Leigh Wells

MONKEY SEE When a monkey watches a researcher bring an object—an ice cream cone, for example— to his mouth, the same brain neurons fire as when the monkey brings a peanut to its own mouth. In the early 1990's, Italian researchers discovered this phenomenon and named the cells "mirror neurons."

 

By SANDRA BLAKESLEE

Cells That Read Minds

A graduate student entered the lab with an ice cream cone in his hand. The monkey stared at him. Then, something amazing happened: when the student raised the cone to his lips, the monitor sounded - brrrrrip, brrrrrip, brrrrrip - even though the monkey had not moved but had simply observed the student grasping the cone and moving it to his mouth.

The researchers, led by Giacomo Rizzolatti, a neuroscientist at the University of Parma, had earlier noticed the same strange phenomenon with peanuts. The same brain cells fired when the monkey watched humans or other monkeys bring peanuts to their mouths as when the monkey itself brought a peanut to its mouth.

Later, the scientists found cells that fired when the monkey broke open a peanut or heard someone break a peanut. The same thing happened with bananas, raisins and all kinds of other objects.

"It took us several years to believe what we were seeing," Dr. Rizzolatti said in a recent interview. The monkey brain contains a special class of cells, called mirror neurons, that fire when the animal sees or hears an action and when the animal carries out the same action on its own.

But if the findings, published in 1996, surprised most scientists, recent research has left them flabbergasted. Humans, it turns out, have mirror neurons that are far smarter, more flexible and more highly evolved than any of those found in monkeys, a fact that scientists say reflects the evolution of humans' sophisticated social abilities.

The human brain has multiple mirror neuron systems that specialize in carrying out and understanding not just the actions of others but their intentions, the social meaning of their behavior and their emotions.

"We are exquisitely social creatures," Dr. Rizzolatti said. "Our survival depends on understanding the actions, intentions and emotions of others."

He continued, "Mirror neurons allow us to grasp the minds of others not through conceptual reasoning but through direct simulation. By feeling, not by thinking."

The discovery is shaking up numerous scientific disciplines, shifting the understanding of culture, empathy, philosophy, language, imitation, autism and psychotherapy.

Everyday experiences are also being viewed in a new light. Mirror neurons reveal how children learn, why people respond to certain types of sports, dance, music and art, why watching media violence may be harmful and why many men like pornography.

How can a single mirror neuron or system of mirror neurons be so incredibly smart?

Most nerve cells in the brain are comparatively pedestrian. Many specialize in detecting ordinary features of the outside world. Some fire when they encounter a horizontal line while others are dedicated to vertical lines. Others detect a single frequency of sound or a direction of movement.

Moving to higher levels of the brain, scientists find groups of neurons that detect far more complex features like faces, hands or expressive body language. Still other neurons help the body plan movements and assume complex postures.

Mirror neurons make these complex cells look like numbskulls. Found in several areas of the brain - including the premotor cortex, the posterior parietal lobe, the superior temporal sulcus and the insula - they fire in response to chains of actions linked to intentions.

Studies show that some mirror neurons fire when a person reaches for a glass or watches someone else reach for a glass; others fire when the person puts the glass down and still others fire when the person reaches for a toothbrush and so on. They respond when someone kicks a ball, sees a ball being kicked, hears a ball being kicked and says or hears the word "kick."

"When you see me perform an action - such as picking up a baseball - you automatically simulate the action in your own brain," said Dr. Marco Iacoboni, a neuroscientist at the University of California, Los Angeles, who studies mirror neurons. "Circuits in your brain, which we do not yet entirely understand, inhibit you from moving while you simulate," he said. "But you understand my action because you have in your brain a template for that action based on your own movements.

Photo Researchers Inc.

Escherichia coli bacteria, better known as E. coli, are prokaryotes.

Photo Researchers Inc.

Giardia lamblia, in orange, is a eukaryote, as are humans.

Evolutionary biologists generally agree that humans and other living species are descended from bacterialike ancestors. But before about two billion years ago, human ancestors branched off.

