Scientists who spent years listening to the communication calls of one of our closest ape relatives say their eavesdropping has shed light on the origin of human language. Dr Adriano Reis e Lameira from Durham University recorded and analysed almost 5,000 orangutan "kiss squeaks".
He found that the animals combined these purse-lipped, "consonant-like" calls to convey different messages. This could be a glimpse of how our ancestors formed the earliest words.
The findings are published in the journal Nature Human Behaviour.
"Human language is extraordinarily advanced and complex – we can pretty much transmit any information we want into sound," said Dr Reis e Lameira.
"So we tend to think that maybe words evolved from some rudimentary precursor to transmit more complex messages.
"We were basically using the orangutan vocal behaviour as a time machine – back to a time when our ancestors were using what would become [those precursors] of consonants and vowels."
The team studied kiss squeaks in particular because, like many consonants – the /t/, /p/, /k/ sounds – they depend on the action of the lips, tongue and jaw rather than the voice.
"Kiss squeaks do not involve vocal fold action, so they’re acoustically and articulatory consonant-like," explained Dr Reis e Lameira.
In comparison to research into vowel-like primate calls, the scientists explained, the study of consonants in the evolution of language has been more difficult. But as Prof Serge Wich from Liverpool John Moores University, a lead author in the study, said, they are crucial "building blocks" in the evolution of language.
"Most human languages have a lot more consonants than vowels," said Prof Wich. "And if we have more building blocks, we have more combinations."
The scientists recorded and analysed 4,486 kiss-squeaks collected from 48 animals in four wild populations.
With thousands of hours of listening as the apes communicated, the researchers found that the animals embedded several different bits of information in their squeaks.
The team compared this to how we might use more than one word to convey the same meaning – saying "car" but also "automobile" and "vehicle"
"They seemed to make doubly sure that the message was received, so they would send the same message with different [kiss squeak combination] signals,"
The scientists say their study suggests that, rather than a concerted effort to form complex words, it might have been this "redundancy" – forming different sounds that had the same meaning, in order to reinforce a message – that drove early language evolution.
Dr Reis e Lameira added: "It’s a way of making sure you don’t end up in a game of Chinese whispers."