A 51-year-old executive came to see me about what he called a focus problem. He had been working on it for two years and had the full apparatus: a nootropic stack, dual n-back training, an unbroken meditation streak, and more magnesium than any one person needs. His memory testing was normal. What he had not mentioned until I asked was that he woke four or five times a night, his resting heart rate had climbed eight points over eighteen months, and he had stopped feeling hungry until about two in the afternoon.
Everything he had been treating sits in the front of the brain. Everything that was actually wrong sits in the back of it.
A study published in Nature Neuroscience on September 18 gives that distinction a developmental basis I did not expect to see in my career. A Stanford Medicine team led by Kyle Loh, PhD, with graduate students Carolyn Dundes and Rayyan Jokhai as co-first authors, reports that the human brain is not one organ. It is two, and they have separate embryonic origins.
What the Stanford Team Actually Found
For decades the field has held that a single progenitor cell early in development gives rise to the whole brain, with the forebrain, midbrain, and hindbrain all branching off from a shared starting point. Loh's group examined mouse embryos during gastrulation, the stage at which the body plan is first laid down, and found two progenitor populations rather than one. One expresses a gene called Otx2 and is committed to becoming the forebrain and midbrain. The other expresses Gbx2 and is committed to becoming the hindbrain, which is the brain stem. The two populations never overlap. They are mutually exclusive from the earliest moment at which they can be identified.
The team then looked at chromatin, which is how a cell packages its DNA to determine which genes stay accessible and which are folded away out of reach. The anterior neural ectoderm, which becomes the forebrain and midbrain, and the posterior neural ectoderm, which becomes the hindbrain, carried fundamentally different chromatin configurations, and those differences lock each progenitor into its respective fate. These are not two branches of one tree. They are two tracks running in parallel that never cross.
Then the researchers asked how old the arrangement is. The same two-origin pattern appears in chickens, in zebrafish, and in acorn worms, which are seafloor animals whose last common ancestor with us lived roughly 550 million years ago. Jellyfish, which diverged from our lineage 600 to 700 million years ago, carry two nervous systems at opposite ends of the body. Loh's interpretation is that evolution took two existing neural systems and pushed them together in space. We did not grow one brain. We inherited two and packed them into the same skull.
Why Nobody Could Grow a Brain Stem in a Dish
This finding explains a failure that has frustrated stem cell biology for thirty years. Laboratories could make forebrain neurons from human pluripotent stem cells reliably. Hindbrain neurons were close to impossible. The reason, on this evidence, is that investigators were trying to coax forebrain and midbrain progenitors into becoming hindbrain cells, and the chromatin state of those progenitors makes that conversion unavailable. They were asking a cell to do something it is constitutionally incapable of doing.
Starting instead from the correct lineage, the Stanford group produced functional human hindbrain motor neurons for the first time. The cells fired action potentials and expressed the segment-identifying proteins of the hindbrain regions that drive the muscles of the face, tongue, and throat.
That matters a great deal for two diseases. Spinal muscular atrophy is a leading genetic cause of death in children under one year of age. ALS is usually diagnosed between forty and seventy. In both conditions, hindbrain motor neurons gradually stop working, swallowing fails, aspiration pneumonia follows, and eventually breathing fails. Until now there has been no way to obtain brain stem tissue from a living patient and no way to grow the cells, so these diseases have been studied largely by inference. There is now a model, and I expect the next decade of brain stem research to look very different from the last one.
What the Hindbrain Actually Controls
Most people reading this do not have ALS. The reason this finding belongs in a conversation about brain performance is what the hindbrain does in a healthy person.
It runs breathing and the chemoreceptor drive behind it. It runs heart rate and the autonomic balance that produces heart rate variability. It houses the arousal nuclei that build sleep architecture, which means it determines whether you get consolidated slow-wave sleep or six hours of fragments. It holds the hunger and satiety circuitry, which is the reason semaglutide works at all, since that drug acts on hindbrain circuits rather than on anything resembling willpower. And it supplies the vagal outflow I have written about before in the context of the gut-brain axis.
Now consider how brain optimization is usually sold. Nootropics, focus protocols, memory training, executive function apps, cognitive games. Every one of those is aimed at the forebrain. An entire industry has built itself around the organ in front and treats the organ in back as background noise.
My patient was asking a forebrain to run on a brain stem that had stopped regulating sleep, autonomic tone, and appetite timing. No supplement addresses that, and no amount of cognitive training will fix it, because the two organs have different biology and they respond to different interventions.
What a Brain Stem Workup Looks Like
When a patient tells me their thinking has slowed, I want forebrain data, and I want hindbrain data as well. That means sleep architecture rather than a sleep score from a ring, which is a distinction I have written about in the context of what a single night of EEG can reveal. It means heart rate variability measured properly, resting and orthostatic vital signs, an assessment of breathing during sleep, and an honest accounting of appetite timing and thermoregulation. Those measurements are part of what our Intensive Brain Health Program is built to collect, and a surprising share of cognitive complaints turn out to hinge on them.
If you are working on this on your own, the useful reframe is to stop asking what will make your thinking sharper and start asking whether the system underneath your thinking is still regulating. Fragmented sleep, a resting heart rate that has drifted upward over a year, appetite that has gone strange, air hunger, and unexplained lightheadedness on standing are all brain stem signals, and they deserve attention before a single nootropic does.
In the Neuroeconomy, cognition is the asset that produces nearly everything else a person builds, and this study is a reminder that the asset rests on a second, older organ that keeps it alive and regulated. That organ has been left out of the brain health conversation for as long as the conversation has existed, and after 550 million years of carrying us, it has earned a seat at the table.
Illustration created with AI image tools.
This article was drafted with the assistance of AI writing tools, then reviewed, edited, and approved by Dr. Sean C. Orr, M.D., who holds full editorial responsibility for its content.
References
- Dundes, C., Jokhai, R., et al., & Loh, K. M. (2026). Two parallel neural ectoderm progenitors contribute to the developing brain. Nature Neuroscience, published online September 18, 2026. https://www.nature.com/articles/s41593-026-02433-7
- Conger, K. (2026, September 18). Human brain is two separate organs, Stanford Medicine-led research finds. Stanford Medicine News Center. https://med.stanford.edu/news/all-news/2026/09/two-separate-brains.html