Neurons are more like snowflakes–no two alike–than anyone realized.

Walt Whitman’s famous line, “I am large, I contain multitudes,” has gained a new level of biological relevance in neuroscience.

As we grow, our brain cells develop different genomes from one another, according to new research from Harvard Medical School and Boston Children’s Hospital. The study, published last week in Science, provides the most definitive evidence yet that somatic (post-conception) mutations exist in significant numbers in the brains of healthy people—about 1,500 in each of the neurons they sampled.

The finding confirms previous suspicions and lays the foundation for exploring the role of these non-inherited mutations in human development and disease. Already, the researchers have found evidence that the mutations occur more often in the genes a neuron uses most. And they been able to trace brain-cell lineages based on mutation patterns.

“This work is a proof of principle that if we had unlimited resources, we could actually decode the whole pattern of development of the human brain,” says co-senior investigator Christopher Walsh, MD, PhD, the HMS Bullard Professor of Pediatrics and Neurology and chief of the Division of Genetics and Genomics at Boston Children’s. “These mutations are durable memory for where a cell came from and what it has been up to. I believe this method will also tell us a lot about healthy and unhealthy aging as well as what makes our brains different from those of other animals.” …