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We’re closer than ever to mapping the entire brain to the microscopic level. Hundreds of neuroscientists across the world recently characterized more than 3,000 human brain cell types as part of the National Institute of Health’s BRAIN Initiative Cell Census Network, publishing almost two dozen papers in four Science journals today. This super-focused attention to detail could unlock many mysteries surrounding that complex organ, such as what happened in our brains to distinguish us from other primates. 

“This is the first large-scale, detailed description of all the different kinds of cells present in the human brain,” says Rebecca Hodge, an assistant investigator at the Allen Institute in Seattle who co-authored multiple studies in the paper package. Her hope is that this brain atlas provides a community resource for scientists to explore how the wide variety of brain cells contribute to health and disease. This first suite of studies shows three main ways the brain map can be used for biology and medicine.

An evolving brain

A human brain atlas can teach us about our evolutionary history. One study published today in Science used single-nucleus RNA sequencing to measure the gene expression of individual brain cells in humans and five other primate species, including chimpanzees and gorillas. In this method, scientists pull out individual cells from a piece of tissue, break them open to expose the genetic messengers inside, then use tags akin to tiny barcodes to identify that material. “This is the main technology used in some of these papers that are coming out, and it’s a technique that’s only been around for the past 10 years,” Hodge says. Getting this genetic profile allows researchers to group clusters of cells into specific types. 

Our cells’ composition and organization is similar to those of our close relatives. However, the biggest differences seemed to occur in a brain region called the middle temporal gyrus, which is involved in processing semantic memory and language. Humans had higher numbers of projecting neurons in this area compared to other species. What’s more, the researchers highlighted a difference in gene expression that promoted synaptic plasticity, which is the ability of neurons to strengthen brain connections. This feature is an important component for learning and memory, and it might explain how humans developed complex cognitive skills.

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