Stanford Scientists Found Your Brain Has Two Origins

For centuries, anatomy textbooks treated the brain as one organ. A Stanford Medicine team now says that picture is wrong: the brain actually has two origins, two ancient nervous systems that evolved separately and were later packed into the same skull, according to Stanford Medicine. The finding, published September 18 in Nature Neuroscience, rewrites a model of brain development that researchers had trusted for decades.

The old model held that a single progenitor cell early in development gives rise to the entire brain. The new work shows the front and back of the brain come from completely different starting cells that never mix. The brain's two origins run on parallel tracks from the earliest moments of embryonic life.

That matters because the two halves do very different jobs. The forebrain handles language, consciousness and abstract reasoning, the parts of thinking that feel most human. The hindbrain, also called the brain stem, keeps the body alive without asking permission: it controls breathing, sleep, heartbeat, hunger, and the face, tongue and throat muscles used for speech and swallowing. Understanding the brain's two origins means understanding why those halves resisted decades of lab work.

The experiment that found the split

The breakthrough came from studying gastrulation, the embryonic stage when the body first takes shape. Graduate students Carolyn "C" Dundes and Rayyan Jokhai, the study's co-first authors, discovered that the hindbrain follows its own developmental path in parallel with the pathway that builds the forebrain and midbrain. It never branches off from it. Senior author Kyle Loh, an associate professor of developmental biology at Stanford, said the front of the brain arises from a totally different progenitor cell than the back.

Working with developing mouse embryos, the team identified two progenitor populations and showed they are mutually exclusive from the earliest stages. One population expresses a gene called Otx2 and is destined to become the forebrain and midbrain. The other expresses Gbx2 and is committed to forming the hindbrain. With two origins, those two cell groups never overlap.

The team then examined chromatin, the DNA packaging that decides which genes a cell can access. The anterior neural ectoderm, the future forebrain and midbrain, and the posterior neural ectoderm, the future hindbrain, carry fundamentally different chromatin configurations. Those differences lock each progenitor into its fate, like travelers on parallel tracks that never cross. The result reframes basic neurodevelopment: the brain's two origins mean it was two systems long before it was one organ.

Why the hindbrain resisted the laboratory

Despite its importance, scientists had struggled for decades to grow human hindbrain neurons in a dish. The new study explains the frustration: previous attempts had been trying to coax forebrain and midbrain progenitors into becoming hindbrain cells, which the discovery of the brain's two origins rules out. Researchers were asking one cell type to become something it was fundamentally incapable of becoming.

Armed with the corrected starting point, the team succeeded where others had failed. They coaxed human pluripotent stem cells into functional hindbrain motor neurons in the laboratory, the first time anyone had done so. The lab-grown neurons fired action potentials and produced proteins that identify the hindbrain segments controlling facial and swallowing muscles, the hallmarks of authentic hindbrain cells.

That unlocks research that was previously near impossible. Scientists cannot take brain stem tissue from living patients, so diseases of the brain stem have been studied mostly from the outside. Growing the cells in a dish gives researchers a working model of the exact neurons that fail. The discovery of the brain's two origins turns a decades-old dead end into a usable tool, and similar painstaking rethinks of lab technique have paid off in other studies of the developing body, such as the adolescent health study that tracked thousands of teens over years.

A merger more than five hundred million years old

The team also traced the split deep into evolutionary history. They found the same two-origins pattern in chickens and zebrafish, and remarkably in acorn worms, tiny ocean-floor creatures that share a distant common ancestor with humans. Jellyfish, which diverged from humans roughly six to seven hundred million years ago, carry two nervous systems at different ends of their bodies. The brain's two origins, in other words, are older than backbones.

Loh said the research suggests evolution took two existing neural systems and pushed them together spatially, keeping a primordial construction even though a single unified organ would probably be more efficient. The brain with two origins is a merger that never fully merged, and it has been running that way since before vertebrates existed.

What it means for disease

The most immediate payoff is in brain stem disorders. Spinal muscular atrophy is a leading genetic cause of death in children under one year of age. ALS, usually diagnosed between the ages of forty and seventy, affects both the forebrain and the hindbrain. In both disorders, certain hindbrain neurons gradually stop working, and patients lose the ability to swallow, which can cause pneumonia when food or liquid reaches the lungs, and eventually the ability to breathe. A recent research explainer on recurrent infection showed how mapping a disease's hidden pathways can open new treatment angles, and the same logic applies here.

There is also an unexpected link to obesity treatment. The hindbrain contains circuits that regulate hunger, which is exactly how weight-loss drugs such as semaglutide work. A clearer map of hindbrain development could sharpen that whole class of therapies. The researchers next want to pin down the developmental origins of the spinal cord and learn precisely how SMA and ALS compromise hindbrain neurons.

Researchers from the California Institute of Technology and the University of California, San Francisco contributed to the study. The deeper lesson is about method: in stem cell biology, the field tends to fixate on producing the final cell type, but the Stanford team found the answer by going back to the earliest stage of development instead. The brain's two origins were hiding in plain sight at the very beginning, waiting for someone to look at the start rather than the finish.