Among the molecules of mental life, they are finding signs of an evolutionary struggle for survival.
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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."
Not so long ago, scientists were certain that genes dictated everything about the brain. But when researchers successfully analyzed the complete human genome three years ago, they discovered that it contained only 25,000 genes — not the 100,000 they had predicted.
Indeed, less than 3% of the genome contained functional genes.
There wasn't nearly enough information in them to account for so many different brain cells and synapses.
Something else had to be at work. But what?
At the Salk Institute, Gage, 53, was consumed by the mystery of the new neurons he had discovered.
In the brain's unexpected ability to renew itself, he saw the potential for repairing brain damage from maladies such as Alzheimer's disease or spinal cord paralysis.
Gage — boyish, unfailingly affable, with a trim, sand-colored mustache and a wave of blond hair that crested over a high forehead — was among the most influential neuroscientists of his generation. He orchestrated his laboratory's research efforts the way an impresario manages an opera company, artfully matching the ambitions of 30 scientists to questions that best challenged their abilities.
Gage deployed tools all but unheard of a generation ago — computerized gene micro-arrays, automated gene sequencers, genetically engineered animals.
His working arrangements were also at the cutting edge.
One section of his Salk laboratory was set aside for experiments with stem cells from human embryos, where work could proceed independent of federal funding and unencumbered by federal policies that restrict such research.