Well-Tooled Primates

The evolutionary roots of our technological prowess may run deep

In a lab in Japan, a macaque monkey eyes a small, plastic rake and performs an act that his wild brethren would never dream of doing. The animal grasps the utensil by its handle and extends it toward a food pellet placed beyond his reach. Slowly, the monkey manipulates the rake so that it drags the morsel close enough that he can grab it and pop it into his mouth. Researchers in the lab suspect that macaques possess an innate neural capacity for manipulating objects that encourages tool use, even if such behavior occurs rarely in the wild.

STONE STRUCK. A researcher demonstrates how to use one stone to pound flakes off another, yielding a sharpened edge in a style developed by our Stone Age ancestors. Stone Age Inst.
IMPLEMENTING COLORS. Brain images show neural regions that became substantially more active as study participants gained experience at making stone tools. The highest activity appears in red. Stout

Meanwhile, at Indiana University in Bloomington, six people smash rocks together in the name of science. At the request of anthropologist Dietrich Stout, each participant chooses a pair of stones from a selection on a cart and strikes them together, again and again, trying to create sharp flakes suitable for use as cutting tools. After four 1-hour sessions, the budding toolmakers produce sharp flakes that look much like the stone tools made by human ancestors as many as 2.5 million years ago.

Brain scans obtained from those participants before and after the toolmaking sessions and from the monkeys as they use the plastic rakes show increases in activity in the same brain area. Furthermore, no activity emerges in the human toolmakers’ neural regions that control planning and memory, intellectual faculties often considered crucial to the evolution of toolmaking.

These related findings support the theory that the evolution of neural areas devoted to object manipulation by ancient primates paved the way for stone-tool making by human ancestors. Our ancient forerunners didn’t think up these technological advances so much as explore their way into them, according to this perspective. The distinction is important because rule following and planning—not to mention self-awareness, imitation, and language skills—flowered after prehistoric humans attained toolmaking expertise.

Researchers who subscribe to these ideas theorize that modern humans are neither blank slates nor carriers of a batch of instincts unique to our species. Instead, via language and cultural traditions, people have collectively molded a shared primate-evolutionary heritage for their own purposes.

“Fairly ancient brain systems were elaborated in new ways when human ancestors began making and using stone tools,” says Stout, now at University College, London. “This process relied on an education of attention, not intellect.”

Monkey business

Although monkey species in some parts of the world spontaneously use sticks or other objects as tools, Japanese macaques seldom do. Yet it took Atsushi Iriki and his colleagues at the RIKEN Brain Science Institute in Saitama, Japan, only about 2 weeks to train adult Japanese macaques to snag food with a rake.

This experience changes the structure of these monkeys’ brains, Iriki’s team found. The alterations then spur the animals to think and act in new ways that have surprising connections to human thought and behavior, Iriki holds.

Iriki suspects that the brain changes tap into “silent precursors of human intelligence in the tool-using monkey brain.” He describes his research in the Dec. 2006 Current Opinion in Neurobiology.

A decade ago, Iriki and his coworkers used hair-thin electrodes implanted in monkeys’ brains to identify neurons in one parietal area, near the brain’s midpoint, that vigorously responded to both visual and bodily sensations. This area also contains what are called mirror neurons, nerve cells that react equally strongly when the animal executes an action and when it observes another animal perform the same action.

Mirror neurons may make it possible to imitate others’ behavior (SN: 9/9/06, p. 163: Copycat Monkeys: Macaque babies ape adults’ facial feats).

Before training macaques to use the rakes, Iriki’s team noted that electrical discharges of other parietal cells peaked when an animal looked at the hand it used for reaching out and grabbing objects. After a macaque learned to use the rake, the same cells spewed impulses when the animal looked anywhere along the trajectory extending from its rake-holding hand to the end of the tool.

This neural shift indicates that macaque rake users incorporate the tool into an internal representation of their bodies and their parts, Iriki proposes.

As a result, tool users gradually come to mentally regard their hands and arms, and then their entire bodies, from a third-person perspective, he says. This achievement boosts the capacity to scrutinize and imitate others’ actions.

In support of this idea, Iriki now finds that the 2-week-long rake training stimulates important brain changes in adult monkeys. His team stained and microscopically examined parietal cells from the brains of five trained monkeys and compared them with corresponding cells from four untrained monkeys.

In the trained group, parietal cells connected to cells in two brain areas that weren’t accessed by those cells in untrained animals. In people, one of those areas, which is near the parietal cortex, fosters a sense of body image and of self, some research has found. The other area, in the frontal brain, contributes to mental flexibility in carrying out familiar tasks.

Tool use by monkeys may even promote a behavior that Iriki regards as an evolutionary precursor of language. When monkeys were first trained to rake in food and then to produce cooing sounds to ask for food, they spontaneously began to emit two acoustically distinctive coos—one for food and another requesting a rake to retrieve food. These findings suggest that human ancestors parlayed stone-tool pursuits into advances in speech, using sounds to label various objects, Iriki asserts.

“Their tool use could have contributed to the emergence of perceiving meaning in language and other higher cognitive functions,” he suggests.

Rock solid

As Stout pored over human-brain data from his novice stone-tool makers last year, Iriki’s studies came to mind. The parietal neighborhoods activated in rake-trained monkeys had geared up when Stout used positron-emission tomography to monitor the volunteers’ brains’ energy use during their toolmaking sessions.

