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Comment by xg15 | Hacker News Reader

> and 3) that evolution modifies existing brain structures in addition to adding new layers.

As I understood it, their point was that evolution does not add new layers, evolution of the brain always happens by modifying the existing structure.

Which is why a "stratified" view of the brain with evolutionary older layers near the center and newer layers near the surface is incorrect.

The implications of this belief then led to incorrect assumptions about both humans and (non-human) animals: That the human brain is an "animal brain plus something else" and therefore automatically superior - and inversely that animal brains are "human brains minus something" and therefore automatically inferior. The article argues against both positions.

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Is the neocortex not an outer layer? That is not present in reptiles? So how is the stratified model incorrect as a high-level structural categorization?

The triune model doesn't claim our brain grows additively in rings like trees. It's merely an observation about the primary evolutionary origins of the three parts. Just those three.

And the superior/inferior characterization is not part of the triune model. You are free to interpret it that way if you want, but it's not part of it, so it's not a rebuttal.

> Is the neocortex not an outer layer? That is not present in reptiles? So how is the stratified model incorrect as a high-level structural categorization?

Not sure about reptiles, but the author write this about mammals:

> Neurobiologists do not debate whether any cortical regions are evolutionarily newer in some mammals than others. To be clear, even the prefrontal cortex, a region associated with reason and action planning, is not a uniquely human structure. Although there is debate concerning the relative size of the prefrontal cortex in humans compared with nonhuman animals (Passingham & Smaers, 2014; Sherwood, Bauernfeind, Bianchi, Raghanti, & Hof, 2012; Teffer & Semendeferi, 2012), all mammals have a prefrontal cortex.

I also read some interesting papers a few years ago about corvids - particular New Caledonian Crows: Those animals do not have a neocortex and hence were thought incapable of many higher-level cognitive tasks, such as planning, tool use, etc. Turned out the crows were in fact capable of them. One hypothesis I read suggested that, as crow brains are structured differently, another structure may have taken the role that the neocortex has in humans.

So even if it's an anatomical distinction, it's an unreliable indicator of mental capacity.

OK, but again -- none of that refutes the triune brain hypothesis at all.

Literally nobody is claiming that only humans have neocortexes.

Nor is anyone claiming it's an indicator of mental capacity. The recent discoveries about crows were interesting, but that doesn't have anything to do with the fact that the neocortex in mammals plays a particular, functional, well-recognized role.

From the wikipedia:

> The triune brain consists of the reptilian complex (basal ganglia), the paleomammalian complex (limbic system), and the neomammalian complex (neocortex), viewed each as independently conscious, and as structures sequentially added to the forebrain in the course of evolution. According to the model, the basal ganglia are in charge of our primal instincts, the limbic system is in charge of our emotions and the neocortex is responsible for objective or rational thoughts.

We disproved:

- the sequentially added nature

- what these zones are responsible for

So in the end we're only left with a semi-arbitrary split of the brain in 3 zones and none of the hypothised implications. That to me refutes the hypothesis as a whole, and we're keeping the zone naming by sheer inertia.

The refutation is of the idea that it’s a strictly chronological layering, with the old layers inside and intact.

The correct view is that while the neocortex is indeed mostly new, the “older” more central structures were modified throughout evolution.

No one thinks the older structures are static. No one is arguing that. It's a simplified model about the origin.

This argument is akin to saying we're not "newer" apes ala

> The refutation is of the idea that it’s a strictly chronological ordering of species, with the old species still inside and intact. The correct view is that while homosapiens are indeed mostly "newer", the “older” apes were also modified throughout evolution

Obviously.

I mean… the quote from TFA that they are arguing against is

> As Paul MacLean (1964), originator of the triune-brain theory, stated,

>> man, it appears, has inherited essentially three brains. Frugal Nature in developing her paragon threw nothing away. The oldest of his brains is basically reptilian; the second has been inherited from lower mammals; and the third and newest brain is a late mammalian development which reaches a pinnacle in man and gives him his unique power of symbolic language.

And they quote other textbooks that are making claims along these lines too; this is right at the beginning of TFA. So I think you are wrong that “no one thinks that”.

Of course they don’t think the old brains are 100% static but there are claims that they are largely conserved.

MacLean doesn't say the brain is an onion with a tiny reptile inside!

> there are claims that they are largely conserved.

That they're not static doesn't necessarily mean that they aren't largely conserved.

It’s an incorrect and misleading model about the origin. Bat wings and bird wings share many traits but the common answer didn’t fly it’s simply convergent evolution. The brain structures of that same common ancestor is only vaguely related to modern structures in mammals and reptiles.

