Neuroscientists long believed you are born with all the neurons you will ever have — in 2025, researchers found newly formed neurons in the brains of adults as old as 78, overturning decades of settled thinking

For most of the twentieth century, the rule seemed simple. You are born with all the neurons you will ever have, and the adult brain makes no new ones. In July 2025, a team at the Karolinska Institutet in Sweden reported finding the cells that give rise to new neurons in the adult human hippocampus, in people as old as 78.

It is a real and important result.

The claim that it “overturns decades of settled science” is the part to handle with care, because the science had not, in fact, been settled for some time.

The old rule, and the long argument

The original dogma goes back to the founders of neuroscience, who treated the adult brain as essentially fixed. That view held for decades, but it had been under pressure since at least 1998, when researchers found signs of new neurons in the adult human hippocampus, a region central to memory.

The argument ran back and forth from there. In 2013, a group that included Jonas Frisén used carbon dating of brain tissue to estimate that the adult hippocampus adds hundreds of new neurons a day. Then in 2018 a prominent study reported close to the opposite, finding new-neuron formation undetectable in adults, and the field split. The sticking point was specific: no one had cleanly identified the dividing precursor cells that ongoing neurogenesis would require. Without them, the sceptics had room to doubt.

What the 2025 study found

That is the gap the new work set out to close. Frisén’s team, reporting in Science, examined post-mortem hippocampal tissue from 35 people ranging in age from newborn to 78. Using single-nucleus RNA sequencing, which reads the gene activity of individual cells, together with methods that pinpoint those cells within the tissue, they traced the whole chain: quiescent stem cells, then dividing progenitors, then immature neurons.

The dividing progenitors are the find that matters. They are the missing cell type, the precursors whose absence had kept the question open, and they were present in the adult brains, including the oldest. “We have now been able to identify these cells of origin,” Frisén said, “which confirms that there is an ongoing formation of neurons in the hippocampus of the adult brain.”

What it does and does not show

It is worth being clear about the limits, because the headline can run ahead of them.

The new neurons are forming in the hippocampus, specifically a part of it called the dentate gyrus, not across the brain as a whole. The old rule still largely holds elsewhere; this is one region, not a general rejuvenation. The study also found large differences between people, with some adults carrying many of these progenitor cells and others almost none, so it is not a uniform feature of growing older. And identifying the cells is not the same as measuring how many working neurons they finally add, or what those neurons do.

What the work does, more precisely than the popular framing, is settle a long and sharp argument in favour of one side. The decades-old dogma was real, but it had been contested for twenty-five years. This is the strongest direct evidence yet that the contested answer was the right one.

Why it matters

The hippocampus is bound up with memory, learning and mood, and it is among the first regions affected in Alzheimer’s disease. If it keeps generating neurons into old age, that is a thread worth pulling, both for understanding healthy ageing and for conditions where the region falters. The wide variation between individuals is its own question, because it is not yet known why some adult brains hold on to these cells and others do not.

None of that is a treatment, and it would be a mistake to read it as one. It is a clearer map of what the adult brain is actually doing.

What to watch

The next step is for other groups to reproduce the progenitor-cell finding with the same modern tools, since reproducibility is precisely what the earlier disagreement lacked. After that, the open questions are what drives the variation between people, and whether the rate of new-neuron formation can be influenced at all. For now, a question that has been argued over since the 1990s has its firmest answer yet.