for event decomposition, or vice versa? To do this, the team played the apes a series of video clips, tracking the apes’ eye movements as they watched.
They found that just like humans, the apes’ attention moved back and forth between agent and patient, which implies that they share our ability to distinguish between the two. This suggests that the capacity to decompose events evolved first, and that it provides a cognitive foundation for language.
Like many animals, apes clearly communicate with one another, and the ways in which they do can be startlingly human-like: They take turns to vocalize, interrupt one another and have individualized voices. Nevertheless, their communication lacks the complexity that characterizes human language. It seems that being able to communicate more effectively would provide an evolutionary advantage—so if they possess the cognitive framework to evolve language, why have apes not done so?
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Wilson explains that the answer to this question remains unclear. “One proposal,” she says, “is that our social cognition played a role [in human language development], and our need for social cooperation drove this externalization of how we perceive and make sense of the world.”
Humans also have significantly larger brains than our closest primate relatives, and one theory is that our complex social interactions—of which language is a key part—is at least part of the reason why. This is another chicken/egg question: Did we evolve large brains to facilitate the use of language, or were we able to develop language because of our large brains? Again, Wilson says, the answer isn’t entirely clear: “One theory of syntax evolution proposes that an increase in our computational ability led to our ability to form complex expressions, which we externalized through speech. So there’s definitely an argument there for brain size playing a role.”
“However,” she continues, “I doubt that we [could] ever say that one led to the other. If larger brains were beneficial for computation that led to language, then there’s likely to have been a selection pressure that continued to drive brain size and communicative complexity in a kind of feedback loop, where the pressures of language require increasing brain size, and increasing brain size is beneficial to language.”
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The paper also notes another possibility. While other animals might be “capable of human-like event decomposition,” they simply “do not have the motivation or resources to communicate about agent-patient relations.” This raises the question of why early humans did have that motivation: How—and why—did language evolve out of more basic methods of communication, like simply grabbing a fellow gorilla’s arm and pointing in the direction of food? Wilson says that again, one theory is that our social cognition may provide an answer, “mov[ing] us beyond communicating about individual entities (such as predator-specific alarm calls or food calls) to communicating about the interaction of different entities.”
But this also raises a more fundamental question: At what point does communication become language, anyway? Wilson says that this question is one over which “linguists and biologists continue to debate,” and that the line isn’t as clear as one might think: “Continuing research on animal communication is constantly re-defining [our] understanding and moving the goalposts of human uniqueness.”
Having said that, she explains that there are several features of human language that set it apart from other forms of communication. “One of these features is compositionality—our ability to combine words of individual meaning into different orders which in turn produce their own specific meaning. Compositionality does exist in animal communication, but so far has only been found in much more simple forms, i.e. two call combinations or gesture combinations that individually, and together, produce different meanings.”
She continues, “Another unique aspect is recursion—our ability to form nested hierarchical structures, which is thought to be the basis of syntax. One theory is that this emerged as a response to a growing lexicon, i.e. there was an upper constraint on a growing number of signals, so syntax allowed us to combine these signals more easily.”
Ultimately, though, the more we learn about animals and the ways in which they communicate—and the cognitive mechanisms that underlie those forms of communication—the more we come to understand that humans are perhaps not as unique as we might like to think we are. “In short,” says Wilson, “we are finding more and more that the difference between human communication and that of other species is one of degree, rather than kind… I would say that at this stage, our understanding of the possible drivers of communicative complexity is still just at the tip of the iceberg.”