Researchers say the human brain is really two organs fused together

For centuries the brain has been treated as one organ. A Stanford Medicine-led team says that is wrong: the front of the brain and the back of the brain arise from two entirely separate progenitor cells that evolved independently over hundreds of millions of years, then were packaged together into what looks like a single structure. The findings were published in Nature Neuroscience on September 18, 2026.

“We’ve shown for the first time that the front of the brain arises from a totally different progenitor cell than the back of the brain,” said Kyle Loh, PhD, associate professor of developmental biology at Stanford and senior author of the study. “Our discovery means that we can now grow neurons from the back of the brain, the hindbrain, in a petri dish and study their functions.”

Two progenitor cells that never overlap

The adult brain has three main regions: the forebrain, midbrain and hindbrain. The forebrain and midbrain handle higher-level functions such as language, consciousness and abstract reasoning. The hindbrain, often called the brain stem, controls the automatic functions that keep a person alive, including breathing, heartbeat regulation, sleep and the muscles of the face, tongue and throat used for speech and swallowing. For decades the accepted model held that a single progenitor cell early in embryonic development gave rise to the entire brain, forebrain and hindbrain alike.

Graduate students Carolyn Dundes and Rayyan Jokhai, the study’s co-first authors, overturned that model by studying mouse embryos during gastrulation, the stage when the body first takes shape. They identified two distinct progenitor cell populations that emerge at the same time but never mix: one expressing a gene called Otx2, destined to become the forebrain and midbrain, and another expressing a gene called Gbx2, committed only to the hindbrain. Examining the chromatin, the DNA packaging that determines which genes a cell can access, the team found the two progenitor populations locked into fundamentally different configurations from the earliest moments of development, according to the Stanford Medicine account of the research. “Previous attempts to make hindbrain neurons likely tried to coax forebrain and midbrain progenitors into hindbrain cells, which our study shows is not possible,” Jokhai said.

Growing brain-stem neurons in a dish for the first time

Once the researchers understood that the hindbrain runs on a separate developmental track, they used human pluripotent stem cells, which can form any cell type in the body, to coax cells down that specific path. The result was functional hindbrain motor neurons grown in the laboratory for the first time, cells that fired action potentials and produced the proteins that mark authentic hindbrain segments controlling facial and swallowing muscles. That is a capability scientists had struggled with for decades, according to the study published in Nature Neuroscience, whose full findings describe two parallel lineage-committed progenitors contributing to the developing brain.

The inability to grow hindbrain tissue has long blocked research into diseases that specifically damage that region, including spinal muscular atrophy, a leading genetic cause of death in children under age 1, and amyotrophic lateral sclerosis, usually diagnosed between ages 40 and 70. In both diseases, hindbrain neurons progressively fail, patients lose the ability to swallow, and eventually lose the ability to breathe. “Now we have a model to better understand these devastating diseases, and work toward regenerative therapies for them,” Jokhai said. “This is a very exciting new frontier in brain research.”

The same split shows up 550 million years back

Loh’s team did not stop at mice and human stem cells. Looking across the evolutionary tree, they found the identical two-origin brain pattern in chickens, zebrafish and acorn worms, tiny seafloor-dwelling creatures that share a distant common ancestor with humans from roughly 550 million years ago. Jellyfish, which diverged from the human lineage even earlier, about 600 to 700 million years ago, carry two separate nervous systems at opposite ends of their bodies, a structural echo of the same division. “I was surprised at our findings because the word ‘brain’ implies a contiguous organ that likely has a singular origin,” Jokhai said. “But even 500 million years ago, there were these separate neural systems, which now almost operate as one, which is very cool.”

Loh framed the pattern as an evolutionary compromise rather than a design flaw. “Our research suggests that evolution took two existing neural systems and pushed them together spatially,” he said. “Having the brain as one organ would probably be more efficient, but we rely on this primordial way to make the brain as two separate pieces.” An unexpected practical link surfaced in the same reporting: the hindbrain houses circuits that regulate hunger, which is the same mechanism that weight-loss drugs like semaglutide act on.

What comes after the discovery

The research drew on collaborators from the California Institute of Technology and the University of California, San Francisco, and was funded by a wide list of sources including the National Institutes of Health, the National Science Foundation, the California Institute for Regenerative Medicine and the Spinal Muscular Atrophy Foundation, among others. Loh’s group says its next step is extending the same developmental tracing to the spinal cord, and using the newly grown hindbrain neurons to study exactly how spinal muscular atrophy and ALS disable them, work that could not have started without first establishing that the hindbrain has its own separate developmental origin.

The finding is also a reminder that a discovery framed as settling an old debate rarely closes every question it touches. Loh’s team has shown where the forebrain and hindbrain come from separately; it has not yet shown how, over hundreds of millions of years of evolution, those two systems learned to coordinate so seamlessly that the split was invisible until researchers went looking for it in embryonic chromatin.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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