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The key cognitive step that allowed humans to become the only animals using language may have been identified, scientists say.

A new study on monkeys found that while they are able to understand basic rules about word patterns, they are not able to follow more complex rules that underpin the crucial next stage of language structure.

For example, the monkeys could master simple word structures, analogous to realising that 鈥渢he鈥� and 鈥渁鈥� are always followed by another word. But they were unable to grasp phrase patterns analogous to 鈥渋f鈥� then鈥︹�� constructions.

This grammatical step, upon which all human languages depend, may be 鈥渢he critical bottleneck of cognition that we had to go through in order to develop and use language鈥�, says Harvard University鈥檚 Marc Hauser, who carried out the study with fellow psychologist Tecumseh Fitch, at the University of St Andrews, Scotland.

鈥淧erhaps the constraint on the evolution of language was a rule problem,鈥� Hauser told 快猫短视频.

Random words

Fitch and Hauser carried out two aural tests on cotton-top tamarin monkeys in which sequences of one-syllable words were called out by human voices.

In the first test, random words were called out in a strictly alternating pattern of male followed by female voices. The monkeys responded to breaks in the male-female rule, by looking at the loudspeaker. This showed that they were able to recognise the simple rule.

In the next test, the grammatical rule dictated that the male voice could call out one, two or three words, as long as the female voice did the same. This type of slightly more complex pattern is called recursive, as it involves a rule within a rule.

This time, the monkeys were unable to recognise any breaks in the pattern. But twelve human volunteers given the same test had no such difficulty, although most were unable to explain what the rule actually was.

鈥淩ecursive ability is uniquely human and affects more than just our language, but most of our behaviour,鈥� says renowned primate language expert David Premack, who wrote an article accompanying the study published in Science. 鈥淔or example, in a classroom we often see child A watch child B watch child C watch the teacher. But in chimps, we see chimp A watch its mother, chimp B watch its mother, chimp C watch its mother鈥︹��

Human flexibility

Premack argues that although recursive ability is not absolutely necessary for language 鈥� non-recursive sentences are possible 鈥� being unable to master recursion may have been a stumbling block that prevented monkeys from developing language.

鈥淢onkeys are also not physically capable of speech, they are unable to properly copy actions and they cannot teach 鈥� all of which are skills required for language,鈥� he told 快猫短视频.

Mastery of the underlying rule of recursion is the key to human flexibility, Premack believes, allowing humans to think in the abstract, use metaphors and comprehend concepts such as time. It probably arose as the brain evolved into a more complex organ, but is not located in a single brain region.

However, it is not known whether modern humans are born with the ability to recognise recursive language patterns. More research into recursive ability in humans and their close relatives chimpanzees needs to be carried out, Hauser says.

Journal reference: Science (vol 303, p 377)