For years now, scientists have debated whether spoken language is a skill unique to Homo sapiens. As far as we can tell, animals can’t talk the way we do, but that hasn’t stopped some scientists from looking to our animal relatives to figure out how we came to speak in the first place. Rhesus macaques are a lab favorite: These chatty, pink-faced, golden-haired macaque, which share 93 percent of their genome with humans, are the focus of a new study in Neuron. Their neural circuitry may be the key to understanding human chatter, even though they can’t utter a word.
In a study released in early July, scientists from the Rockefeller University argue that the neural networks monkeys use to express emotion and recognize faces may be the same brain networks that gave rise to the human capacity for speech. Researchers Stephen Shepherd, Ph.D. and Winrich Freiwald, Ph.D. used MRI scans of rhesus macaque brains, taken as the animals watched video clips of other macaques making communicative facial expressions, to draw these conclusions. When the subject monkeys were shown clips of other macaques smacking their lips towards them in friendship, they would mimic the gestures — but only when the monkeys from the clips appeared to be looking directly at them. After all, eye contact, in humans as in macaques, is key to social communication.
In the brains of the monkeys watching the videos, reactive lip-smacking was accompanied by a change in brain activity. The scans suggested that making direct eye contact lit up neural circuits sensitive to social context and information processing, which in turn suggested that the monkey brains contain the cognitive functions necessary for social communication.
The most exciting part, however, was a very unexpected change in brain activity: When the monkeys saw a friendly lip-smack, a part of their brain resembling Broca’s area — the brain area in humans associated with speech — lit up. In humans, this part of the prefrontal cortex becomes activated right before words are spoken, aids the flow of sensory information from the temporal cortex, and helps the motor cortex choose which words to spit out.
This observation, the team notes, suggests that “monkey facial expressions like lip-smacks might be evolutionary precursors to human speech.”
In the past, other researchers didn’t take the gestures or brains of monkeys seriously, arguing that their actions and physiology were too elementary to represent the roots of human communication. But over the past 40 years, much of research focused on the origin of human speech has centered on the brains of monkeys. In 2016, a study in Science Advances made waves with its assertion that the only thing holding monkeys back from speaking like humans can is their brains.
That study contradicted previous work arguing that the reason monkeys can’t talk is because their anatomy won’t allow it. This new study brings the roots of the human-monkey linguistic divide back to the brain.
To get to the root of this scientific mystery, Freiwald and Shepherd nw plan to measure the electrical activity of the individual neurons that were activated during their first experiment. A more detailed understanding of monkey brains, they argue, is a crucial step towards being ready to interpret the blasts of information within our own heads.