Stanford Medicine researchers discovered that the human brain evolved as two independent organs rather than a single unified structure. Published on September 18, 2026, in Nature Neuroscience, the findings reveal that the forebrain and hindbrain arise from completely distinct progenitor cells, overturning a foundational model of embryonic development.
For centuries, the human brain has been viewed as a contiguous organ sharing a single point of origin during embryonic development. That long-standing model is now overturned by a Stanford Medicine-led research team, which established that the human brain actually consists of two distinct neural systems that evolved independently over hundreds of millions of years.
Evolutionary Origins in Ancient Neural Systems
Tracing the anatomy back through evolutionary time reveals the ancient roots of this dual structure. The research team identified the same two-origin pattern in chickens, zebrafish, and acorn worms—tiny ocean-floor organisms sharing a distant common ancestor with humans. Jellyfish, which diverged from human lineages roughly 600 to 700 million years ago, possess 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. 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.
Kyle Loh, via Stanford
The more primitive hindbrain, situated at the base of the skull, governs automatic survival functions including respiration, sleeping, heartbeat regulation, and hunger urges, alongside facial, tongue, and throat muscles essential for swallowing and speech. Meanwhile, the forebrain enables complex capabilities such as mathematics, poetry, and abstract thought.
Overcoming Decades of Laboratory Frustration
For decades, developmental biologists struggled to generate functional human hindbrain neurons in laboratory settings, a roadblock that severely hindered investigations into brain stem disorders. Previous attempts typically tried coaxing forebrain and midbrain progenitors into hindbrain cells—an outcome now proven biologically impossible because those precursor cells lack the capacity to make the switch.
Previous attempts to make hindbrain neurons likely tried to coax forebrain and midbrain progenitors into hindbrain cells, which our study shows is not possible.
Rayyan Jokhai, via Stanford
By shifting focus to the earliest stage of embryonic shape formation known as gastrulation, the team successfully directed human pluripotent stem cells to become working hindbrain motor neurons.
Therapeutic Implications for Spinal Muscular Atrophy and ALS
The discovery provides a viable laboratory model to investigate degenerative conditions affecting the brain stem, where obtaining living tissue has historically proven nearly impossible. Conditions such as spinal muscular atrophy and amyotrophic lateral sclerosis involve the gradual failure of hindbrain neurons.
Spinal muscular atrophy stands as a leading genetic cause of death among children under one year of age, while amyotrophic lateral sclerosis typically strikes adults between the ages of 40 and 70. Both diseases eventually strip patients of their ability to swallow and breathe. The ability to culture these specific neurons in a dish offers an unprecedented pathway for mapping disease pathology and testing future regenerative therapies.