We all know that parrots can talk. Some people may have even seen elephants, seals, or whales mimicking speech sounds. So why can’t our closest primate relatives speak like us? Our new research suggests they have the right vocal anatomy but not the brainpower to use it.


Scientists have been interested in understanding this phenomenon for centuries. Some have argued that nonhuman primates didn’t have the right-shaped body parts to make the same sounds as we do, and that human speech evolved after our speech organs changed. But comparative studies have shown that the form and function of the larynx and vocal tract is very similar across most primate species, including humans.


This suggests that the primate vocal tract is “speech ready” but that most species don’t have the neural control to make the complex sounds that comprise human speech. When reviewing the evidence in 1871, Charles Darwin wrote, “the brain has no doubt been far more important.”


Along with Jeroen Smaers from Stony Brook University in New York, I have been investigating the relationship between the number of different calls that each primate species can make and the architecture of their brains. For example, Golden pottos have only ever been recorded using two different sounds, while chimpanzees and bonobos use around 40.


The results indicate a positive correlation between the relative size of the cortical association areas and the size of the vocal repertoire of primates. In simple terms, primates with bigger cortical association areas tended to make more sounds. But, interestingly, a primate’s vocal repertoire was not lixed to the overall size of its brain, just the relative size of these specific areas.


We also found that apes have particularly large cortical association areas, as well as a bigger hypoglossal nucleus than other primates. The hypoglossal nucleus is associated with the cranial nerve that controls the muscles of the tongue. This suggests that our closest primate relatives may have finer and more voluntary control over their tongues than other primate species.


By understanding the nature of the relationship between vocal complexity and brain architecture, we hope to identify some of the key elements that underlie the evolution of complex vocal communication in our ancestors, ultimately leading to speech.