A new study finds evidence of an important neural speech pathway in macaques.
- Researchers find traces of something like our arcuate fasciculus in macaque brains.
- Since the last ancestor we shared with macaques was 25-30 million years ago, this would push speech way back.
- The study suggests human speech began in the auditory cortex and eventually extended to include the executive-function areas of the brain.
The arcuate fasciculus and the auditory cortex<img type="lazy-image" data-runner-src="https://assets.rebelmouse.io/eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpbWFnZSI6Imh0dHBzOi8vYXNzZXRzLnJibC5tcy8yMzE3NzAwOC9vcmlnaW4uanBnIiwiZXhwaXJlc19hdCI6MTYxMjExMzE3NX0.3tVh8sWuOdfsWWxu_an5-opBLdy7ag616NhSrnOsaTU/img.jpg?width=980" id="f2ff0" class="rm-shortcode" data-rm-shortcode-id="491baf85f91e992ac0e2407158520d1e" data-rm-shortcode-name="rebelmouse-image" data-width="982" data-height="929" />
Image source: Human Brain MRI Data and Connectome Atlas /wikimedia<p>The fuss is about a neural pathway in humans called the arcuate fasciculus, or AF, that traverses our prefrontal cortex and frontal lobe. Recent <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3071430/" target="_blank">research</a> suggests it has connections to other brain regions as well.</p><p>"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 <em>Newsweek</em>.</p><p>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. <a href="https://www.newsweek.com/origin-human-brain-network-language-20-million-years-scientists-1498967" target="_blank">Says</a> 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."</p>
Not just us<img type="lazy-image" data-runner-src="https://assets.rebelmouse.io/eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpbWFnZSI6Imh0dHBzOi8vYXNzZXRzLnJibC5tcy8yMzE3NzAxMS9vcmlnaW4uanBnIiwiZXhwaXJlc19hdCI6MTYxMTk2MjYyN30.ejPIZESh11Q1m5DlyZLvlI1i_TXOCbczHem3zw2Nqkc/img.jpg?width=980" id="3c45b" class="rm-shortcode" data-rm-shortcode-id="5609a51cb22bac3075a6ae59404ff3aa" data-rm-shortcode-name="rebelmouse-image" data-width="1440" data-height="960" />
Image source: Steven Diaz/Unsplash<p>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.</p><p>"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.'"</p>
Do macaques talk, then?<img type="lazy-image" data-runner-src="https://assets.rebelmouse.io/eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpbWFnZSI6Imh0dHBzOi8vYXNzZXRzLnJibC5tcy8yMzE3NzAxNS9vcmlnaW4uanBnIiwiZXhwaXJlc19hdCI6MTY2MjQxNTk5Nn0.HqlbLesqREYhFirwH0xOgayYDsd_wqKkomOeQpyxC1o/img.jpg?width=980" id="b54bc" class="rm-shortcode" data-rm-shortcode-id="a377d5fdf6b87df26c682f1a5b77592e" data-rm-shortcode-name="rebelmouse-image" data-width="1440" data-height="960" />
Image source: Jonathan Forage/Unsplash<p>Well, in a sense, yes. While we haven't observed words and sentences in these <a href="https://en.wikipedia.org/wiki/Old_World_monkey" target="_blank">Old World monkeys</a>, they do <a href="https://www.nih.gov/news-events/nih-research-matters/hints-language-origin-rhesus-monkey" target="_blank">communicate with vocal sounds and with gestures</a>, signaling information about food and about imminent danger. Other research has identified what appears to be a <a href="https://bigthink.com/robby-berman/researchers-discover-monkeys-have-a-speech-ready-vocal-tract" target="_self">speech-ready vocal tract</a>.</p><p>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."</p>
But/and...<p>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.</p><p>Joint senior author neurologist <a href="https://www.ncl.ac.uk/medical-sciences/people/profile/timgriffiths.html" target="_blank">Timothy Griffiths</a> 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."</p>
Not only do these monkeys use tools, they're developing new, better tools to adapt to their environment.
- 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.
Analyzing the stone tools<img type="lazy-image" data-runner-src="https://assets.rebelmouse.io/eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpbWFnZSI6Imh0dHBzOi8vYXNzZXRzLnJibC5tcy8xOTYyODIwNS9vcmlnaW4ucG5nIiwiZXhwaXJlc19hdCI6MTY0MzMzNzkzMX0.pzdxXsJleydwNOaqQh7x5Tcdtm0LUHGaw2X7J0mqefs/img.png?width=980" id="551ea" class="rm-shortcode" data-rm-shortcode-id="1d045580de1ba49fe53ce396b3f17970" data-rm-shortcode-name="rebelmouse-image" />
Examples of the capuchin monkeys' stone tools with typical percussive damage.
Photo: Falótico et al., 2019<p>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.</p><p>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. </p><p>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.</p>
What's the significance?<span style="display:block;position:relative;padding-top:56.25%;" class="rm-shortcode" data-rm-shortcode-id="6b9c9913ca1604ed06e34465811b07c8"><iframe type="lazy-iframe" data-runner-src="https://www.youtube.com/embed/_MgHBvp1uwk?rel=0" width="100%" height="auto" frameborder="0" scrolling="no" style="position:absolute;top:0;left:0;width:100%;height:100%;"></iframe></span><p>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 <a href="http://humanorigins.si.edu/evidence/behavior/stone-tools/early-stone-age-tools" target="_blank">human tool use</a>, 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.</p><p>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.</p><p>"This capuchin excavation shows that this species of primate in Brazil has its own individual archaeological record," he told <em><a href="https://www.nationalgeographic.com/science/2019/06/capuchin-monkeys-used-stone-tools-3000-years-oldest-outside-africa/" target="_blank">National Geographic</a></em>. "They have their own antiquity to their tool use." </p>
Big and strong? That's not what makes an alpha male, says primatolgist Frans de Waal.
- The cultural notion of an alpha male as a strong, mean aggressor is rampant but wrong. The reality is more complex.
- Frans de Waal notes two types of alpha males: Bullies and leaders. In chimpanzee society, the former terrorizes the group while the latter mediates conflict.
- The reign of alpha male bullies usually ends poorly in the wild. Chimpanzee bullies get expelled or even killed by their group, while leader alphas are somewhat democratically kept in power, sometimes for as long as 12 years.
Primatologist Frans de Waal explains the primal instinct that unites humans and chimpanzees.
- Humans throw temper tantrums when they feel frustrated, lose power, or sense a threat to their status or security.
- Chimpanzees exhibit the same behavior; alpha male chimps who lose their status throw tantrums to elicit sympathy from their group, hoping to have their power restored.
- But that tactic almost never works, notes primatologist Frans de Waal. An important lesson for humans from chimps.