Stanford scientists find the brain develops from two distinct cell lineages, reshaping brain evolution research and offering new models for ALS and SMA.

Introduction

For centuries, scientists have viewed the human brain as one unified organ that coordinates everything from breathing and heartbeat to memory, language and consciousness. A new Stanford Medicine study is challenging a fundamental assumption about how that organ develops.

Researchers have found evidence that the brain originates from two distinct populations of progenitor cells, rather than from one common developmental source. One population gives rise to the forebrain and midbrain, while the other develops into the hindbrain, which includes the brainstem and controls many essential automatic functions.

The study, published September 18, 2026, in Nature Neuroscience, could change how scientists understand brain development and evolution. More importantly, the discovery has already helped researchers produce functional hindbrain motor neurons from human stem cells in the laboratory, potentially improving research into neurological diseases such as amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA).

Brain Development May Begin on Two Separate Tracks

The Stanford team focused on one of the earliest stages of embryonic development, known as gastrulation. This is a critical period when cells begin taking on specialized developmental identities.

Researchers studying mouse embryos identified two different populations of early brain progenitor cells.

The first population expressed the Otx2 gene and was destined to form the forebrain and midbrain. These regions are involved in functions including consciousness, language, reasoning and other higher-level processes.

The second population expressed the Gbx2 gene and was committed to forming the hindbrain. The hindbrain is responsible for numerous essential functions, including breathing, heartbeat regulation, swallowing and other automatic processes.

According to the researchers, the two populations remained mutually exclusive rather than mixing together and later separating into different brain regions.

This challenges the long-standing model that a single early progenitor population produces the entire brain.

Otx2 and Gbx2 Reveal a Fundamental Developmental Divide

One of the most important findings involved the genetic and molecular identity of the two cell populations.

Scientists examined chromatin, the DNA-packaging system that determines which parts of a cell's genetic material are accessible. They found major differences between the cells destined to form the front and back portions of the brain.

These differences appear to establish the developmental fate of the cells at a very early stage.

In practical terms, cells heading toward the forebrain and midbrain were already following a different biological program from cells destined to become hindbrain tissue.

The finding may also explain why researchers have struggled for years to generate certain hindbrain neurons from stem cells. Earlier approaches often attempted to manipulate cells that were already committed to forebrain or midbrain development.

The Stanford research suggests that the key may be starting with the correct progenitor population much earlier in the developmental process.

This Does Not Mean Humans Have Two Brains

The headline that the brain is "two separate organs" requires important context.

The research does not mean that an adult human has two physically independent brains. The brain remains a connected structure that operates as an integrated system.

Instead, the Stanford researchers are describing two distinct developmental and evolutionary origins within the structure commonly called the brain.

The research also has nothing to do with the popular "left brain versus right brain" concept. The study concerns the developmental distinction between the forebrain/midbrain and hindbrain, rather than the two cerebral hemispheres.

Nature also reported that co-author Kyle Loh described the brain as one organ that is built from two different parts that connect and work together.

An Evolutionary Story That May Go Back More Than 550 Million Years

The researchers did not stop with mouse embryos.

They examined the developmental pattern across several species, including chickens, zebrafish, macaques and acorn worms. The same broad two-origin pattern was observed across these organisms, suggesting that the developmental architecture may have extremely deep evolutionary roots.

The researchers also compared the findings with the nervous systems of more distant animals. Stanford's account notes that jellyfish, which diverged from the lineage leading to humans hundreds of millions of years ago, possess two nervous systems located at different ends of their bodies.

The findings support a hypothesis that ancient nervous systems may have been brought together during evolution and subsequently became integrated into the complex brain found in vertebrates today.

However, the evolutionary interpretation remains a scientific model based on developmental and comparative evidence rather than evidence that ancient animals literally possessed a modern human-style brain divided into two organs.

Stanford Scientists Successfully Grow Hindbrain Neurons

One of the most significant practical outcomes of the research came from stem-cell experiments.

