Stanford University researchers published a study in Nature Neuroscience on September 18, revealing that the human brain is technically built from two distinct organs that evolved independently over hundreds of millions of years. Led by developmental biologist Kyle Loh, the team found that the forebrain and hindbrain emerge from completely separate progenitor cells rather than a single evolutionary path.
Mapping Separate Embryonic Progenitor Cells
Conventional biological models long assumed that the brain developed as a single unified entity from one core cellular population. Graduate student and first author Rayyan Jokhai and co-researcher Carolyn Dundes analyzed early embryonic gastrulation phases in mouse models to test this assumption. They mapped two distinct progenitor populations that do not overlap from the earliest stages observed. The first population expresses the Otx2 gene and forms the forebrain and midbrain, while the second population expresses the Gbx2 gene and develops into the hindbrain.
This division stems from fundamentally different chromatin configurations—the DNA, RNA, and protein mixtures packaging long DNA molecules. Because these configurations are distinct right from the start, the hindbrain follows a separate, parallel development track rather than branching off the forebrain path. Jokhai explained that previous laboratory efforts failed to grow hindbrain neurons because they likely tried to force forebrain and midbrain progenitor cells to change their identity.
Cultivating Functional Motor Neurons in the Lab
Armed with this developmental mapping, the Stanford team successfully transformed human pluripotent stem cells into functional hindbrain motor neurons in a laboratory setting. These lab-grown cells successfully fired action potentials and produced region-specific protein markers associated with controlling facial muscles and swallowing. Orbitindonesia.com reported that this advance directly impacts research into severe neurological conditions like spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS).
For decades, studying diseases that degrade the brainstem remained severely restricted. Hindbrain tissue samples are nearly impossible to harvest from living patients, and lab cultivation had hit a wall. SMA stands as a leading genetic killer of children under one year old, while ALS typically strikes adults between ages 40 and 70. Both diseases damage the hindbrain and disrupt automatic functions like breathing and swallowing.
Tracing Animal Evolution
The research team traced these dual developmental pathways across multiple species to understand how ancient organisms assembled their nervous systems. They identified similar divergent pathways across a range of animals including zebrafish, chickens, and acorn worms, covering a history of evolution. Even more distant creatures like jellyfish, which split from the human lineage long ago, possess two separate nervous systems located at opposite ends of their bodies.
Kyle Loh noted that evolution effectively pushed two pre-existing, separate nervous systems into close spatial proximity until they operated nearly as a single organ. This ancient merger allowed the hindbrain to manage essential life-support functions like breathing, heart rates, sleep, and hunger. Meanwhile, the forebrain gained the evolutionary freedom to experiment and develop higher-level cognitive traits, creative expression, and memory systems.
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