Modern reptiles, birds, and humans have experienced the same amount of evolution. There’s similar traits, but we don’t call whale flippers hands because that’s what the structures was in some of their ancestors or even what it is in related species. Similarly we don’t call human hands flippers because that’s what the structure started out as even further back and it’s still preforming that function in modern fish.

The human visual cortex is larger than most creatures entire brain, it’s a wildly different structure than you find on a frog which plays a wider role in cognition than just decoding vision.

Putting this particular theory aside the generalization that evolution is a nested hierarchy of structures is pretty obvious on the macroscopic level. Traits like breathing oxygen, having symmetrical bodies, backbones, limbs. These are all examples of structures building on top of one another sequentially and conserved across species. Sure each of these subcomponents continues to specialize but the core functionality remains largely the same.

Going back to your analogy saying visual cortex of frog is wildly different from a human visual cortex. That's similar to saying a frog backbone is wildly different from a human backbone. Sure that's true but there's also a shared common core functionality largely conserved across time

Again the issue is the theory is misleading. Those traits are conserved through evolution largely because they’re useful, but if you look at say owl skulls you find wild asymmetries because that’s more useful even our lungs are very different from each other. Hagfish lost backbones and snakes lost limbs because nothing is sacred to evolution.

So yes frogs and humans both have a visual cortex, because we both have eyes. It really doesn’t explain anything beyond that point. The human visual cortex doesn’t even map to the same structures as it expanded into nearby ones.

>It really doesn’t explain anything beyond that point.

I think taking a macroscopic view like this points at interesting types of analysis. Evolution is analogous to a hill climbing algorithm navigating towards local optima in a probability field. Macroscopic evolutionary trends give a low resolution peek into the underlying probability distributions.

That’s not what this theory is suggesting.

Frogs, birds, etc have very distinct spherical visual cortexes. Humans and squirrels don’t have that kind of spherical structure. So, other than naming where vision is processed the same thing the overlap isn’t particularly strong even at the most basic structural level.

It’s perfectly reasonable to extrapolate from studied organisms to species that share recent common ancestors. But reptiles don’t represent the probability distribution, they represent different results from that distribution under different evolutionary pressures. Extrapolation backwards and our common ancestor was very different from all decedents.

But you could say that whale flippers are 'basically' their hands.

Every scientific model and analogy is flawed in some way, but many are still useful.

How exactly is this theory useful? Yes, frogs and humans process visual information because they both have eyes. That’s about all those structures have in common.

For nearly all of human history everyone thought that those frog vs. human eyes were completely separate and designed by some magical being living up in the cloud.

By noticing that while they have significant differences, those structures also have significant similarities, we were able to correctly answer one of the greatest questions of all time: where did humans come from?

Of all models, this one is particularly useful.

To be useful models must accurately represent some aspect of reality, Reptiles don’t represent the brain of our common assessor particularly well. So this model is just wildly wrong without any predictive power.

Compare the giant spheres used by snakes, bird, frog etc to process visual information and try and map that to a rodent, bat, or human. Hint you’re not going to be able to locate an obvious sphere because the fundamental morphology is just different. You can say we both have areas of the brain devoted to processing vision but so do fruit flies.

Eyes are a good example of conservation and path dependence, development is initiated by a fairly highly conserved gene:

https://en.wikipedia.org/wiki/PAX6#Species_distribution

Being reasonably conserved has minimal predictive power. Yes, knocking it or a closely related gene out out harms eye formation in a wide range of species but it says little about what specific structure ends up being formed.

> No one thinks the older structures are static.

It would seem enough people did to warrant this person to write about the topic.

- The superior/inferior characterize actually is part of the triune model, Maclean's book is replete with language like "advanced" vs "primitive".

- The point of the criticism is not that the neocortex is not a "layer" at all, but that it is not the case that if you were to remove the neocortex layer, you would essentially get the brain of a lower-order animal--but this is what is implied by the triune theory.

> Is the neocortex not an outer layer?

It seems like this is what they are refuting. It is not so much a new layer as it is an evolution of existing structure.

Of course it's a layer. It's a layer that itself is made up of 6 sublayers. This is not up for debate:

https://en.wikipedia.org/wiki/Neocortex

It clearly states:

> The six-layer cortex appears to be a distinguishing feature of mammals; it has been found in the brains of all mammals, but not in any other animals.

What it evolved out of is entirely irrelevant -- everything evolves out of something else in some fashion.

While the neocortex does have distinct layers, the neocortex itself is not “layered” around the rest of the brain - it’s deeply integrated all over the place with the rest of the brain and nervous system. There is no hierarchical relationship between the neocortex and different systems it integrates with.

