Researchers have discovered an earlier origin to the human language pathway in the brain, suggesting that its origins lie around 25 million years ago – 20 million years earlier than previously thought.
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.
For neuroscientists, this 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 study lead author Chris Petkov from Newcastle University in the UK.
For the findings, published in the journal Nature Neuroscience, the international teams of European and US scientists carried out the brain imaging study and analysis of auditory regions and brain pathways in humans, apes and monkeys. 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. I admit we were astounded to see a similar pathway hiding in plain sight within the auditory system of nonhuman primates," Petkov added.
Left side of this brain pathway was stronger and the right side
The study also illuminates the remarkable transformation of the human language pathway. A key human unique difference was found: the human 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.
The study relied on brain scans from openly shared resources by the global scientific community. It also generated original new brain scans that are globally shared to inspire further discovery.
Also since the authors have predicted that the auditory precursor to the human language pathway maybe 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 study senior author Timothy Griffiths.