Scientists may have analysed almost 5,000 orangutans "kiss squeaks" - purse-lipped consonant-like calls that appear to convey different messages to understand how human ancestors formed their earliest words.
Researchers from Durham University in UK spent years listening to the communication calls of one of our closest ape relatives.
The team studied kiss squeaks in particular because, like many consonants, 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 are acoustically and articulatory consonant-like," said Reis e Lameira, from Durham University.
The scientists recorded and analysed 4,486 kiss-squeaks recorded from 48 animals in four wild populations.
They 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," said Lameira.
The 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.
The findings are published in the journal Nature Human Behaviour.