Monkeys understand grammar too

Chimpanzees

Chimpanzees also understand simple grammatical rules. (Image: University of Warwick)

A grammatically sophisticated language is considered unique to humans. The basis for this could have been possessed by the last common ancestor of humans, monkeys and great apes around 40 million years ago. This is suggested by a new study that shows that all three species are able to understand grammatical relationships between the building blocks of language.

We use language to exchange information and thoughts, we can create a common culture and develop technologies. The basis for this is the ability to understand the relationship between words in a set of sentences. When this ability developed in our evolutionary history is still unclear. Previous studies have indicated that New World monkeys have some understanding of grammatical structures. However, it was controversial whether this ability goes back to the common ancestor of apes and humans, or whether it later developed independently in apes and humans.

Artificial grammar

“Research into language development is of key importance in understanding what it means to be human,” says Stuart Watson of the University of Zurich. Together with his colleagues, he tested humans, New World monkeys and great apes for their grammatical understanding using a uniform experimental design. In particular, it was about whether the human and animal subjects understand the relationship between words that belong together and are separated by other parts of a sentence.

An example of such a grammatical construction is the sentence “The dog that bit the cat ran away.” Here it is clear that the run refers to the dog, although there are several words in between. In order to test the understanding of such “non-adjacent dependencies”, the researchers developed an artificial grammar in which the sentences consist of sounds instead of words. In this way they could use the same tasks for all three species, although they do not have a common communication system.

Confused by grammatical errors

In several training units, they taught Brazilian marmosets, chimpanzees and humans that certain tones are always followed by certain other tones, even if the sequences that belong together were separated by other tone sequences. This simulates that in human language, for example, we expect a verb like “ran away” after a noun like “the dog”, even if another part of the sentence (“who bit the cat”) is inserted in between.

For the experiment, the researchers played their test subjects, in addition to the previously learned tone sequences, new sequences that either followed the same rules or violated them. The human test subjects then asked them which sequences they had found correct and which they had found incorrect. Instead, the researchers observed the behavior of the marmosets and chimpanzees. In fact, it turned out that if a sequence of tones violated the previously learned rules, the monkeys looked at the loudspeaker for much longer than with known and unknown sequences that followed the rules. Apparently they were irritated because they noticed mistakes in grammar.

Chimpanzees as a link

“The results show that all three types are able to handle non-adjacent dependencies. So the ability is probably widespread in primates, ”says Watson’s colleague Simon Townsend. “This indicates that this critical feature of language already existed in our last common ancestor.” The knowledge about chimpanzees provides an important link. Since the developmental branches of humans and chimpanzees separated much later than those of humans and New World monkeys, it is likely that the ability did not arise through convergent evolution, but actually goes back to the last common ancestor.

The last common ancestor of the three species lived around 40 million years ago. Basic grammatical skills therefore developed millions of years before the actual language. Grammatical structures can also be observed in the communication of today’s monkeys. However, these are usually very simple and only consist of two parts. Further studies must clarify whether the monkeys not only understand a more complex sentence structure, but can also actively use it.

Source: Stuart Watson (University of Zurich) et al., Science Advances, doi: 10.1126 / sciadv.abb0725

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