Published on September 18, groundbreaking research led by Stanford Medicine reveals that the human brain is not a single, unified organ, but rather two distinct nervous systems that evolved independently over hundreds of millions of years, fundamentally overturning decades of developmental biology models.
Overturning a Decades-Old Developmental Model
For generations, the scientific community operated under the prevailing model that a single progenitor cell early in embryonic development gave rise to the entire human brain. According to findings published in Nature Neuroscience on September 18, that foundational theory is incorrect. The human brain actually consists of two ancient nervous systems packaged together during development: a primitive system handling vital autonomic functions and a newer system responsible for advanced cognition, poetry, and mathematics.
Kyle Loh, PhD, associate professor of developmental biology and senior author of the study, noted the structural reality of this discovery. “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 explained, pointing out that this breakthrough allows researchers to finally grow hindbrain neurons in a petri dish.
Forebrain vs. Hindbrain: Two Evolutionary Pathways
To understand the magnitude of this discovery, one must look at the structural division of the adult brain. The brain traditionally contains three primary regions: the forebrain, the midbrain, and the hindbrain. The forebrain manages higher-level thinking, language, and consciousness. Conversely, the hindbrain—positioned at the back of the skull as the brain stem—regulates automatic survival mechanisms including breathing, heartbeat, sleeping, and swallowing.
During the gastrulation stage of embryonic development, graduate students and co-first authors Carolyn Dundes and Rayyan Jokhai discovered that the hindbrain follows an entirely separate developmental path running parallel to the pathway forming the forebrain and midbrain. Through examining mouse embryos, the team identified two mutually exclusive progenitor cell populations that never overlap from the earliest developmental stages:
- Forebrain and Midbrain Progenitors: Express the Otx2 gene, committing them to higher cognitive functions.
- Hindbrain Progenitors: Express the Gbx2 gene, dedicating them exclusively to building the primitive brain stem.
By analyzing the chromatin and DNA packaging within these distinct cell populations, the Stanford team mapped how differently these two ancient systems are organized right down to the molecular level.
Unlocking New Avenues for Neurodegenerative Disease Research
For decades, laboratories worldwide struggled to generate human hindbrain neurons in vitro. This technical barrier stalled vital research into severe neurological conditions impacting the brain stem. Spinal muscular atrophy (SMA), which stands as a leading genetic cause of death in children under one year of age, stems directly from hindbrain neuron degradation. Similarly, amyotrophic lateral sclerosis (ALS)—commonly diagnosed between ages 40 and 70—attacks both forebrain and hindbrain circuits, eventually robbing patients of their ability to swallow and breathe.
With Stanford’s new capability to culture hindbrain neurons independently, biomedical researchers now possess the necessary cellular models to study these devastating conditions directly. By isolating the distinct developmental origins of these two separate organs, the scientific community moves past a historical roadblock, paving the way for targeted therapies in neuromuscular and neurodegenerative diseases.
