Although many different animal species can communicate by making sounds and gestures, humans are the only ones that can speak. The origin of this ability has always fascinated people, and previous research has identified that, in order to speak, humans needed to develop large brains, a tongue with a particular shape and position in the larynx, closed rows of teeth and a smooth palate. And then they needed fine motor control over their speech organs.
Fully functional speech anatomy first appears in fossil humans only from around 50,000 years ago, and is absent from all great apes and earlier hominins, including Neanderthals. There are many different theories about how human speech originated, and non-human primates have been studied for decades in search of clues to cast light on this question. However, the calls of non-human primates are composed primarily or exclusively of voiced vowel-like sounds, while humans mix up vowels and consonants.
New research from scientists at the University of Warwick has now revealed that orangutans, the most arboreal of the great apes, produce consonant-like calls more often and of greater variety than their African, ground-dwelling cousins (gorillas, bonobos and chimpanzees). This runs contrary to expectations since the African apes are most closely related to humans and could be expected to have calls that reflect this.
In this study, Professor Adriano Lameira, Associate Professor of Psychology at the University of Warwick, investigated the origins of human spoken language, which is universally composed of vowels that take the form of voiced sounds, along with voiceless sounds in the form of consonants.
“Existing theories of speech evolution have thus far, focused exclusively on the connection between primate laryngeal anatomy and human use of vowels,” explained Professor Lameira. “This doesn’t explain though, how voiceless, consonant-like sounds became a fundamental component of every language spoken around the globe. This raises questions about where all the consonants, that compose all the world’s languages, originally come from.”
In order to understand the origin of consonant sounds in the human lineage, Professor Lameira compared patterns of consonant-like vocal production in the vocal repertoire of three major great ape lineages that survive today from a once-diverse family – orangutans, gorillas, bonobos and chimpanzees. These great apes have call repertoires that consist of both consonant-like and vowel-like sounds, but there is great variation in the way the apes use these sounds in nature.
“Wild gorillas, chimpanzees and bonobos don’t use a huge variety of consonant-like calls,” explained Lameira. “Gorillas, for example, have been found to use a single, particular consonant-like call, but this is only prevalent in some gorilla populations and not others. Some chimpanzee populations produce one or two consonant-like calls associated with a single behavior, for example while they are grooming, but these same grooming calls are uncommon in other chimpanzee populations.”
“Wild orangutans, however, use consonant-like calls universally and consistently across different populations and for multiple behaviors, much like humans do with speech. Their vocal repertoire is a rich display of smacking, clicks, kiss-sounds, splutters and raspberries.”
Professor Lameira has observed orangutans in their natural habitat throughout the last 18 years and thinks their arboreal lifestyle and feeding habits could help to explain the complexity and sophistication of their consonant-like calls.
“All apes are accomplished extractive foragers. They have developed complex mechanisms to access protected or hidden foods like nuts or plant piths, which often requires either meticulous use of hands or tools. Apes, such as gorillas and chimpanzees, need the stability of the ground in order to successfully handle these foods and use tools, however orangutans are largely tree-dwelling, and access their food up in the canopy, where at least one of their limbs is constantly used to provide stability among the trees,” said Professor Lameira.
“It is because of this limitation, that orangutans have developed greater control over their lips, tongue and jaw and can use their mouths as a fifth hand to hold food and maneuver tools. Orangutans are known for peeling an orange with just their lips so their fine oral neuro-motoric control is far superior to that of African apes, and it has evolved to be an integral part of their biology.”
The findings of this study, published in the journal Trends in Cognitive Sciences, suggest that despite being more closely related to humans, African apes may not have evolved the prerequisites for using consonant-like sounds because they spent more time on the ground.
Arboreal, rather than terrestrial, lifestyles appear to have driven great apes to develop different vocal repertoires with large and varied inventories of consonant-like calls. If the habit of living in trees did indeed provided the stimulus to develop and use consonant-like sounds, then this also suggests that our own evolutionary ancestors might have lived a more tree-dwelling lifestyle than previously thought.
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