Chimpanzees are our closest primate relatives, yet we are fluent conversationalists and they do not speak at all. What is the evolutionary reason behind this huge difference? Chimpanzees, bonobos, gorillas, and orangutans can learn to communicate with humans through pictures or sign language, however, the vocal sounds they produce go hardly beyond coos and grunts. It’s puzzling that they can’t learn to speak any words even when they are raised by humans from birth. Do chimpanzees simply lack the vocal tract anatomy necessary to produce varied sounds or do they have less neural control over their vocal tracts?
A widely discussed hypothesis regarding anatomical differences in the vocal tract is the “descended larynx” hypothesis. According to this hypothesis, humans can make much more varied sounds compared to non-human primates because their larynxes sit in a lower position in their throats. A lower larynx position results in an elongated vocal tract, which supposedly enables humans to produce a larger range of vocal sounds. However, recent research did not support this hypothesis. When researchers examined the dynamic flexibility of the vocal tract anatomy in rhesus macaque monkeys using x-ray videos (what their vocal tract does while they make sounds), it turned out that monkeys are capable of producing at least 5 clearly distinguishable vowels, which is sufficient to produce most human languages worldwide. Besides, animals that have completely different vocal anatomy can still imitate readily intelligible speech. The most famous example is the African gray parrot Alex, who could produce over 100 words. Likewise, the Korean elephant Kosik was able to mimic speech sounds by sticking his trunk in his mouth, similar to how humans whistle with their fingers in their mouth. Of course, imitating speech is not necessarily an act of communication. Parrots may produce whole sentences, however, we don’t know how much they understand the concepts behind words. Chimpanzees have much more sophisticated social and cognitive skills, and they can actually understand concepts to a certain degree when they communicate using pictures or sign language. So the fact that they cannot communicate using speech sounds must be down to physical rather than intellectual limitations.
If the vocal tract anatomy of the chimpanzees is not the limiting factor for producing speech sounds, it stands to reason that it must be the neural control over their vocal tracts. Indeed, recent findings revealed that humans have direct synaptic connections from motor cortical regions to motor neurons that control the larynx, while chimpanzees lack these connections. Most animals use vocal sounds for communication, to signal potential threats in the environment, call for mating partners or scare away other animals. However, these are all innate calls, meaning that they are not learned and come from birth. Just like laughing, crying or screaming in humans, innate calls in animals are usually uncontrolled and reflexive. Only a group of animals, known as vocal learners, have the ability to control their voices to imitate novel vocal sounds. Vocal learners include songbirds, parrots, seals, whales, bats and elephants, but not chimpanzees or any other non-human primates. But how do vocal learners control their voices? Interestingly, it was found that songbirds, a species that is evolutionarily very distant from humans, possess neural circuitry that is very similar to humans for controlling their vocal organs. However, to validate the importance of this neural circuitry in vocal control, future research should test whether similar connections exist in other vocal learner animals as well.
What do we know about the evolution of vocal control in humans? Roughly around 8 million years ago humans diverged from chimpanzees and evolved neural control over their vocal tract. With the advancement of molecular genetics, scientists now can compare the human genome with that of non-human primates and extinct hominins to figure out what evolved exactly. So far, the most considered genetic factor that is involved in the oral motor control of speech is the FOXP2 gene. The role of FOXP2 in speech was first discovered in a British family who showed severe speech and language impairments in multiple generations due to a mutation of the gene. Recent genetic data suggests that a modern variant of FOXP2 gene evolved around 500,000 years ago and was shared with Neanderthals but not with chimpanzees. This is a very exciting finding because it not only sheds light on our evolutionary differences with chimpanzees but also suggests that perhaps extinct hominins also possessed some form of speech ability.
We are now closer to answering what evolved in humans differently than in chimpanzees that enabled us to speak. Future studies will better distinguish the neural circuitry that enables vocal control, identify more relevant genetic factors by closely examining clinical cases with speech and language deficits, and compare these factors in hominids and primates. An interdisciplinary approach that combines comparative neuroscience, genetics and paleontology research will surely be the key to pinpoint the evolution of speech.
This blog piece was written by Müge Özker Sertel.
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Nim Chimpsky (1973-2000) was a chimpanzee who was taught sign language and raised as if he was a human as part of an experiment to see if a chimpanzee could be taught to communicate with humans.