The reptilian equivalent to the neocortex is the dorsal ventricular ridge which evolved separately and in parallel. This presents two problems to the hypothesis: first the much simpler DVR serves much of the same purpose as the neocortex which was completely unknown at the time and second the most interesting bird species (the smartest ones) often don’t have an equivalent structure at all. There isn’t even a clear relationship between intelligence, complexity, evolutionary age, etc. After 250 million years of evolution any similarities are accidents of random convergence.

Also, it's not clear why we should accept that the brain would develop with locked-in "old strata" to a degree that we do not see in all sorts of other organs and systems.

As much as people joke about having a separate stomach for ice-cream, I've never heard anyone suggest that their "lizard stomach" would handle certain foods.

Because the brain physically, anatomically, has these "old strata". Lizard brains are pretty much just a basal ganglia. Humans have a basal ganglia, and then extra stuff on top. Mammals, and only mammals, have a neocortex "strata" to their brain as well.

I am not qualified to argue for or against the Triune brain, but it seems easy to see why the brain is different from the stomach in this regard.

> Because the brain physically, anatomically, has these "old strata". [...] Humans have a basal ganglia, and then extra stuff on top.

IMO the key is distinguishing between these two ideas:

1. There is a gross anatomical structure that can be linked to ancient ancestors with certain characteristics.

2. Those structures in living creatures are somehow "not really modern" or are unusually tied to the needs or limitations of those ancient ancestors.

Consider your fingers: They originate from fin-bones ~380 million years ago, yet (unlike "lizard brain") nobody talks about possessing "fish fingers" except as a fried food product. We also don't create narratives explaining our finger operations or design in terms of what ancient fish required or were capable of.

The Pterodactylus flying lizards are named literally for their "winged fingers". I suspect that we talk about this stuff when discussing evolutionary biology just fine, it's just that the "lizard brain" is a popsci term that the great-unwashed have latched onto.

In addition to this... Sure, evolution modifies existing structures, but if you compare a cat's heart and stomach to a pig heart and stomach, the difference is not that big, even though the nearest evolutionary ancestor was tens (hundreds?) of millions of years ago. Once a structure is in place and works well, you can certainly tweak it, but it's easier to mostly just keep it.

Humans have a neocortex, but if you play with a cat or dog, you can recognize and understand the emotions they are feeling. Fear, anger, joy, anxiety, relaxation, etc. That suggests the structures responsible for those things in us are probably not that different from them.

> Humans have a neocortex, but if you play with a cat or dog, you can recognize and understand the emotions they are feeling. Fear, anger, joy, anxiety, relaxation, etc. That suggests the structures responsible for those things in us are probably not that different from them.

And yet birds don't have a neocortex, and yet they share all the same behaviors that mammals share (at least in some species). I don't know if there is some creature with a neocortex that doesn't share all of these behaviors, but at least we can see that the neocortex may be sufficient but is not required for a species to have them.

Agreed. Mammals don’t even have the same metabolic strategy as reptiles - we’re endothermic while reptiles can’t even self regulate body temperature. Other systems fundamental to life like our reproductive strategies are also completely different.

The idea that major organs are strictly conserved over 250 million years while something as fundamental as homeostasis drastically diverges is frankly a bit wackadoo.

> evolution of the brain always happens by modifying the existing structure

Hmmm, my impression was that evolution took an existing structure and stuffed a bunch of new functionality into it, making it much larger and more complicated. Although the structure already existed in some form, it didn't have the same function. I think it's reasonable to think of this process as "adding" something, even if the structure itself is not entirely new. And human brains pretty clearly _are_ more intelligent than reptile brains!

Here's an analogy: Ancient organisms had light-sensitive "eyespots", and many of our distant cousins on the evolutionary tree (e.g. present-day flatworms) still have eyespots. Evolution gradually modified these existing structures into eyes. I think it's reasonable to think of eyes as a "new" organ that humans have and flatworms don't; and our eyes are clearly superior to flatworms' eyespots.

Evolution and adding complexity aren't the same thing. For example, evolution can also work in reducing complexity. Animals that spend generations living underground in caves will lose their eye sight over a long enough time period.

Point being its not about adding, it's about adapting

In _this_ case, the case of the human brain, I think it's fair to say that evolution started with the reptile brain and added something (even if that "something" is "new function and complexity" rather than "a completely new physical structure").

I agree there are other cases where evolution removes/simplifies things. Evolution is adaptation, and that adaptation can consist of adding, changing, and/or removing.

Eyes are far superior to eyespots in certain ways (accuracy, for one), but far inferior in others (energy consumption, size, durability). They are newer than the eyespots of the last common ancestor, but they could well be older than the eyespots of some flatworm species alive today.

"As I understood it, their point was that evolution does not add new layers, evolution of the brain always happens by modifying the existing structure."

The human brain is a highly complex set of distinctive organs. Saying there are different types of brains forming the human nervous system is literally what the biology teaches us.

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