An international team of researchers led by Newcastle University Medical School has discovered an earlier evolutionary origin to the human language pathway in the brain, pushing back its origin by at least 20 million years. Previously, a precursor of the language pathway was thought by many scientists to have emerged more recently, about 5 million years ago, with a common ancestor of both apes and humans.
Auditory dorsal and ventral pathway strength and lateralization in macaques, chimpanzees and humans. Left: bar plots showing the individual results and mean fiber counts for the left and right dorsal and ventral pathways for each of the three species. Right: schematic summary of the dorsal (purple) and ventral (dark green) pathway results in macaques, chimpanzees and humans, overlaid on previous observations (light yellow and light green). Image credit: Balezeau et al, doi: 10.1038/s41593-020-0623-9.
For neuroscientists, this discovery is comparable to finding a fossil that illuminates evolutionary history.
However, unlike bones, brains did not fossilize. Instead neuroscientists need to infer what the brains of common ancestors may have been like by studying brain scans of living primates and comparing them to humans.
“It is like finding a new fossil of a long lost ancestor. It is also exciting that there may be an older origin yet to be discovered still,” said Newcastle University’s Professor Chris Petkov, lead author of the study.
In the study, Professor Petkov and colleagues analyzed auditory regions and brain pathways in humans, apes and monkeys.
The scientists relied on brain scans from openly shared resources by the global scientific community. They also generated original new brain scans that are globally shared to inspire further discovery.
They discovered a segment of this language pathway in the human brain that interconnects the auditory cortex with frontal lobe regions, important for processing speech and language.
Although speech and language are unique to humans, the link via the auditory pathway in other primates suggests an evolutionary basis in auditory cognition and vocal communication.
“We predicted but could not know for sure whether the human language pathway may have had an evolutionary basis in the auditory system of nonhuman primates,” Professor Petkov said.
“I admit we were astounded to see a similar pathway hiding in plain sight within the auditory system of nonhuman primates.”
The study also illuminates the remarkable transformation of the human language pathway.
The key human unique difference is that the left side of this brain pathway was stronger and the right side appears to have diverged from the auditory evolutionary prototype to involve non-auditory parts of the brain.
Also since the study authors predict that the auditory precursor to the human language pathway may be even older, the work inspires the neurobiological search for its earliest evolutionary origin –the next brain ‘fossil’ — to be found in animals more distantly related to humans.
“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,” said Newcastle University’s Professor Timothy Griffiths, senior author of the study.
The findings were published in the journal Nature Neuroscience.
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F. Balezeau et al. Primate auditory prototype in the evolution of the arcuate fasciculus. Nat Neurosci, published online April 20, 2020; doi: 10.1038/s41593-020-0623-9
