Suzana Herculano-Houzel spent most of 2003 perfecting a macabre recipe—a formula for brain soup. Sometimes she froze the jiggly tissue in liquid nitrogen, and then she liquefied it in a blender. Other times she soaked it in formaldehyde and then mashed it in detergent, yielding a smooth, pink slurry.
Herculano-Houzel had completed her Ph.D. in neuroscience several years earlier, and in 2002, she had begun working as an assistant professor at the Federal University of Rio de Janeiro in Brazil. She had no real funding, no laboratory of her own—just a few feet of counter space borrowed from a colleague.
“I was interested in questions that could be answered with very little money [and] very little technology,” she recalls. Even so, she had a bold idea. With some effort—and luck—she hoped to accomplish something with her kitchen-blender project that had bedeviled scientists for over a century: to count the number of cells in the brain—not just the human brain, but also the brains of marmosets, macaque monkeys, shrews, giraffes, elephants, and dozens of other mammals.
Her method might have seemed carelessly destructive at first. How could annihilating such a fragile and complex organ provide any useful insights? But 15 years on, the work of Herculano-Houzel and her team has overturned some long-held ideas about the evolution of the human mind. It is helping to reveal the fundamental design principles of brains and the biological basis of intelligence: why some large brains lead to enhanced intelligence while others provide no benefit at all. Her work has unveiled a subtle tweak in brain organization that happened more than 60 million years ago, not long after primates branched off from their rodent-like cousins. It might have been a tiny change—but without it, humans never could have evolved.