New research suggests that the ability to mentally connect related elements often cited as a hallmark of human language may be deeply rooted in primate evolution.

In laboratory experiments, monkeys have shown the same ability to incorporate phrases into other phrases, scientists reported in Science Advances on June 26. Many linguists consider this ability, known as recursion, the basis of grammar (SN: 12/04/05) and therefore unique to humans.

However, “this study shows that there is the ability to express recursive sequences in animals that do not learn a language,” says Stephen Ferrinho, a psychologist at Harvard University.

Recursion allows one to elaborate a sentence such as “This pandemic is awful” into “This pandemic, which has put so many people out of work, is awful, not to mention a health risk.

Ferrigno and his colleagues tested recursion in apes and humans. Ten adults in the United States recognized recursive symbol sequences in nonverbal tasks and quickly applied this knowledge to new sequences of elements. Not so much, but to a large extent, 50 American preschoolers and 37 adults Tsimane villagers from Bolivia who did not have a math or reading school did so too.

These results mean that the ability to detect recursion must occur early in life and does not require formal training.

Humans didn’t have three rhesus monkeys for this task. However, after additional training, two of these monkeys showed recursive learning, says Ferrigno’s group. On average, one of the two animals formed a new recursive sequence, more than about three-quarters of preschoolers and about half of Bolivian villagers.
In contrast to previous studies of apes and birds, where it was difficult to prove that sequence learning requires recursive insights, a new study presents a single sentence. I looked for recursive knowledge that would allow me to learn. It looks like “the cat the dog was chasing ran away.” The first two sentences, “cat” and “dog”, must correspond to the last two sentences, “hunting” and “running”, respectively. The two middle sentences are together, as is the last sentence.