Complex linguistic features
© S. Ferrigno, Harvard University
Evidence is piling up to suggest humans are not as different from animals as many like to think.

New research adds more fuel to the debate, showing that a complex ability thought to be a hallmark of human language is not only innate across different ages and cultures, but can also be picked up by monkeys.

The study, published in the journal Science Advances, tested the ability to embed a smaller phrase within another phrase in three- to five-year-old children, US adults, native Bolivian adults and three rhesus macaques named Horatio, Beyoncé and Coltrane.

Nesting phrases, or recursion, enables us to organise ideas in language, by creating the structure of a pair within another pair, for example.

"You can take a phrase like 'the cat meowed', and nest 'the dog chased' in the centre of the sentence to make 'the cat the dog chased meowed'," explains lead author Stephen Ferrigno from Harvard University, US.

"To understand the meaning of this sentence, the inner noun needs to be matched to the inner verb. The same is true with the outer noun/verb pair."

Being able to nest something within another related element underpins other unique human abilities like intricate tool use, music, social skills and maths.

Until now, it's proven difficult to test this ability in humans and animals linguistically because they could find other ways to correctly order phrases.

To get around this, the researchers developed a non-linguistic sequencing task to learn and apply the nested structure using images of colourful brackets.

Participants were trained to touch images on a computer screen in a centred, embedded order, for example, "{,[,],}". They were then asked to order new, untrained lists to test their ability to generalise this.

All humans, including the young children, spontaneously cottoned onto the structure and applied it to the new lists, suggesting the ability is innate. The monkeys were able to do it too, albeit after an extra learning trial.

The universality of this ability has been "hotly" debated, the authors write, and theorised to not only be exclusive to our species but "even the sole difference that separates humans from non-human animals".

The work progresses our understanding about what makes human thinking unique, says Ferrigno.

"Our findings show that the capacity to represent nested sequences is present in an animal that can never and will never learn language," he adds.

"This suggests that this ability is more evolutionary ancient than language and could have been the precursor to the development of human grammar.