Stanford Finds the Human Brain Is Two Separate Organs

A Stanford Medicine-led study in Nature Neuroscience concludes the brain forms from two distinct progenitor lineages, upending a decades-old single-origin model.

A Stanford Medicine-led study published in Nature Neuroscience on Sept. 18, 2026, concludes that the human brain is not one organ with a single developmental origin but two distinct organs that arose side by side. Researchers led by senior author Kyle Loh traced the earliest stages of embryonic development and found two progenitor cell populations that never overlap: one expressing the gene Otx2, destined to build the forebrain and midbrain, and another expressing Gbx2, committed solely to forming the hindbrain, or brain stem. "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," Loh said.

For roughly 70 years, developmental biologists subscribed to the idea — traced by StudyFinds to embryologist P. D. Nieuwkoop's 1952 proposal — that a single progenitor cell gives rise to the entire brain. The new work shows instead that two parallel lineages run side by side from gastrulation, the stage when the body first takes shape. According to StudyFinds, lineage tracing in mouse embryos found that of 494 tagged cell clusters, about 63% landed in the forebrain or midbrain, roughly 33% in the hindbrain, and only about 4% straddled the boundary. The finding explains why standard lab protocols kept failing to produce hindbrain cells: as co-first author Rayyan Jokhai put it, earlier attempts likely tried to coax forebrain and midbrain progenitors into hindbrain cells, which the study shows is not possible.

The team examined chromatin — the packaging of DNA that determines which genes are reachable — in the two progenitor populations. Stanford Medicine reports that the anterior neural ectoderm (future forebrain and midbrain) and posterior neural ectoderm (future hindbrain) have fundamentally different chromatin configurations, effectively locking each lineage into its fate like travelers on parallel tracks. In human stem cell tests, StudyFinds reports the split was stark: about 95% of front-type cells became forebrain cells and 82% midbrain, while back-type cells refused those paths at under 1%; conversely, roughly 97% of back-type cells adopted hindbrain identities versus fewer than 2% of front-type cells. The commitment emerged within about two days of stem cell growth across four cell lines.

The dual-origin pattern appears to be ancient. Stanford Medicine notes that jellyfish, which diverged from humans about 600 to 700 million years ago, already carry two separate nervous systems at opposite ends of their bodies. "Our research suggests that evolution took two existing neural systems and pushed them together spatially," Loh said. The forebrain handles language, consciousness and abstract reasoning, while the hindbrain controls breathing, heartbeat, sleep, hunger, and the facial and swallowing muscles — two ancient systems now packaged to operate as one.

The practical payoff is immediate. Stanford Medicine reports the team used the new roadmap to grow functional human hindbrain motor neurons from pluripotent stem cells for the first time — cells that fired action potentials and carried markers of hindbrain segments controlling facial and swallowing muscles. That fills a critical gap: brain stem tissue cannot be taken from living patients, stalling research into spinal muscular atrophy, a leading genetic cause of death in children under 1, and ALS, typically diagnosed between ages 40 and 70, in which patients progressively lose the ability to swallow and breathe. Neuroscience News adds an unexpected link to metabolism: the hindbrain houses the hunger-regulating circuits targeted by weight-loss drugs like semaglutide, so lab-grown tissue could let pharmacologists study those pathways directly.

The team now wants to pin down the developmental origins of the spinal cord and learn exactly how SMA and ALS compromise hindbrain neuron function, with an eye toward regenerative therapies. Caveats remain: StudyFinds reports that evidence the two cell types cannot switch fates comes mainly from lab dishes rather than living embryos, lineage tracing was done in mice, the origin of the cerebellum is unresolved, and a brief-lived single "pan-brain" progenitor cannot be ruled out. Stanford has also filed patent applications related to the neural differentiation methods, per StudyFinds. "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."