Using the newly identified developmental principles, researchers were able to guide human pluripotent stem cells into functional hindbrain motor neurons.

The laboratory-grown cells demonstrated characteristics expected of genuine hindbrain neurons. They produced electrical activity and expressed proteins associated with hindbrain regions involved in controlling facial and swallowing muscles.

This represents an important step because obtaining and studying specific human brainstem neurons can be extremely difficult.

Researchers can now potentially create these cells in controlled laboratory environments and investigate how they function, how they become damaged and how they respond to potential treatments.

Possible Implications for ALS and SMA Research

The discovery could be particularly relevant to neurological diseases that damage motor neurons.

Amyotrophic lateral sclerosis (ALS) can progressively damage motor neurons involved in movement and essential functions such as swallowing and breathing.

Spinal muscular atrophy (SMA) is another serious motor-neuron disorder and is a major genetic cause of death in infants.

Both conditions can involve neurons associated with the brainstem. Until now, scientists have faced significant challenges in obtaining suitable human hindbrain cells for laboratory research.

Having a method to generate these neurons from human stem cells could allow researchers to build more precise laboratory disease models, test potential medicines and investigate the biological mechanisms underlying these disorders.

The work could also have implications for research into brain circuits involved in hunger and appetite because the hindbrain contains neural circuits involved in regulating these functions.

What the Study Proves and What It Does Not

The findings represent an important change in the understanding of early brain development, but they should not be interpreted as proof that humans literally have two independent brains.

The strongest experimental evidence came from mouse embryos and laboratory-grown human stem cells. The researchers also found related developmental patterns in other species.

That distinction matters because directly observing early human embryonic brain development at the relevant stages is not equivalent to tracing the same process in living human embryos.

The study therefore provides evidence for a two-lineage developmental model while leaving additional questions for future research.

Key Highlights

• Stanford Medicine researchers identified two distinct early brain progenitor populations.
• Otx2-expressing cells develop into the forebrain and midbrain.
• Gbx2-expressing cells develop into the hindbrain.
• The two populations were found to remain separate during early development.
• Researchers identified distinct chromatin configurations associated with the two developmental paths.
• The pattern was observed across species including mice, chickens, zebrafish, macaques and acorn worms.
• The evolutionary pattern may extend back more than 550 million years.
• Scientists successfully generated functional human hindbrain motor neurons from pluripotent stem cells.
• The technique could improve laboratory research into ALS and SMA.
• The discovery concerns developmental origin and evolution, not two physically separate adult human brains.

Why This Discovery Matters

The significance of the research goes beyond rewriting a chapter of biology textbooks.

For decades, scientists have tried to reproduce different types of human neurons in the laboratory. The new findings suggest that success may depend on understanding a cell's developmental identity at a much earlier stage than previously appreciated.

Rather than simply asking how to turn an existing neural cell into a desired neuron, researchers may need to recreate the developmental pathway that produced that neuron in the first place.

That approach could lead to more accurate disease models and potentially improve the testing of future treatments for neurological disorders.

Professional Disclaimer & Third-Party Reference

This article is based on publicly available information from Stanford Medicine, Nature Neuroscience, Nature and other standard third-party science and news references. The research findings and medical implications described above are presented for informational purposes only and should not be interpreted as medical advice, a confirmed treatment, or proof of a future therapy.

Conclusion

Stanford researchers have provided evidence that the brain's front and back regions follow two distinct developmental paths from the earliest stages of embryonic formation. The discovery offers a new perspective on how the brain evolved and may explain why certain brainstem neurons have been so difficult to reproduce in the laboratory.

The ability to generate functional hindbrain neurons from human stem cells could become particularly valuable for studying diseases such as ALS and SMA. While the research does not mean humans literally have two separate brains, it reveals that the organ we know as the brain may have a much more complex developmental history than previously understood.

As scientists continue investigating how these two developmental systems became integrated, the discovery could open another chapter in the study of human brain development, evolution and neurological disease.