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 "the" and "a" are always followed by another word. But they were unable to grasp phrase patterns analogous to "if... then..." constructions.

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

"Perhaps the constraint on the evolution of language was a rule problem," Hauser told New Scientist . Yahoo