Origin of Human Brain Network Crucial for Language Emerged 20 Million Years Earlier Than We Thought, Scientists Say

Researchers have shed new light on the language pathway in the human brain, suggesting that its origins lie around 25 million years ago—twenty million years earlier than previously thought.

Brain pathways, or neural pathways, are circuits of interconnected neurons (nerve cells) that enable the transmission of electrical signals between different parts of the brain. In humans, one of these networks called the "arcuate fasciculus" pathway is known to be crucial for language.

"This is a pathway that interconnects brain regions that are important for language. If this pathway or some of these regions it interconnects are damaged because of stroke or brain degeneration a person might immediately (because of stroke) or progressively (because of dementia) lose the ability to understand or to produce language," Chris Petkov, lead author of a Nature Neuroscience study from Newcastle University in the U.K., told Newsweek.

It has previously been assumed that an early precursor to the arcuate fasciculus emerged around five million years ago when humans and chimpanzees last shared a common ancestor. But according to the study, this precursor may have emerged at least 20 million years earlier, further than the split from the common ancestor of humans and macaque monkeys.

"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," Petkov said in a statement.

Because brains don't fossilize, unlike bones, the only way for scientists to get an idea of what the brains of common ancestors may have been like is by doing imaging studies of the brains of living monkeys and apes, and comparing these to humans. So for their study, Petkov and his team examined images of brain pathways and regions responsible for auditory functions in humans, apes and monkeys.

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"Whether monkeys have a homolog (a precursor) of this pathway was highly controversial," Petkov told Newsweek. "Thinking further about the basis for the controversy, when we started the project we also wondered whether such a pathway in monkeys was missed because scientists had not looked in the correct place. We predicted that a missing correspondence to humans might be hiding, so to say, in the auditory system. So that is where we looked first. The analogy here is that we may have been looking in the wrong place for the missing brain 'fossil.'"

In humans, the researchers identified a segment of the language pathway that connects the auditory cortex—which processes auditory information—to regions in the frontal cortex of the brain that are responsible for speech and vocal production. Furthermore, they found that the brains of both apes and monkeys also have their own version of this pathway.

"To be honest, we were really quite surprised that the auditory system has this privileged pathway to vocal production regions in frontal cortex," Petkov said. "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."

brain, synapses
Stock image: Illustration of synapses in the brain. iStock

"Since only humans have speech and language as we know them, this new information and the link by way of the auditory system indicates that the ancestors to humans and monkeys, who would not have had language either, had sophisticated hearing and vocal communication abilities as do extant apes and monkeys," he said. "These abilities and the ancestral brain pathway supporting them appear to have been the evolutionary basis upon which the language pathway further evolved in our human ancestors for language."

The study also indicates that the ancestral pathway was present in a common primate ancestor that lived about 25 million years ago, although the researchers were not able to identify its origins.

"This too is exciting in that there may be more brain 'fossils' yet to be discovered with even earlier evolutionary origins," Petkov said. "We might predict that the auditory system of even more distantly related primate species would either show that 25 million years ago was when this key pathway first developed—if more distantly related species have a very different pathway. Or it may be discovered that the origin of this pathway traces back even further if another brain "fossil" is found," he said.

According to the researchers, the latest results could have implications for the treatment of certain neurological problems in humans, and the team are now working with clinical partners to carry out further studies in these fields.

"Neurology patients that have stroke or brain degeneration that affects their language or memory abilities could now benefit from additional research to understand how the auditory segment of this pathway is affected," Petkov said.

"This new discovery is encouraging because it indicates that we should take a closer look at the auditory segment of the the language pathway in the human brain and maybe even find others. Language can be spoken, written or signed, but if some of these pathways are more or less intact after stroke that might provide a venue for more accurate prognosis of language recovery and new ideas on rehabilitation to improve language abilities," he said.

"Other benefits are that the work with monkeys means that for the first time we have a model of this pathway that we can study in fundamental ways not possible in humans. Animal research has helped to advance treatments for Parkinson's, for example, which required work with monkeys. But to date an animal model for aspects of human language was not thought possible, in part because the language pathways link just has not been there," he said.

Origin of Human Brain Network Crucial for Language Emerged 20 Million Years Earlier Than We Thought, Scientists Say | Tech & Science