No other primate constructs sentences. Chimpanzees employ dozens of distinct sounds, but each remains static – a danger cry is always a cry, never initiating a longer thought.
The brain region responsible for these signals resides in virtually the same location across all primates, humans included.
A recent study compared the pathway linking this area to other parts of the body in three animal species, uncovering two specific alterations in humans that appear to enable language. The findings were published in Nature Communications.
The deliberate production of sound activates two frontal brain areas: one positioned high and centrally, the other lower and laterally. Both of these brain regions are present in apes, chimpanzees, and humans. These two areas, the arcuate fasciculus, are interconnected and are currently considered central to speech production.
A research team led by Marco Catani from Gabriele d’Annunzio University of Chieti-Pescara (UdA) in Italy questioned whether this pathway looks different.
Speech unfolds in a particular order. The correct sounds must occur in the correct sequence, rapidly, and this order is re-established for each new thought. If two brains are so similar, the difference must lie in their structure.
To compare the structure of neural networks across species, the team utilized tractography, a method that maps brain networks as three-dimensional lines.
In scans, the network appears like a visible rope of threads, and software tracks and measures each one. Three species. One comparison.
When comparing, the three versions told different stories. In humans, the tract was clearly larger and had a leftward bias, resting more heavily in the hemisphere housing most language.
The growth was uneven. Most of the extra bulk rested at the front of the tract, with a portion connected to the prefrontal cortex—the region responsible for planning and sequencing.
Earlier research had linked this cable to fluent thought and action.
The arcuate fasciculus was not the only pathway that had grown. Along with it, a second pathway, the superior longitudinal fasciculus, had expanded—the route by which sound travels from auditory areas of the brain to regions responsible for speech.
Both pathways lead to the same frontal brain region: one relays information perceived by the ears, and the other processes information sent by the mouth.
The co-growth links hearing and speech in a tighter loop. A separate paper focusing on the anatomy of this tract suggests the same idea.
Increased size does not always equate to better quality, and scans cannot reveal the network’s functionality. The increased size indicates more space to travel between regions that remain weakly connected in other primates.
Whether this provokes the discussion or is a consequence of it, the images themselves cannot yet say. Anatomy shows more, not the function of each part.
For this, the research team examined humans losing speech due to a degenerative condition called primary progressive aphasia—a disease in which speech disappears as brain tissue wears away over time.
The damage spreads unevenly. Different patients exhibit damage in different parts of the tract.
This uneven pattern represents a natural experiment. Patients with damage at the back of the neural tract experienced the greatest difficulty with speech fluency—the smooth, even flow of words.
They knew what they wanted to say; the problem was expressing it. At the front of the book, wear manifested as impaired syntax—the grammar that dictates which word goes where. Word order broke down, and sentences lost their structure, even as the words themselves remained in place.
No one had previously divided this tract into two parts so clearly: grammar at the front, fluency at the back.
Connecting all these elements, a picture of how language emerged begins to take shape. The frontal brain regions responsible for sound generation were already present in our ancestors, shared by apes and the great apes alike.
The available evidence suggests that evolution did not create an organ for language out of nothing. It repurposed an older system.
Borrowing an old structure for a new application is a common occurrence in evolution, and the authors cite language as an example.
Presumably, the neural network that once handled simple auditory signals was repurposed in humans to combine sounds into words and ordered sentences. This enlarged, left-biased neural network tract is the visible change.
This idea aligns with a number of recent studies of parts of the human frontal lobe that lack clear counterparts in other primates.
One study describes a frontal center that appears to be unique to each individual. No single study proves this story definitively, but anatomical features and language impairments now point toward the same perspective.
For medical practitioners, the division into the anterior and posterior portions holds practical significance.
A patient losing grammar and a patient losing fluency may experience impairments in different parts of the same tract, and scans can help differentiate them. The findings may help guide what further research should be done.
What was unclear before this work now has a firmer foundation. The human version of this speech tract is larger and left-biased, its front portion has grown, and its two halves are responsible for different aspects of speech.
In the context of studying the origins of language, this finding touches on a long-standing debate concerning something tangible to hold onto.
Researchers should stop asking when humans began speaking and start asking how an electrical circuit could have been rewired to transmit grammar.
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