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In the classic children's book "Caps for Sale," a band of monkeys steals the caps off the top of a peddler's head while he sleeps. He shakes his fists and stomps his feet at the monkeys only to have them "ape" his movements right back at him. In disgust, he finally pulls his one remaining cap from his head and throws it on the ground. The monkeys, of course, do the same and the peddler reclaims his caps.

We call this kind of imitation "aping" and "parroting." And yet, it might be more appropriate to call it "humaning." We are far better at "aping" than any ape. Whole books have been written, in fact, claiming that our ability to imitate is what makes us uniquely human. Scientists, philosophers and literati have argued that imitation is key to language learning, empathy and many other cultural advances that separate us from the apes.

That link between imitation and humanity is at the crux of a new trend in evolutionary science: theories claiming that a class of nerve cells, dubbed "mirror neurons" for their ability to mirror the actions of others, were the spark that allowed our hominid ancestors to branch off from apes. Monkeys and apes also have these neurons (see page 49) but, claim the theories, the humanoid brain capitalized on them in new ways that allowed them to move beyond simple imitation to more complex imitation, and that in turn blossomed into language, music, art, tool-making and even empathy.

University of California, San Diego, neuroscientist and mirror-neuron proponent Vilayanur Ramachandran, MD, PhD, gives these cells credit for causing the big bang of human development, otherwise known as the "great leap forward." That leap occurred somewhere around 50,000 years ago, when human culture experienced a sudden explosion of technological sophistication, widespread cave art, clothes, stereotyped dwellings and the like.

While many researchers think Ramachandran goes too far with his claims, a large and growing group is intrigued enough by mirror neurons that more researchers have begun to investigate their potential role in human evolution.

"They are a mechanism of connecting me with you, one person with another, and therefore may play a vital role in communication and social interaction," says psychologist Sarah-Jayne Blakemore, PhD, of the Institute of Cognitive Neuroscience at University College London.

Explaining the 'big bang'

When University of Parma, Italy, neuroscientist Giacomo Rizzolatti, MD, and his team discovered mirror neurons in monkeys in the early 1990s, several researchers immediately saw the potential to link mirror neurons to human evolution. But it was Ramachandran's provocative big bang essay published in 2000 on the Edge Web site (www.edge.org)--an online salon for scientists and other intellectuals--that really got people's attention.

"I predict that mirror neurons will do for psychology what DNA did for biology," wrote Ramachandran. "They will provide a unifying framework and help explain a host of mental abilities that have hitherto remained mysterious and inaccessible to experiments."

In particular, he says, mirror neurons primed the human brain for the great leap forward by allowing us the ability to imitate, read others' intentions and thereby learn from each other.

So why did we develop these sophisticated abilities and not monkeys or apes? Ramachandran says that's the most common question he's asked when he talks on this topic. His response? "Wings evolved from forelimbs, but no one wonders why humans didn't evolve wings even though we also have forelimbs," he says.

Besides, he adds, his theory states that mirror neurons were necessary but not sufficient for humans to evolve these more sophisticated traits.

"There was a fortuitous combination of things--mirror neurons being one--working together, and evolution took advantage of that," says Ramachandran. Then, once we had the ability to imitate, and learn through imitation, transmission of culture could continue by leaps and bounds.

Although Ramachandran's theory is mostly that--by his own admission he went out on a limb--researchers from diverse fields have been attracted to mirror-neuron theories of human evolution because they are testable.

If, for example, a trait like human empathy evolved from mirror neurons, it stands to reason that people who lack empathy would have a dysfunctional mirror-neuron system. That's exactly what preliminary research on autism--a disorder that leaves people incapable of understanding others' feelings--is showing (see page 52).

Language evolution

An area that's found a ready home for mirror neurons is the study of language evolution. Rizzolatti and University of Southern California neuroscientist Michael Arbib, PhD, spearheaded the movement in 1998 with a theory of how mirror neurons for grasping could have advanced from the simple cells seen in monkeys to a more sophisticated network allowing for the symbolism and syntax of human language with speech building on a scaffolding of manual gesture.

