A structure critical to our brain's core language pathway, found only in humans and apes, has now also been identified in monkeys, according to a controversial new study - suggesting the origins of language may have appeared 20 to 25 million years earlier than previously thought.
Compared to other animals, the human brain is uniquely adapted to language. Our ability to produce speech, listen, and communicate with one another is unparalleled, and to understand why, we need to know how we got here.
Unfortunately, brain tissue doesn't survive over evolutionary timescales, so it's hard to know when the first building blocks for language appeared in our distant past. Today, if we want to locate this missing brain 'fossil', scientists must largely rely on our living cousins.
So far, brain imaging studies in chimpanzees have revealed a similar language circuit to humans, but the idea that monkeys may also contain something comparable remains highly disputed.
Now, some researchers claim that's because we've been looking in the wrong spot. While neuroscientists have been focused on the prefrontal cortex and the temporal lobes - where this pathway exists in humans and apes - the origins of our language may actually lie in the auditory cortex of rhesus macaques.
"I admit we were astounded to see a similar pathway hiding in plain sight within the auditory system of nonhuman primates," says comparative neuropsychologist Chris Petkov from Newcastle University in the UK.
"It is like finding a new fossil of a long lost ancestor."
If the researchers are right, and they've truly found this missing link, the first neural building blocks for the evolution of language may have appeared much earlier than we thought.
The last common ancestor to macaques and humans lived roughly 25-30 million years ago, and that's way earlier than the ancestor we share with chimps, which lived only 5 million years ago.
In humans, speech is generally produced and perceived along a core language pathway, known as the arcuate fasciculus (AF), which spans the prefrontal cortex and the temporal lobe.
Over the years, however, we've come to realise this circuit is far more complex than we once thought. It's connected to many other regions of the brain, and some research suggests the auditory cortex plays a key part.
Comparing the brains of humans, apes, and monkeys alongside new imaging data, the new study was able to identify the AF in the auditory complex of both halves of the human brain, and determine it was more developed on the left side than the right.
They then showed a similar (if less pronounced) pathway existed in the same areas in both macaque and chimp brains.
Noting how the connection evolved differently in humans, the authors argue our system for language seems to have moved away from relying so heavily on the auditory pathway, involving more temporal and parietal areas of the brain.
"To be honest, we were really quite surprised that the auditory system has this privileged pathway to vocal production regions in frontal cortex," Petkov told Newsweek.
"That in itself tells us that there is something special about this pathway. The link to projection from the auditory system to frontal cortex regions, which in humans supports language, is fascinating. So we were honestly surprised to find it there and see that both apes and monkeys have their own version of this pathway."
The observations certainly fit with the idea that language adaptations may have arisen from primate auditory pathways, which the authors say involve spatial processing, as well as sound and vocal patterning.
While human language is wholly unique, the results suggest we aren't the only ones with sophisticated levels of auditory cognitions and vocal communication.
Macaques are known to communicate about food, identity, or danger with various vocalisations, and perhaps it's this auditory connection that allows them to do that, at least in part.
"This discovery has tremendous potential for understanding which aspects of human auditory cognition and language can be studied with animal models in ways not possible with humans and apes," says neurologist Timothy Griffiths at Newcastle University.
Such knowledge would also be huge for treating human patients with neurological issues. Language can be spoken, written or signed, so even if a stroke patient can no longer communicate via one of these ways, perhaps they have other intact language pathways that can still be accessed.
It's an exciting premise, but there's still a lot that needs to be verified. Brain imaging studies can't tell us everything we need to know, and further research will need to explore this auditory pathway in greater detail.
Even Petkov himself admitted to Inverse that this missing brain fossil is "highly controversial". But given how much good it could do, he thinks it's "important to resolve."
The authors suggest further imaging studies be done on the brains of other monkeys to see how far back in time they can trace this missing link.
The study was published in Nature Neuroscience.