A mouse's brain immune cells finish maturing in about three weeks. The human equivalent takes four to eight years.
That gap, reported by scientists at Columbia's Zuckerman Institute in a Neuron study published July 28, 2026, is the first demonstration that human microglia follow the same unusually slow developmental clock long documented in human neurons. And the researchers traced it to a specific culprit: SRGAP2, one of several dozen genes that exist in duplicate copies only in humans.
The implication is that the cells long cast as the brain's janitorial and security staff were also swept up in whatever made human brains distinctive.
A Gene Ten Times More Active Where Nobody Expected It
Franck Polleux's lab has been working on SRGAP2 for more than 15 years. Its earlier finding concerned neurons: human-specific copies of the gene increase the number of synapses a neuron forms and make those synapses mature very slowly, producing cells with denser, stronger connections to their neighbors.
Then lead author Carlos Diaz-Salazar checked where the human-specific duplicates were most abundant. They turned up in microglia at nearly ten times the level found in neurons.
"So the question was, 'Why on Earth is this gene so active in microglia?'" Polleux was recalled in a Zuckerman Institute announcement.
Microglia make up 5-10% of brain cells. For decades, the assumption was that they patrolled for pathogens and cleared debris. Work over roughly the last 20 years has complicated that: microglia also help neurons decide which synapses to keep and which to discard during development, and they can turn the responsiveness of circuits up or down.
If a gene controlling developmental tempo in neurons is even more active in the cells that sculpt those neurons' connections, the timing of the two processes becomes the interesting question.
Four to Eight Years Versus Three Weeks
Using experiments in mice and in human cells, the team showed that the human-specific SRGAP2 duplicates are what stretch human microglial maturation to four to eight years. Remove SRGAP2 from microglia, and they mature faster. Mouse microglia, which lack the human duplicates, wrap up in roughly three weeks.
The pattern has a name. Neoteny describes prolonged, delayed development, and the human brain is an outlier among mammals in how long it takes to finish developing. That extended timetable is widely thought to underlie human cognitive capacity, because a brain still under construction remains open to being shaped by experience.
What the new work adds is that neurons are not doing this alone.
"This gene helps control the developmental tempo of neurons, and nature has also selected it to control the development of microglia that are so crucial to neuron development, so they are in sync during development," said Diaz-Salazar, now a researcher at the Hospital del Mar Medical Research Institute in Barcelona.
Synchronization is the operative idea. If microglia matured on a mouse-like schedule while human neurons took years, the cells pruning synapses would finish their job before the synapses they were meant to refine had formed.
The Claim the Study Does Not Make
The finding is striking, and it is also easy to overread.
Nothing here demonstrates that slower microglial maturation causes higher intelligence. The paper's title refers to neotenic features of microglia maturation and their impact on synaptic development, which is a narrower claim: SRGAP2 duplicates slow these cells down, and that slowing affects how synapses develop. An earlier preprint version of the work framed it the same way.
Linking a cellular timetable to cognition requires a chain of evidence that does not yet exist. Human intelligence is not a trait any single gene or cell type has been shown to produce, and comparative claims about brain evolution rest on inference rather than experiment.
Diaz-Salazar's own phrasing was appropriately hedged. He said the slow development "may help" human microglia influence the brain in ways that enable human cognitive abilities.
The evolutionary framing is a hypothesis that the data make more plausible. It is not a result.
Why Neurologists Are Paying Attention
The disease angle may prove more consequential than the evolutionary one.
Microglia have become central to research on neurodevelopmental and neurodegenerative disorders over the past decade. Their involvement in synaptic pruning connects them to conditions where circuit wiring goes awry, and their inflammatory capacity connects them to Alzheimer's disease and related conditions.
If human microglia operate on a fundamentally different developmental schedule than the mouse microglia used in most laboratory research, that is a practical problem for translating findings.
"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," Polleux said.
A four-to-eight-year maturation window also overlaps with early childhood, a period when several neurodevelopmental conditions become apparent. Whether that overlap is meaningful is unresolved, and the study does not test it.
The team's stated next step is mechanistic: working out precisely how SRGAP2 slows development in neurons, microglia, and other brain cells.
The work was supported by a Revson Senior Fellowship in Biomedical Sciences, a career award from the National Institute of Neurological Disorders and Stroke, and an award from the NOMIS Foundation. The authors reported no competing interests.
Key Questions Answered
What are microglia? They are the most abundant immune cells in the brain, making up 5 to 10 percent of brain cells. They defend against pathogens, clear damaged neurons, help prune synapses during development, and can adjust the responsiveness of circuits.
What did the study find? Human-specific duplicate copies of the gene SRGAP2 cause human microglia to take four to eight years to mature. Mouse microglia mature in about three weeks.
Does this prove that slow microglia make humans smarter? No. The researchers say the slow tempo may help microglia shape the human brain, but no experiment here links microglial timing to intelligence.
Why does synchronization matter? SRGAP2 duplicates also slow the maturation of human neuronal synapses. Matching timetables mean microglia are still shaping circuits during the years when human synapses are forming.
What is neoteny? It is prolonged, delayed development. The human brain matures unusually slowly compared with that of other mammals, and this extended period is thought to support advanced cognition.
What is the medical relevance? Microglia are implicated in neurodevelopmental and neurodegenerative diseases. Knowing that human and mouse microglia mature on very different schedules matters for interpreting animal research.
