As far as we know, humans alone are capable of speech as we know it, with words and sentences. This has to do, scientists believe, with a pathway in the brain we possess. Now a new and controversial study reports the presence of this same pathway, albeit in less pronounced form, in macaques. Given that our last shared ancestor with these monkeys was 25-30 million years ago, the study suggests that speech may date back to at least that far, much longer ago than the previous estimates of five million years.
Pinpointing such evolutionary milestones is tricky, since brain tissues don't survive as fossils, leaving us to examine the contemporary brains of our closest relatives, such as primates, to piece these puzzles together. The study's lead author, comparative neuropsychologist Chris Petkov of Newcastle University in the UK compares the study of our living cousins is like finding a lost fossil.
The arcuate fasciculus and the auditory cortex
Image source: Human Brain MRI Data and Connectome Atlas /wikimedia
The fuss is about a neural pathway in humans called the arcuate fasciculus, or AF, that traverses our prefrontal cortex and frontal lobe. Recent research suggests it has connections to other brain regions as well.
"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," Petkov tells Newsweek.
For the study, international teams of European and US scientists pored through new imaging data of humans looking for evidence of this pathway in other regions. They found a segment of it, unexpectedly, in the auditory complexes of both brain hemispheres, though most strongly identifiable in the left one. Says Petkov, "To be honest, we were really quite surprised that the auditory system has this privileged pathway to vocal production regions in frontal cortex." He adds, "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."
Not just us
Image source: Steven Diaz/Unsplash
Things got even more interesting when Petkov and his colleagues began searching for the AF in apes and monkeys. In their auditory cortexes, too, the researchers saw what appeared to be something similar to, though less distinct than, the human AF. Their interpretation of the finding is that speech may have begun in the auditory cortex and in humans over time spread outward to encompass the prefrontal cortex and frontal lobe whose executive function allowed us to develop basic sound communication into sophisticated speech.
"Whether monkeys have a homolog (a precursor) of this pathway was highly controversial," says Petkov. "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.'"
Do macaques talk, then?
Image source: Jonathan Forage/Unsplash
Well, in a sense, yes. While we haven't observed words and sentences in these Old World monkeys, they do communicate with vocal sounds and with gestures, signaling information about food and about imminent danger. Other research has identified what appears to be a speech-ready vocal tract.
Finding an AF-like pathway in macaques may not even represent their earliest development, notes Petkov, who points out, "there may be more brain 'fossils' yet to be discovered with even earlier evolutionary origins. Or it may be discovered that the origin of this pathway traces back even further if another brain "fossil" is found."
But/and...
Not everyone will be onboard with Petkov's conclusions, which he admits are "highly controversial." Still, if they turn out to be valid, even beyond the "wow" factor, who knows where further identification of AF-related pathways could lead, potentially including new ways to work around interruptions in brain circuitry that may affect patients with speech-based disorders.
Joint senior author neurologist Timothy Griffiths says, "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."
- Archaeologists dug into the ground of an area of a Brazilian national park known to be frequented by capuchin monkeys.
- They found that over the past 3,000 years, the stone tools that the monkeys use have evolved and changed, marking the first time this kind of development has been observed in a non-human species.
- The findings underscore the intelligence of the capuchin monkeys and serve as a parallel to our own development.
As it turns out, humans might not be the only species that's developing new technology. Researchers traveled to a popular spot in the Serra da Capivara National Park in Brazil known as Caju BPF2, an open-air site within the Baixão de Pedra Furada (BPF) valley. The spot, however, isn't a popular destination for tourists — its most frequent visitors are Sapajus libidinous, or capuchin monkeys.
One of the defining traits of the capuchin monkey is its innovative tool use. Because of this, capuchins are often characterized as among the more intelligent primates. At the Caju BPF2 site, capuchin tool use mainly features the use of quartzite stones for a variety of tasks, such as cracking nuts, processing seeds and fruits, stone-on-stone percussion, and sexual displays — specifically, female capuchins flirt by throwing rocks at potential mates. While the use of tools is a unique feature of an animal species alone, it appears that the capuchins' use of stone tools has developed over the last 3,000 years. Researchers believe the type of stone tools the capuchins use are changing depending on whether the monkeys are eating soft, low-resistance foods like seeds or hard, high-resistance foods like cashews.
"It's likely that local vegetation changes after 3,000 years ago led to changes in capuchin stone tools," says archaeologist Tomos Proffitt, who co-authored the paper. Alternatively, it's possible that different capuchin groups that occupied the site over time may have had different food preferences and correspondingly different techniques for processing them.
Analyzing the stone tools
Examples of the capuchin monkeys' stone tools with typical percussive damage.
Photo: Falótico et al., 2019
To determine this, the researchers dug several feet down and recovered stones that they referred to as "hammerstones" or "anvils." These could be identified by their sizes, since they tended to be larger than the pebbles naturally found in the ground, and they have small impact marks covering their surfaces. It was clear that these were specifically capuchin tools and not those of ancient humans, since humans tend to use sharpened, knapped stones instead of conveniently shaped natural rocks.
Then, the researchers radiocarbon-dated the stone tools to determine their age. Altogether, the tools came from four distinct periods in capuchin history, with a maximum age of around 3,000 years. By comparing the periods with the stones' characteristics, the researchers were able to identify what tasks the capuchins were using these tools for.
The earliest tools were small and covered in many impact marks, suggesting that they were used for processing many small, soft foods, like seeds. Then, the tools grew larger, indicating that the capuchins had shifted their diet to one consisting of hard-shelled nuts and fruits. Beginning about 100 years ago, the stone tools shifted to a moderate size, which modern capuchins use to crack open cashews.
What's the significance?
It might seem like pounding rocks against rocks can hardly be considered tool use or that the fact that these stones changed size over time isn't significant. However, using rocks to pound things is the first example of human tool use, too. What's more, West African chimpanzees have been observed using large, heavy stones to crack tough-shelled nuts and small stones to break softer-shelled nuts, lending credence to the researchers' theory that the size variation of the capuchin stone tools corresponded to a change in diet.
The real contribution of this study, however, is that it's the first to record an on-going variation in non-human tool use. West African chimps may use different tools for different tasks, but there's no evidence that this has changed over the past few millennia. To Proffitt, this is an exciting find.
"This capuchin excavation shows that this species of primate in Brazil has its own individual archaeological record," he told National Geographic. "They have their own antiquity to their tool use."
