According to research published Sept. 18 in Nature Neuroscience by Stanford Medicine, the human brain is not a single unified organ that develops from one shared origin, but is instead built from two distinct ancient nervous systems that evolved separately over hundreds of millions of years.
How Two Ancient Nervous Systems Form the Human Brain
For decades, standard neuroscience models held that the entire forebrain, midbrain, and hindbrain trace back to a single early progenitor cell population. According to Stanford Medicine associate professor of developmental biology Kyle Loh, the new findings overturn that premise. The human brain actually combines two separate systems. One system builds structures for vital automatic functions like breathing and heartbeat regulation, while the other produces regions responsible for language, mathematics, abstract thought, and reflection on our own existence.
Did you know? According to Stanford Medicine researchers, jellyfish diverged from humans about 600 to 700 million years ago and possess two nervous systems positioned at different ends of their bodies, pointing to a very ancient evolutionary split.
Early Embryonic Divergence and Gene Expression
The split occurs during gastrulation, one of the earliest stages of embryonic development when the basic structure of the body begins taking shape. Graduate student and co-first author Rayyan Jokhai, alongside co-first author Carolyn Dundes, discovered that the hindbrain follows its own pathway from the beginning in parallel with the forebrain and midbrain rather than emerging as a later branch. Studying mouse embryos, the team identified two different populations of brain progenitor cells. One population expresses the Otx2 gene to form the forebrain and midbrain, while a second population expresses the Gbx2 gene to form the hindbrain. Chromatin packaging further confirmed that anterior and posterior neural ectoderm cells commit to separate developmental paths from the start.
This developmental divide explains a long-standing roadblock in neuroscience: why scientists struggled for decades to create human hindbrain neurons in a lab. Previous attempts likely tried to coax forebrain and midbrain progenitors into hindbrain cells, a cellular fate they were never capable of adopting. By recognizing the separate pathway, the Stanford team successfully guided human pluripotent stem cells into functional hindbrain motor neurons that produced electrical signals. According to Stanford Medicine, this breakthrough creates new opportunities to investigate severe brain stem disorders, including spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS).
Evolutionary Origins Spanning 550 Million Years
To trace how far back this dual origin extends, the research team examined chickens, zebrafish, and acorn worms. Looking across more than 550 million years of evolutionary history, the data showed the same two-origin arrangement. “Our research suggests that evolution took two existing neural systems and pushed them together spatially,” Loh said. While a single-origin organ might appear more efficient, human biology relies on this primordial way to make the brain as two separate pieces.
Frequently Asked Questions
What is the main discovery of the Stanford Medicine study?
According to Stanford Medicine researchers, the human brain develops from two separate progenitor cell populations rather than a single unified origin, effectively combining two ancient nervous systems into one organ.
How does this discovery impact ALS and SMA research?
By identifying the distinct developmental pathway of the hindbrain, scientists can now grow functional human hindbrain motor neurons in a petri dish. This provides a new model to study diseases like spinal muscular atrophy and amyotrophic lateral sclerosis that affect the brain stem.
What genes determine the split in early brain development?
Research shows that progenitor cells expressing the Otx2 gene form the forebrain and midbrain, while cells expressing the Gbx2 gene develop into the hindbrain during early embryonic gastrulation.
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