The study's authors believe their findings, published in the journal Biology Letters, could be a "critical step" towards unlocking the mystery of how speech evolved in humans.

Humans are thought to open their mouths between two and seven times per second while talking, with each open-close cycle corresponding to a syllable.

This speech-like rhythm has also been observed in other monkey species such as gibbons, orangutans and macaques.

However, the lip smacks of African great apes, the closest species to humans, had never been studied.

The researchers analysed the video recordings of these primates from Edinburgh Zoo and Leipzig Zoo in Germany, alongside footage collected from wild communities in Uganda, including the Kanyawara and the Waibira.

They found that chimpanzees made lip smacks while grooming another, opening and closing their mouths at an average of four times per second.

According to the experts, these findings confirm that human speech has "ancient roots within primate communication".

Study author Dr Adriano Lameira, from the department of psychology at the University of Warwick, said: "Our results prove that spoken language was pulled together within our ancestral lineage using 'ingredients' that were already available and in use by other primates and hominids.

Australian Associated Press