Human Brain Composed of Two Separate Organs, Stanford Study Reveals
New research suggests the brain evolved by merging two ancient nervous systems, a discovery that could advance understanding of neurological diseases.
The human brain, long considered a single organ, is actually composed of two distinct nervous systems, according to a new study from Stanford Medicine. This finding challenges conventional understanding and may open new pathways for studying neurological conditions.
Scientists have identified two primary components within the brain. One, a more primitive system, manages automatic functions essential for survival, including breathing, sleeping, regulating heartbeat, and hunger. The second system is responsible for higher-level cognitive functions such as language, consciousness, and abstract reasoning.
"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," said Dr. Kyle Loh, an author of the study. "Our discovery means that we can now grow neurons from the back of the brain, the hindbrain, in a petri dish and study their functions."
The adult brain is typically described as having three main regions: the forebrain, midbrain, and hindbrain. The forebrain is associated with complex thought processes, while the hindbrain controls vital autonomic functions and aspects of speech and swallowing. Historically, generating human hindbrain neurons in laboratory settings has proven difficult, limiting research into diseases affecting the brain stem, such as spinal muscular atrophy and amyotrophic lateral sclerosis (ALS).
The research team investigated developing mouse embryos to understand brain development, observing that the hindbrain follows a distinct developmental path from other brain regions. Cells in the forebrain and midbrain express the gene Otx2, while hindbrain cells express Gbx2. Researchers also noted fundamental differences in DNA packaging between these regions.
Dr. Rayyan Jokhai, a co-author, explained that previous attempts to create hindbrain neurons likely failed because they tried to convert forebrain or midbrain cells, which is not possible according to their findings. This new understanding has enabled the creation of functional hindbrain motor neurons in the lab for the first time.
This breakthrough could accelerate research into debilitating diseases like ALS. "Now we have a model to better understand these devastating diseases, and work toward regenerative therapies for them," Dr. Jokhai stated. "This is a very exciting new frontier in brain research."
To trace the evolutionary origins of this two-part brain structure, the researchers examined evolutionary history over 550 million years. They found evidence of a similar two-origin brain pattern in chickens, zebrafish, and even acorn worms, creatures that share a distant common ancestor with humans.
"Our research suggests that evolution took two existing neural systems and pushed them together spatially," Dr. Loh added. "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."