A dogma nobody had tested carefully
The claim under dispute is a basic one about the adult human brain: does it ever manufacture new neurons, or is a person’s full complement of brain cells fixed at birth or shortly after? For much of the twentieth century, neuroscience treated the fixed-supply answer as settled fact, an assumption strong enough that even direct evidence against it was set aside for years rather than pursued. Peter Eriksson and Fred Gage’s 1998 report, published in Nature Medicine, reopened that question by identifying immature nerve cells forming in the human hippocampus, the brain structure most associated with memory. Their finding, and the string of animal work that had led up to it, argued that the adult brain retains at least some capacity to generate new neurons rather than only losing the ones it started with.
Evidence ignored for twenty years
The path to the 1998 human finding ran through animal research that had itself struggled for acceptance. Joseph Altman reported evidence of new neurons forming in the rat cerebral cortex in 1962 and in the hippocampus in 1963, but the scientific community largely ignored those findings for roughly two decades. Interest revived in the 1980s when Shirley Bayer and Michael Kaplan demonstrated the same process in rats and Fernando Nottebohm showed it occurring in birds, prompting a wave of renewed investigation through the 1990s. Against that backdrop, Eriksson and Gage’s human study, identifying immature nerve cells in adult hippocampal tissue, extended a pattern already established in other species to people, though the material available does not detail exactly what technique they used to identify those cells.
The human claim arrives
What has held up, at minimum, is that adult neurogenesis is real and substantial in other mammals and in birds, a point the later human controversy has not seriously challenged. In humans, a 2013 study using carbon-14 dating, a method that estimates the age of cells from radioactive carbon absorbed during nuclear testing decades earlier, put the rate of new hippocampal neuron formation in adults at roughly seven hundred cells a day, a striking and specific figure that briefly looked like it had settled the matter in humans as well as in animals. That estimate stood as the field’s working number for several years and is still cited as one of the more carefully derived pieces of evidence for ongoing human neurogenesis.
A number: roughly 700 a day
What has not held up cleanly is the confidence that followed the 2013 estimate. A 2018 paper by Sorrells and colleagues in Nature argued that adult hippocampal neurogenesis drops sharply after childhood to undetectable levels in adults, directly challenging both the 1998 finding and the 2013 estimate. Their central objection was methodological: a protein marker, doublecortin, widely used to identify newly formed neurons, had in some cases been shown to stain cells with an already mature structure, meaning studies relying on it may have mistaken existing mature cells for new ones. A 2022 assessment reinforced that scepticism and raised a further question, whether findings from rodent neurogenesis research, which is well established, can be extended to human biology at all, leaving the field without an agreed answer even for its most basic claim.
A 2018 paper says otherwise
Whether the adult brain can grow new neurons bears directly on how much recovery from injury, ageing decline or psychiatric illness might realistically be helped by encouraging that process. Fred Gage’s laboratory has gone on to study how factors such as physical exercise appear to influence hippocampal neurogenesis in animal models, work whose relevance to humans depends heavily on which side of the ongoing dispute turns out to be right. If adult human neurogenesis is real and substantial, therapies aimed at boosting it become a plausible route to treating memory loss or depression. If it is largely absent, as the 2018 paper argued, then treatments premised on stimulating new human neuron growth are targeting a process that may not meaningfully exist, a distinction with real consequences for where research funding and clinical hope get directed.
Still unresolved, five years on
Yes, and it is worth reading precisely because it resists a tidy conclusion. This is not a case of an old idea being quietly confirmed or debunked; it is a genuine, ongoing disagreement between researchers using different methods, on a question that sounds like it should have a simple yes-or-no answer. Anyone who has absorbed the confident popular version, that the adult brain definitely grows new neurons, or the equally confident opposite, will find the actual state of the evidence considerably less settled than either claim suggests. The value here is less a fact to take away than an honest picture of how a major neuroscience question can remain open for more than two decades of active investigation.