Well-tooled primates: the evolutionary roots of our technological prowess may run deep

Science News,  Feb 10, 2007  by Bruce Bower

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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.

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).

Before training macaques to use the rakes, Iriki's team tinted that electrical discharges of other parietal ceils 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.