The region of the brain that became the language pathway in humans was believed to have emerged about 5 million years ago, but a new study pushes its origins to at least 25 million years ago.
As reported in Nature Neuroscience, a team from multiple universities compared the brain scans of three chimpanzees and three macaques with those of three male humans. They focused on the arcuate fasciculus (AF) pathway, which is believed to be crucial for language. This pathway connects the auditory cortex, a region that processes sounds, to the frontal lobe, which is important for speech and language, among other things.
The presence of an analogous pathway in monkeys is a hotly debated topic. The team wondered if previous studies had been looking in the right places. Since the scientists claim they found the pathway in their new analysis, the AF must have already been present in the last common ancestor between us and macaques.
The last common ancestor between humans and chimps is believed to have lived between 5 and 6 million years ago. The last common ancestor between humans, chimps, and macaques is five times as old, between 20 and 30 million years ago. As brains don’t fossilize, their evolution is estimated by looking at similarities and differences between closely and not-so-closely related species.
“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,” senior author Professor Chris Petkov, from Newcastle University, said in a statement.
“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.”
The work also found a key difference between our AF and those of nonhuman primates. In toddlers as well as apes, the AF is equally underdeveloped but as we grow older, the pathway starts looking different. In a human adult brain, the use of language produces two important changes: the left side of the pathway is stronger and the right side has more connections to brain regions not related to auditory processes. This allows for information, in this case language, to perhaps move more quickly across the different areas of the brain.
“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. The study has already inspired new research underway including with neurology patients," said joint senior author Professor Timothy Griffiths, a consultant neurologist at Newcastle University.