This haunts me. Gorillas and chimps are crazy smart and can be taught sign language, so why can’t they speak? They have human-like mouths and they’re obviously vocal, so why don’t they at least have the vocabulary of a toddler?
In:
I wrote this up about a year ago, and I’ll post it again
Chimpanzee and Bonobo vocal chords/tracts are capable of producing human speech. The reasons that they do not speak are not because they are physically incapable of doing it. When scientists used computers to model the vocal tract of chimpanzees, the computer models demonstrated that the issue with chimpanzees isn’t that the vocal tracts don’t work to produce human speech. [Here](https://www.youtube.com/watch?v=Hq0H1kEFNAk&feature=youtu.be) is an example of simulated macaque vocal chords producing human speech. (Warning: This is pretty spooky since its computer generated)
In fact, it turns out that chimpanzees, like the vast majority of other animals, can’t learn new sounds at all, and that’s why they cannot speak; teaching chimpanzees/bonobos gestural communication works a lot better than trying to teach them to talk. Many chimpanzees/bonobos like Washoe, Nim, and Kanzi have successfully learned a few hundred words in sign language, but they can’t learn spoken language since they never learn to produce new sounds– the only species that can do this to my knowledge are humans, many species of birds, dolphins, elephants, seals and bats. (I’ve been corrected about this multiple times and have edited in the better info. I don’t know if it’s good form to credit the people who told me this or not)
I can’t really speak for songbirds, but **the reasons why humans are able to produce speech are deeply ingrained in the human brain**. What I mean by this is that it’s not just a blanket “we’re smarter than chimpanzees, so we can speak”.
Individuals who suffer from microcephaly often have brains about the same size as chimpanzees, but every one of these individuals, while they often have speech problems, are better at language than even the smartest chimp. The reason that we’re able to speak and that other animals can’t is because our brains are wired differently.
To be able to understand this, you have to be able to understand kind of the basics of human speech production.
Neurologists have figured out that if you damage the posterior of an area of the brain called the superior temporal gyrus on the left side of the brain in humans, they become unable to comprehend speech. This area is called Wernicke’s area, and is thought to be strongly implicated in speech comprehension.
Wernicke’s area has a really strong connection to a region in the frontal lobe of the brain that, when damaged, causes individuals to no longer be able to produce speech. This area, named Broca’s area, is strongly implicated in speech production.
The neuronal tract between Wernicke’s and Broca’s area is called the arcuate fasciculus. Damage to it causes individuals to become unable to repeat words. IE, they can process the word in Wernicke’s area, but they cannot get the information to Broca’s area to be repeated. Wernicke’s area also has projections to areas around it that are thought to be involved in other aspects of language like grammar.
So when asking about why humans can talk and why other primates can’t, you have to look at Wernicke’s and Broca’s area. Macaques actually have fairly well developed Wernicke’s areas, and are thought to be involved in functional reference calling. Functional reference describes how macaques give different warning calls based on what kind of predator it sees. So, for example, a macaque gives a different call when it sees an eagle vs when it sees a leopard. Damaging a macaque’s Wernicke’s area will prevent it from comprehending these functional reference calls.
*However*, damaging a macaque’s Broca’s area will **not** interfere with its ability to make any calls at all. This supports the finding that functional reference calls are actually involuntary. They just don’t have the area of the brain dedicated to producing speech like we do.
Neurons in the brain are clustered into units called “cortical columns”. The individual cortical columns between humans and chimpanzees are about the same, except in two area. In Wernicke’s area, humans have much thicker cortical columns than chimpanzees do, suggesting that, in a simplified explanation, that humans dedicate more “brain power” to speech comprehension than chimpanzees do. The same is true for Broca’s area, and on top of that, a human’s Broca’s area is also much larger than a chimpanzees.
Additionally, brain imaging studies have shown that the human arcuate fasciculus, as well as the connections between Wernicke’s area and the other semantic areas around it, are incredibly more developed than in other species. [Here](https://imgur.com/a/wgA4F) is a schematic for the differences between them. As you can see, the connections are very weak in macaques, slightly stronger in chimps, but much, much stronger in humans.
So the question as to why primates are incapable of speech kind of boils down to the fact they don’t really have the brain connections needed to produce speech or to be able to put together the individual words needed for language to make meaning.
Additionally, Broca’s area is not just involved in “generating words to say” but also involved in the motor aspects of speech. In this way, it is true that chimpanzees do not have the neurons needed to make control their throats and mouth enough to produce speech.
But why exactly do our brains develop differently like this? This is a tough question to answer, and it will require a much greater knowledge neurodevelopment than we do now. However, one interesting finding is the FOXP2 gene. I don’t know too much about it, but the FOXP2 gene is a regulator gene that controls the expression of other genes. Additionally mutations in the FOXP2 gene cause movement disorders in the mouth and face, and disrupts the production of speech. Individuals with a mutation also have smaller Broca’s areas. Very interestingly, our FOXP2 protein is distinctly different from those of almost all other primates, who have very similar FOXP2.’
**Edit: Another copy and paste**
The target audience of this response obviously isn’t literal 5 year olds. One of my pet peeves is that people who write on ELI5 often have no idea what they are talking about, and simplify their answers to the point of uselessness. My goal was to write a response that took a bit of effort to read, but would be as complete and accessible as I could make it. The diction, tone, and length of this post were all written with a casual audience in mind. If you’re confused by anything, I am more than happy to elaborate– I wrote this to hopefully help people learn something about neuroscience, not to seem smart, so if I slipped up and got too technical somewhere, just let me know. I am happy to edit my post.
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