At ZME Science, Tudor Tarita tells us that our current brains may have started as two separate organs in different life forms five hundred million years ago:

Stanford-led researchers found that the forebrain and midbrain appear to arise from one embryonic cell lineage, while the hindbrain—in charge of autonomic and motor functions—develops from another. The researchers argue that these two developmental programs may descend from separate neural systems that evolution eventually pushed together.

Stanford Medicine went so far as to describe the finding as evidence that the brain is really “two separate organs.”

“Scientists Say the Human Brain May Have Started as Two Separate Organs That Fused Together,” September 24, 2026

This is not a left brain/right brain thing. It is more like claims for a reptilian brain underlying our mammalian one.

But how would the two brains have merged? “The Stanford team argues that, somewhere along the lineage leading toward vertebrates, evolution may have brought two pre-existing neural systems into closer physical proximity until they became the tightly integrated brain we know today.”

How likely is that biologically? “Ultimately, the study suggests that evolution may have assembled one extraordinarily integrated organ from developmental programs that began separately. And we can even see this ancient boundary as the brain first takes shape.”

If so “evolution” is behaving like a designer, not a random process. The paper at Nature is not open access.

No surprise, there is pushback against this model

At Ars Technica, John Timmer responds in some detail, but here is the gist:

By the time the first cells that are committed to develop into neurons show up, the vertebrate embryo already knows its head from its tail, and its back from its belly …

So if cells in the head make a different collection of signaling molecules from those in the tail, this can transfer positional information to the developing nervous system, telling it where to form the brain and where to form the spinal cord…

In other words, as soon as cells know they’re going to eventually develop into neurons, they know whether they can potentially form part of the hindbrain and not the mid- or forebrain. (This is, roughly, where the idea of “two brains” in the press release comes from.)

“Contrary to some reports, you don’t have two brains,” September 23, 2026

He adds, “While the new work says nothing about how many brains you actually have (it’s one with several distinct regions within it), it provides another piece for the puzzle of how the incredible complexity of our brains gets generated from a flat sheet of cells that, just a few hours before, would have happily developed into our skin instead.”

That’s pretty remarkable engineering and it makes more sense just to leave “evolution” out of it.