LA JOLLA, Calif. — Alysson Muotri was looking for brain cells that glow in the dark.
With growing frustration, the 31-year-old Brazilian cancer biologist stared through his microscope at slides of brain tissue for any evidence his experiment had succeeded. His eyes ached.
Maria Marchetto, 28, took pity on her husband. Let me look, she said. In a darkened room at the Salk Institute for Biological Studies here, she began to scrutinize the tissue samples for firefly flecks of fluorescent light.
Together, the couple stalked an elusive sequence of DNA hidden in the heredity of every human cell. The wayward strand appeared to seek out developing brain cells and, like a virus, arbitrarily alter their genetic makeup.
In this way, it might be partly responsible for the infinite variety of the mind.
In debates over creationist doctrines, evolutionary biologists often are hard-pressed to explain how nature could make something as intricate as the human brain. Even Alfred Wallace, the 19th century biologist who discovered natural selection with Charles Darwin, could not accept that such a flexible organ of learning and thought could emerge by trial and error.
No two brains are exactly alike, despite their overall anatomical similarity. Each brain changes throughout a lifetime, altered by experience and aging. Even the simplest mental activities, such as watching a moving dot, can involve slightly different areas in different people's brains, studies show.
Underlying every personal difference in thought, attitude and ability is an astonishing variety of brain cells, scientists have discovered.
Some neurons fire only when they perceive a straight vertical line, others when they are exposed to a right angle. Some respond to the emotions in a facial expression or to social cues. Others retain a memory long after conscious recollection has faded.
To respond so selectively to experience, each of these cells must vary incrementally from its neighbors, as singular as a face in a crowd.
Yet what could generate such diversity?
If Muotri's suspicion was correct, a peculiar string of biochemicals caused the billions of neurons in each person's brain to develop in distinctly different ways, so that even identical twins could develop minds of their own.
Muotri and Marchetto searched hundreds of slides for any sign that the DNA sequence had altered brain cells. Each tissue sample took an hour to analyze under ultraviolet light.
When Marchetto closed her eyes, she could see the glowing afterimage of neurons.
The spidery cells, she would say later, crawled through her sleep.
In every human brain, there are as many neurons as there are galaxies in the known universe -- about 100 billion, drawn from 10,000 different cell types and woven into a three-dimensional tapestry, with threads of neural interconnections that number in the trillions.
Each one is tinder for the spark-of-life experience.
Memories are made of this gray matter. So are inspiration and imagination.
Electrochemical currents of intellect and emotion race though living labyrinths of neurons at 200 mph. When they are blocked, diverted or damaged, abilities atrophy. Personality disintegrates.
By exploring the life and death of these cells, researchers hope to learn how biochemistry becomes thought.
Among the molecules of mental life, they are finding signs of an evolutionary struggle for survival.
In the womb, brain cells increase at a rate of 250,000 a minute. The total doubles after birth. By age 3, a child's brain, on average, has twice as many neurons and neural connections, and is twice as energetic, as an adult's.
Already, a ruthless competition is underway.
Throughout developing brain circuits, neurons and synapses vie for sensory stimulation -- the electricity of touch, vision, taste, hearing and smell. Some thrive, while others atrophy for want of exposure to life's raw experience, to be eliminated at a rate of thousands per second.
By adulthood, more than half the neurons a brain possessed in early childhood will have died.
For many years, scientists were convinced that the brain quickly lost its ability to produce new neurons. But in the last decade, independent research teams at the Salk Institute led by Fred W. Gage and at Princeton University by Elizabeth Gould showed that even middle-aged minds generated thousands of new neurons every day in areas crucial to learning and memory.
Inside the Darwin machine of the brain, therefore, the survival struggle of neurons and synapses lasts a lifetime.
In this competition, the forces of variation and selection that shape a species also sculpt each brain, neuron by neuron, creating the biological truth of individuality.
"The neurons are never identical," Muotri would say. "They are all slightly different."