Scientists Say the Human Brain Might Be Made of Two Distinct Nervous Systems That Got Squished Together

Two separate types of progenitor cells give rise to the front and back of the organ, according to a new study. The findings can help researchers study human diseases that affect the hindbrain, since its nerve cells have previously been difficult—if not impossible—to make in the lab

Margherita Bassi | Daily Correspondent

September 22, 2026 11:54 a.m.

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The brain is an incredible organ, powering basic functions like breathing while also giving rise to human consciousness. That might be because it’s actually made of two nervous systems that are squished together in our skulls, according to new research.

In a study published September 18 in the journal Nature Neuroscience, researchers reveal that the front of the brain, called the forebrain, and the back of the brain, or the hindbrain, form independently during embryonic development. The findings hint that our ancestors had two distinct nervous systems that, over millions of years of evolution, became tightly packed.

“Our research suggests that evolution took two existing neural systems and pushed them together spatially,” study co-author Kyle Loh, a developmental biologist at Stanford University, tells New Scientist’s Alice Klein. “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.”

The forebrain and hindbrain have different responsibilities. While the forebrain processes higher-level cognitive processes, such as language and abstract reasoning, the hindbrain, also called the brainstem, oversees essential automatic functions like hunger cues, sleep and regulation of the heartbeat. The latter area also controls the muscles of the face, tongue and throat.

The brainstem is extremely important and is implicated in diseases like spinal muscular atrophy, a genetic condition that can affect infants, and amyotrophic lateral sclerosis, also known as Lou Gehrig’s disease, typically diagnosed in mid- to late-life. But scientists have long struggled to coax human pluripotent stem cells—which can become almost any type of cell—into turning into hindbrain nerve cells, hampering efforts to study this part of the brain.

So Loh and colleagues took a close look at mouse embryos roughly a week after conception. RNA analysis revealed two populations of brain progenitor cells. One expresses a gene called Otx2 and becomes the forebrain and another part called the midbrain, and one expresses a gene called Gbx2 and becomes the hindbrain. The two cell types also package their DNA differently, the team found, further hinting that they always remain independent of each other.

These distinct progenitor cells were seen in chickens, zebrafish and sea creatures called acorn worms, too, suggesting the trait is evolutionarily conserved. On the other hand, jellyfish have two nervous systems on different sides of their bodies. Their ancestors diverged from the lineage that produced humans around 600 million to 700 million years ago, a bit before the ancestors of the marine worm split off from our lineage roughly 550 million to 600 million years ago. That hints the dual nervous systems became jam-packed at some point in between.

What’s more, the discovery points to why previous attempts to create human hindbrain nerve cells, or neurons, in the lab have been unsuccessful, study co-author Rayyan Jokhai, a developmental biologist at Stanford, says in a statement. Scientists probably tried to generate them from forebrain and midbrain progenitors, he says, which is not possible, according to the study. Jokhai, Loh and their colleagues used their newfound understanding to become the first team to make human pluripotent stem cells turn into working hindbrain motor neurons.

Not everyone agrees that the brain originated as two nervous systems. Alex Pollen, a neurobiologist at the University of California, San Francisco who didn’t work on the research, tells Nature’s Lynne Peeples that it’s hard to prove that the two distinct cell populations they found don’t come from a single, short-lived type of progenitor cell.

Developmental biologist Cecilia Moens, of the Fred Hutchinson Cancer Center, isn’t convinced the two progenitor cells’ destinies are fully determined, reports the outlet. Moens, who wasn’t involved in the study, says that she has witnessed cells swap “fates,” even just for brief periods of time.

Nevertheless, the study still represents a crucial step on the journey toward understanding and treating conditions that affect the brainstem. “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,” Loh says in the statement.

Margherita Bassi is a freelance journalist and trilingual storyteller. Her work has appeared in publications including BBC Travel, Discover magazine, Live Science, Atlas Obscura and Hidden Compass.