Human brain is two separate organs, Stanford Medicine-led research finds

What a way to oversell the discovery.

The reality is:

    * Different brain regions have different functions: ancient knowledge.
    * Different regions contain distinct cell types and molecular programs: also well established.
    * Anterior and posterior brain structures may trace back to fundamentally separate progenitor lineages that were independently specified very early in evolution and development: that's the interesting new result.

> They identified two different brain progenitor cells. One, which expresses a gene called Otx2, is destined to become the forebrain and midbrain. The other, which expresses a gene called Gbx2, is committed to forming the hindbrain. They showed that these two cell populations never overlap; they are mutually exclusive from the earliest stages of development.

Very cool. Never thought the brain could have two completely different ancestors

The really exciting thing in this article is that this research led them to a new technique for growing brain stem cells in vitro, which has been very difficult until now. If that isn't being oversold, it's big news. That's going to make future research into diseases like ALS a lot easier.

bioRxiv preprint July 2025 [0]

The copyright holder for this preprint is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under a CC-BY 4.0 International license.

[0] https://www.biorxiv.org/content/10.1101/2025.07.02.662771v2

PDF is free to download.

Fun fact: 80% of the brain's neurons are in the cerebellum, not the cortex.

In other words: only <20% of pur neuron budget goes to higher cognition (thinking, language, seeing, hearing). 80% is spent on motor skills.

> The new research finding shows that the human brain consists of two ancient nervous systems cleverly packaged together — a more primitive part that regulates our hearts’ beating, our breathing and other functions, and another that makes us distinctly human, capable of poetry, mathematics and wondering about our own origins.

I'm not sure the actual article in Nature Neuroscience [1] actually says that. I remember "Your Brain Is Not an Onion With a Tiny Reptile Inside" [2] from a few years ago, but I'm not sure where the dissonance comes from: my own misunderstanding, the press release using a popular but flawed model to communicate with laymen, or these two articles being in direct contradiction with each other?

[1]: https://www.nature.com/articles/s41593-026-02433-7 [2]: https://journals.sagepub.com/doi/10.1177/0963721420917687

Related https://www.newscientist.com/article/2589739-our-brain-evolv... https://news.ycombinator.com/item?id=49755533

I don’t think the medical community has believed as the article claims for nearly a century.

Most animal brains have three very different organs in the brain: the cerebrum, cerebellum, and the limbic system (which itself is further divided into numerous differentiated types of tissues)

Hm, I’m of two minds on this one.

I thought this was widely known, at least for males, where the second one can be found in the lower half of the body.

All the comments I see are about is it two organs, or a composite organ.

Surely the interesting thing is the ability to grow hindbrain cells in vitreo. That's what's potentially going to lead to the cure of lots of things, not whether 1=2.

So, what we colloquially call 'reptile brain' turns out to be somewhat accurate.

What is the difference between saying that the brain is two organs, as in the title, or a "composite organ" as in the actual paper? What makes an organ a single autonomous organ?

that's funny when I was in junior school, 15 years ago, I said in the classroom during science class, that we have 2 brains. everyone including my teacher were laughed at me.

I can see how one of these is the gut feelings origin, and other bodily feelings.

We are going to have to deprecate an idiom or two. "I'm in two minds about something" now gives the wrong meaning.

The title is factually incorrect and is not what the research actually found:

https://www.nature.com/articles/s41593-026-02433-7

Title: "Two parallel neural ectoderm progenitors contribute to the developing brain".

"Hence, we postulate the brain is a composite organ emanating from two lineage-restricted progenitors; these dual progenitors may be evolutionarily conserved across 550 million years from hemichordates to mammals."

It's cool, but the headline is really pretty misleading.

The new result seems to be that researchers have identified different progenitors for fore/midbrain vs. hindbrain and have found that they can be identified by some specific developmental gene markers. That's genuinely cool.

However, organs are not typically defined by a single tissue characterized by a progenitor that can be labeled with a single gene marker. I suppose it's fine if on a functional and anatomical level you want to split the fore/midbrain and hindbrain into two different "organs", but whether you think of the brain as 1 organ or 2, it really doesn't change much in terms of how you would think about these regions.

It isn't at all surprising that different regions of the brain have localized functions; some of the earliest evidence of this was how focal injuries (like strokes) have characteristic deficit patterns depending on brain region affected, and obviously there has been a lot of work since then further showing the organization, function and development of different brain regions.

And that's why we oftentimes talk about "brains" in the plural

Wintermute and neuromancer!

I went to see Michael Gazzanigga talk about hemispheric separation some years ago. Fascinating. The two hemispheres can operate separately.

A bicameral mind cool

This isn't peer reviewed, just a press release. 10 other "papers" will be published in the next 2 weeks counterfinding anything in this post.

Science moves forward yet again with a few backward steps.