Arbib and Rizzolatti formed their theory after they, along with Scott Grafton, MD, collaborated on a study in humans using positron-emission tomography, which measures which areas of the brain are working during specific tasks. They found that an area of the brain called Broca's area lit up when people grasped an object and when they watched someone else grasp the object, similar to what Rizzolatti saw in monkeys. This finding was even more interesting because it showed that Broca's area--a part of the brain traditionally associated with speech--was also involved in organizing intentional actions and in allowing people to understand the intentional actions of others.

"The fact that Broca's area was traditionally thought of as a speech area, and the fact that the deaf community had long insisted that language was more than speech [that the brain processes signed language in the exact same way it processes spoken language], led to our evolutionary view that 'language is within our grasp,'" says Arbib, referring to the title of a 1998 article in TRENDS in Neurosciences (Vol. 21, No. 5, pages 188-194) he co-authored with Rizzolatti.

Indeed, the discovery of mirror neurons provided a neurological basis for an increasingly popular idea that syntax and language grew out of manual action, which has its own kind of very basic grammar, an idea promulgated by University of California, Los Angeles, psychologist Patricia Greenfield, PhD, through her research in the 1970s (see sidebar): When someone grabs a ball, for example, "grasp" is the verb, "ball" is the object and the person is the subject. Arbib's mirror system hypothesis argues that mirror neurons provided the foundation on which language developed from a simple gestural protolanguage (itself building on a scaffolding of pantomime), to a verbal protolanguage, to a fully functional language. At the heart of the model is the uniquely human ability for complex imitation. Being able to imitate another person's performance--by recognizing a large set of movements as a combination of familiar actions--is not only key to a child's ability to acquire language, but also to an adult's ability to use language, argues Arbib. And this ability was built on the more rudimentary mirror system seen in monkeys and apes.

Not everyone agrees with the details of Arbib's language-evolution theory, but many have begun to accept the overall claim that mirror neurons formed a foundation for language evolution.

"First and foremost, language is an action," says University of Chicago psychologist Bennett Bertenthal, PhD, "Speech is an action that involves the tongue, lips and vocal chords. So the fact that these different actions--speech and other instrumental actions--seem to have an overlapping neural basis is fascinating."

That said, Bertenthal and psychologist David McNeill, PhD, believe that verbal language and gesture evolved in tandem in Broca's area, rather than one after the other, as Arbib asserts. The key point, they assert, is how the brain moved from understanding actions as actions to understanding actions as symbols--how an upward movement can translate into the idea of "upness." Mirror neurons, says McNeill, do this by allowing someone making a gesture or speaking a word to understand the social implications of their actions. In other words, mirror neurons allowed our preverbal ancestors to understand that their actions had a social significance.

Others won't even go that far. University of Hawaii linguist Derek Bickerton, PhD, doesn't deny that mirror neurons are an intriguing discovery. In fact, he admits they may have played a role in the evolution of some of the first words. But the theories put forth so far, particularly by Arbib, fail to answer the most critical questions about language evolution, he says. In particular, how did the brain invent syntax?

"There's a tendency in certain circles to look for some magic bullet or quick fix to say, 'That's the cause of language, not that,'" says Bickerton. "It's not that easy."

Despite the critics, Rizzolatti's discovery of mirror neurons a decade ago has inspired researchers from computer science to philosophy to investigate their potential role in human evolution. And some critics worry that the intense focus will take resources away from other promising avenues of research. Others, however, believe the mirror revolution, as Arbib calls it, is healthy for the field.

"Probably the most exciting aspect of the mirror-neuron story is that it's facilitated more multidisciplinary thinking," says Bertenthal. "People from computer modeling to psychology, to linguists, to neurophysiology have thought about how mirror neurons in primates would enable us to better understand how humans were endowed with certain abilities and cognitions. That can only put evolutionary theories on more solid ground."

Indeed, the area is now establishing itself, says Ramachandran. "The game is afoot," he notes. "The next stage is to tailor experiments to test our hypotheses."

Beth Azar is a writer in Portland, Ore.

Further Reading

  • Arbib, M. (2005). From monkey-like action recognition to human language: An evolutionary framework for neurolinguistics. Behavioral and Brain Sciences, 28, 105-167.

  • Arbib, M. (Ed.). (2005). Action to language via the mirror neuron system. New York: Cambridge University Press.

  • McNeil, D. (2005). Gesture and thought. Chicago: University of Chicago Press.

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