However, stone-tool making ignited brain regions aside from the seemingly ancient network that Iriki observed in macaques, Stout reports in a 2007 Neuropsychologia (vol. 45, issue 5). Toolmaking practice enlivened brain areas that regulate handgrips and that mediate visual attention in people.

An additional parietal area displayed pronounced activity as toolmakers gained skills. Studies directed by neuroscientist Guy A. Orban of K.U. Leuven Medical School in Belgium suggest that this region is present in people, but not in apes or monkeys. It boosts three-dimensional perception and enhances the clarity of moving images—handy attributes for toolmakers.

For Stout, these findings suggest that ancient toolmaking rested on extensive practice that improved people’s visual analysis of rocks and the fluidity of the actions needed to produce sharp flakes. Earlier work indicated that novices learned by doing and by getting feedback from experienced workers, not by following rules (SN: 4/12/03, p. 234: Available to subscribers at The Stone Masters).

Neuroscientist Scott H. Frey of the University of Oregon in Eugene sees Stout’s results as consistent with studies of more-mundane tool use. Using functional magnetic resonance imaging to track blood-flow changes in the brain, Frey’s group has found that people who are planning to, say, eat with a spoon or pound with a hammer galvanize a widespread network of regions in the left brain, including frontal and parietal tissue.

When actually using such implements, the same individuals display activity largely relegated to the parietal areas emphasized by Stout. “These regions are involved in transforming sensory information into motor commands,” Frey says.

He suspects that expert stone-tool makers also call on a broad network of left brain structures, including frontal areas linked to planning and memory.

Psychologist Arthur Glenberg of the University of Wisconsin–Madison suggests that further research examine whether parietal responses are stimulated by stone-tool making itself or by practicing any set of goal-directed actions.

Changing spaces

If Iriki and Stout are right, then our prehistoric ancestors didn’t invent stone tools out of evolutionary whole cloth. Instead, groups bound by cultural traditions turned a humble neural inheritance into a unique aptitude for toolmaking and other technological pursuits.

That scenario rings true to linguist Stephen C. Levinson of the Max Planck Institute for Psycholinguistics in Nijmegen, the Netherlands. He and his colleagues study how people perceive their locations and orientations relative to those of external objects and plan routes from one spot to another. This mental faculty, known as spatial cognition, contributes to toolmaking and tool use.

In the Nov. 14, 2006 Proceedings of the National Academy of Sciences, Levinson’s group asserts that all people innately consult environmental cues to locate themselves in space and to navigate from point A to point B. However, assumptions built into some cultures and languages transform this innate tendency into a preference for using oneself, rather than one’s surroundings, as a spatial reference point, the researchers say.

The researchers emphasize that some languages, including English, tend to describe object locations in terms relative to an observer’s viewpoint, such as front, back, right, and left. Other languages generally use terms for absolute directions—north, south, east, and west—or refer to familiar landmarks.

In one experiment, the researchers studied 12 adults and 12 children, ages 8 to 10, who spoke Dutch, a language that, like English, uses mainly relative spatial terms. Another 12 adults and 12 children came from an African hunter-gatherer group that typically uses absolute spatial descriptions.

Each volunteer sat in front of a table and watched an experimenter alongside the table place a token under one of five cups positioned like dots on a die—two on the bottom, one in the middle, and two on top. Participants then moved to the opposite side of the table and to another set of cups and indicated where they thought a second token might be hidden.

In a series of trials, Dutch adults and kids rapidly learned where the tokens were and made few errors if the tokens in the two tests maintained position relative to a participant’s viewpoint, such as starting out on the bottom left-hand side and again being bottom left after the volunteer moved to the new viewing position. However, their performance declined sharply if the tokens maintained absolute position, such as being located under the northwestern cup—which started out on the lower left and then was upper right after the participants repositioned.

In contrast, the hunter-gatherers excelled at finding hidden tokens that maintained absolute position and stumbled on the other condition.

The researchers then administered a simpler version of the hidden-token test to 12 German 4-year-olds attending preschool as well as 5 orangutans, 7 gorillas, 7 pygmy chimpanzees, and 11 common chimps. Although German adults tend to use relative spatial terms, both the preschoolers and the apes located tokens more readily and accurately when using environmental cues—either absolute or landmark based.

Levinson’s team theorizes that apes and people possess an innate tendency to navigate by tracking features of their surroundings. An observer-centered viewpoint develops slowly during childhood only when cultivated by language and culture, the researchers propose.

Psychologist Nora S. Newcombe of Temple University in Philadelphia expresses skepticism about that conclusion. Mobile individuals skillfully use both viewer-centered and environment-centered spatial strategies when necessary, she says. For instance, landmarks are essential to speakers of relative languages when they’re planning alternative routes to a destination and to speakers of absolute languages when, on occasion, dead reckoning leads them astray.

Similarly, researchers will need to use a variety of strategies as they wend their way along the path from rake-wielding monkeys to tool-producing people. There’s still a long distance to go, but a few neural landmarks now light the way.

Bruce Bower has written about the behavioral sciences for Science News since 1984. He writes about psychology, anthropology, archaeology and mental health issues.