Human brains are extraordinary. They allow us to compose symphonies, solve complex equations, and ponder our own existence. Yet the biological blueprint behind this cognitive prowess has remained one of science’s most tantalizing mysteries. Now, researchers at Columbia University’s Zuckerman Institute have uncovered a crucial piece of the puzzle—and it involves a gene that exists only in humans.
The study, published in the journal Neuron, reveals that human microglia—the brain’s resident immune cells—mature at an astonishingly slow pace compared to other animals. While mouse microglia reach maturity in roughly three weeks, human microglia take four to eight years . This extended developmental timeline, the researchers discovered, is orchestrated by human-specific copies of a gene called SRGAP2.
The Backstory: A 15-Year Quest
The story begins over a decade ago, when the Polleux laboratory began investigating what makes the human brain unique from an evolutionary standpoint. Their focus settled on SRGAP2, one of several dozen genes that underwent duplication specifically in the human lineage after our ancestors diverged from the chimpanzee lineage.
Previous work by Dr. Franck Polleux revealed that human-specific copies of SRGAP2 do something remarkable in neurons: they increase the number of synapses—the connections between brain cells—while simultaneously slowing their maturation. This combination produces a denser, stronger network of neuronal connections, potentially enhancing the brain’s ability to process and store information.
But in the new research, lead author Dr. Carlos Diaz-Salazar noticed something unexpected. Human-specific copies of SRGAP2 were nearly ten times more abundant in microglia than in neurons. “So the question was, ‘Why on Earth is this gene so active in microglia?’” recalled Dr. Polleux .
The Molecular Brake: A 2024 Discovery
The 2026 microglia study builds on a crucial 2024 finding that revealed how SRGAP2 actually slows synapse development at the molecular level. In a paper published in Cell Reports, researchers led by Nora Assendorp identified a protein called CTNND2 (also known as delta-catenin) as a major partner of SRGAP2 in the brain.
Think of synaptic development like a car. In non-human species, SRGAP2A acts like a gas pedal, pushing synapses to mature quickly. When humans evolved their unique SRGAP2C copy, it began blocking SRGAP2A—essentially lifting off the gas. But that wasn’t enough to explain the dramatic slowdown. The 2024 study revealed the missing piece: with SRGAP2A suppressed, levels of CTNND2 rise, acting like a brake that further slows synaptic maturation.
CTNND2 is particularly intriguing because mutations in its gene have been linked to severe intellectual disability, autism, and epilepsy . The 2024 study showed that CTNND2 works by helping another protein called SYNGAP1 accumulate at synapses. SYNGAP1 is also associated with intellectual disability when mutated. Together, these proteins form a molecular network that controls the pace of brain development—and in humans, this network is uniquely tuned to go slow.
This finding was significant because it provided the first direct molecular link between human brain evolution and genes involved in neurodevelopmental disorders. As Prof. Pierre Vanderhaeghen noted, “the same genes that are involved in the evolution of the human brain also have the potential to modify the expression of specific brain diseases”.
The Neoteny Connection
Both the 2024 and 2026 studies fit into a long-standing evolutionary theory called neoteny—the idea that humans retain juvenile traits into adulthood. Compared with other mammals, human brains develop over an unusually prolonged period, and scientists have long suspected that this extended developmental window underlies our advanced cognitive abilities .
The research suggests that SRGAP2 may coordinate this slow developmental pace across multiple cell types, ensuring that neurons and microglia mature in synchrony. This synchronization appears to be critical: by keeping microglia in a prolonged immature state, the brain may allow for extended periods of synaptic refinement and circuit optimization.
The 2024 Cell Reports study demonstrated that when CTNND2 is knocked down in human neurons, synapses mature too quickly—producing an overabundance of connections at a stage when they should still be forming slowly . This mirrors what scientists observe in certain forms of autism spectrum disorder, where accelerated synapse development may disrupt normal circuit formation .
The significance of these findings extends beyond evolutionary curiosity. Microglia have recently been implicated in neurodevelopmental disorders such as autism and schizophrenia, as well as neurodegenerative conditions like Alzheimer’s disease. If human microglia possess unique characteristics that distinguish them from those of other animals—as this study suggests—then understanding those differences could be crucial for developing accurate disease models and effective treatments.
“We want to understand all the elements that help make up the human brain to understand what makes us unique from an evolutionary standpoint,” said Dr. Polleux. “Because scientists have recently found that microglia are involved in neurodevelopmental disorders and neurodegenerative diseases, our findings get us a step closer to understanding what makes human microglia special in the context of brain diseases” .
The researchers are now working to uncover the precise molecular mechanisms through which SRGAP2 promotes neoteny in microglia and other brain cells. They also aim to determine how this prolonged developmental window functionally impacts synaptic connectivity and information processing .
As Dr. Diaz-Salazar noted, the findings reveal that “neotenic microglial maturation modifies the timing of synaptic development, linking microglial developmental programs to the timing of circuit formation” . In other words, the slow pace of brain development isn’t just a side effect of complexity—it may be the very mechanism that enables it.