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.
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.
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.
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.
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.
The correct view is that while the neocortex is indeed mostly new, the “older” more central structures were modified throughout evolution.
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.
> 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.
That they're not static doesn't necessarily mean that they aren't largely conserved.
Importantly modern reptiles and humans have experienced the same amount of evolution. There’s vaguely 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 for example is larger than most creatures entire brain, it’s a wildly different structure than you find on a frog.
Every scientific model and analogy is flawed in some way, but many are still useful.
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.
As I have mentioned before these structures are very different, 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.
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
So yes frogs and humans both have a visual cortex, because we both have eyes. It really doesn’t explain anything beyond that point.
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.
It would seem enough people did to warrant this person to write about the topic.
> 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 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.
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.