Living in trees may have given great apes vocal skills for consonants

Our complex speech may have originated from life in the trees. The first analysis of the evolution of consonants suggests their roots may be linked to an arboreal lifestyle, hinting that our ancestors spent more time in trees than currently thought.

All human language uses a combination of vowels and consonants to transfer information. Most primates communicate almost exclusively using vowel-like calls, but non-human great apes produce consonant-like sounds to varying degrees.

This raises the question of where consonants come from, says Adriano Lameira at the University of Warwick in the UK. To find out, Lameira combed through existing literature to see how common consonants are among the great apes and if this could shed light on their evolutionary origin.

He found that wild orangutans, which spend most of their time in the forest canopy, produce a greater number and variety of consonant sounds than wild gorillas, chimpanzees and bonobos, which live on the ground. All orangutans produce these sounds, but in other great apes they aren’t universal.

“Orangutans have this rich repertoire of kiss sounds, scrapes and clicks and raspberries and smacks,” says Lameira. They typically use these sounds while building nests, or communicating with their young, or as alarm calls.

Lameira thinks that living in the trees may explain why orangutans have evolved this broad vocal repertoire. Great apes are adept at extracting hidden or protected foods, like nuts, a skill that often requires the use of tools. While foraging up in the canopy, however, orangutans must always use at least one arm to maintain stability. They have therefore developed more complex control of their lips, tongue and jaws to use their mouths as a “fifth limb” — orangutans can peel an orange just by using their lips, for example.

As an evolutionary side effect, this advanced motor skill gave orangutans an increased ability to produce consonant-like sounds, argues Lameira. This could mean that our early ancestors developed consonant sounds while hanging around in the trees, too.

“There’s a growing sense that our dependency on trees was much larger and deeper than we think,” says Lameira, which goes against the idea that humans started walking upright as they moved into the savannah. “Within that mosaic that was emerging, we might actually have stayed where the trees were and crossed from one patch to the other as quickly as we could.”

The link between feeding and vocal communication doesn’t apply to smaller tree-dwelling primates such as monkeys, says Lameira,because their size and their tails make them more stable on tree branches and they don’t forage in the same way.

“The arboreal origin of consonants is an interesting hypothesis worth testing,” says Chris Petkov at Newcastle University, UK, though he questions some aspects. He says many monkeys use grunts, which are akin to consonants. And as humans aren’t tree-dwelling, there must be other reasons why consonants persisted, such as growing social networks driving an expansion of call types. These hypotheses could be tested by characterising consonant-like vocalisations more systematically across species, he says.

“Given that we do not know what led to the evolution of consonants, I think testing this hypothesis can potentially provide some insights,” says Serge Wich at Liverpool John Moores University in the UK. “Of course, we have to remain very cautious that even if there would be a relationship that this does not mean causality as there could be other factors involved.”

Sign up to Our Human Story, a free monthly newsletter on the revolution in archaeology and human evolution