This new group, called eukaryotes, also gave rise to other animals, plants, fungi and protozoans. The differences between eukaryotes and other organisms, known as prokaryotes, are numerous and profound. Dr. Lynch, a biologist at Indiana University, is one of many scientists pondering how those differences evolved.

Eukaryotes are big, compared with prokaryotes. Even a single-celled protozoan may be thousands of times as big as a typical bacterium. The differences are even more profound when you look at the DNA. The eukaryote genome is downright baroque. It is typically much bigger and carries many more genes.

Eukaryotes can do more with their genes, too. They can switch genes on and off in complex patterns to control where and when they make proteins. And they can make many proteins from a single gene.

That is because eukaryote genes are segmented into what are called exons. Exons are interspersed with functionless stretches of DNA known as introns. Human cells edit out the introns when they copy a gene for use in building a protein. But a key ability is that they can also edit out exons, meaning that they can make different proteins from the same gene. This versatility means that eukaryotes can build different kinds of cells, tissues and organs, without which humans would look like bacteria.

When explaining this complexity, most scientists have proposed variations on the same thing: natural selection favored it because versatility gave a reproductive advantage. But Dr. Lynch argues that natural selection had little to do with the origin of the eukaryote genome.

"Everybody thinks evolution is natural selection, and that's it," Dr. Lynch said. "But it's just one of several fundamental forces."

In a paper accepted for publication in the journal Molecular Biology and Evolution, Dr. Lynch argues that eukaryotes' complexity may have gotten started by chance.

Natural selection is the spread of genes as a result of their ability to raise the odds of survival and reproduction. But when the peculiar features of eukaryotes first arose as accidental mutations, Dr. Lynch argues, they were probably harmful.

Once an intron was wedged into the middle of a gene, a cell had to be able to recognize its boundaries in order to skip over it when making a protein. Some mutations to the intron made it difficult for the cell to recognize those boundaries. If the cell couldn't edit out the intron, it produced a defective protein. If natural selection had been strong in early eukaryotes, all introns would have been eliminated.

Evolutionary biologists have long recognized that natural selection is a matter of probability, not destiny. Just because a mutated gene raises the odds that an individual will reproduce is not a guarantee that it will spread in a population.

Think about flipping a coin. It has 50 percent chance of coming up heads or tails. If you flipped it twice, you wouldn't be surprised to get two heads. But you would be surprised if you flipped it 1,000 times and got 1,000 heads.

Likewise, natural selection works more effectively as populations get bigger. In small populations, it is not so reliable at spreading beneficial genes and eliminating harmful ones.

When natural selection is weak, genes can become more common simply thanks to chance.

The random spread of genes is known as genetic drift. Dr. Lynch argues that genetic drift is much stronger in eukaryotes than in prokaryotes. Several factors are responsible, including the bigger size of eukaryotes. Even a single eukaryote cell may be 10,000 times as large as the typical bacterium. Far fewer eukaryotes can survive in a given space than prokaryotes, leading to smaller populations of eukaryotes.

Dr. Lynch argues that early eukaryotes experienced strong genetic drift. Their population may have shrunk. Natural selection became weak, and genetic drift became strong. Genes that were slightly harmful to the proto-eukaryotes became widespread.

Although these changes may have been caused by genetic drift, they created opportunity for natural selection to create adaptations. Exons could be spliced to create proteins adapted for different jobs. Genes could be switched on in different places, to help build new organs. Complex multicellular organisms - like humans - could emerge.

Natural selection has produced useful adaptations in eukaryotes. If it hadn't, Dr. Lynch said, "we wouldn't be here."

Prokaryotes never got the chance to evolve this complexity because their populations were so large that natural selection blocked the early stages of its evolution. "There was one lucky lineage that became us eukaryotes," Dr. Lynch said.

Dr. Lynch dismisses claims by creationists that complexity in nature could not be produced by evolution, only by a designer.

"In fact, a good chunk of what evolutionary biologists study is why things are so poorly designed," he said. "If we needed a bigger genome, there would be a brighter way to build it."