Count Your Neurons, Count Your Blessings: How a Big Brain Made Us Human

Be thankful this season not just for family and food, but also for the long evolutionary path that produced our unusually large, neuron-rich brains and the long, slow lives they support.

Brains, neurons and a human story

Thanks in part to ancestors who began processing and cooking food many generations ago, humans became the one species able to sustain a remarkably high number of cortical neurons. Those extra neurons are linked to a prolonged childhood and, for many people, lifespans that can approach a century — traits that make human life distinct.

More cortical neurons mean more than abstract intelligence: they let us seat four generations around a table to trade gossip and advice, turn fragmented facts into reliable knowledge, and learn the subtle social rules that smooth group life. They also underlie the technologies and communication practices that bring people together in person and across vast distances.

Counting neurons to tell a better evolutionary story

Early in my career I discovered that many people still believed the myth that we use only 10% of our brains. That’s not true: we use all of our brain, though different regions are active at different times for different tasks. The confusion was partly fueled by repeated textbook claims — for example, the oft-repeated figure of 100 billion neurons — that were more estimates than direct measurements.

To settle the question, I developed a reliable, rapid method to count brain cells and spent 15 years collecting and analyzing brains. The resulting data revealed real patterns: primates concentrate more neurons in the cerebral cortex than most other mammals, regardless of total brain size. Some large-brained mammals, like elephants, have large but relatively sparse neurons; primate neurons are smaller, and bird neurons smaller still, allowing some birds to pack many cells into small brains. But none outnumber the largest primate cortices — ours included.

Why cortical neurons matter

We focus on cortical neurons because the cortex enables cognition that goes beyond simple stimulus–response: it lets species learn from the past, plan for the future, and build culture. Neurons are the building blocks of information processing; the more cortical neurons a species has, the greater its cognitive flexibility and potential complexity — regardless of raw brain size.

My work also shows a clear link between cortical neuron number and life history. Species with more cortical neurons take longer to mature — like assembling a mansion from a truckload of Lego pieces takes longer than building a small house from a few blocks — and, for reasons not yet fully understood, tend to live longer. In that sense, gaining cortical neurons is a two-for-one evolutionary payoff: more mental capacity paired with more lifetime to develop and use it.

The energetic cost — and the cooking solution

Those extra neurons come with a high energy cost. If humans had remained strictly on a raw-food diet like other primates, we would need to spend more than nine hours each day foraging and eating to sustain roughly 16 billion cortical neurons — time that would leave little room for invention or reflection. Our great ape relatives, who rely largely on raw foods, have at most half as many cortical neurons and spend over eight hours a day obtaining food.

Human ancestors found a crucial workaround: food processing and cooking. Stone tools and controlled use of fire made food easier to digest and calories easier to extract. With more calories available in less time, successive generations could support larger cortices. As cortical neuron counts rose, childhoods lengthened and adult lifespans increased, creating an extended window for learning from parents, grandparents and elders. Culture and technology could therefore accumulate and compound across generations.

What it means for us today

Modern human life is shaped by the interaction between neuron-rich brains and cultural practices that transmit knowledge across generations. Being born with many cortical neurons gives individuals the potential for long, slow learning; whether that potential is realized depends on social institutions — families, schools, communities — that convene people to share experience, stories of success and failure, and collective knowledge.

By Suzana Herculano-Houzel, Vanderbilt University. She is the author of The Human Advantage: How Our Brains Became Remarkable.