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

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. 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.

Earlier this month, researchers revealed that they created mice with half-human brains. The hybrid animals can help scientists study human neurological conditions like cerebral palsy. For example, low oxygen during development can cause cerebral palsy, but typical mice aren’t vulnerable to hypoxia in the same way people are.

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.