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> 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 ne

by xg15 3y ago

> 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.

Terr_ 3y ago
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.
- deleted 3y ago
  [deleted]
- dahfizz 3y ago
  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.
  - snek_case 3y ago
    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.
    - tsimionescu 3y ago
      > 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.
  - Terr_ 3y ago
    > 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.
    - samplatt 3y ago
      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.
- civilitty 3y ago
  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.
deleted 3y ago
[deleted]
crazygringo 3y ago
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.
- shevis 3y ago
  > 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.
  - crazygringo 3y ago
    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 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.
    - civilitty 3y ago
      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.
wheelerof4te 3y ago
"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.
timmaxw 3y ago
> 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.
- overgard 3y ago
  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
  - timmaxw 3y ago
    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.
- tsimionescu 3y ago
  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.

by xg15 3y ago

Terr_ 3y ago

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.

deleted 3y ago
[deleted]
dahfizz 3y ago
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.
- snek_case 3y ago
  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.
  - tsimionescu 3y ago
    > 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.
- Terr_ 3y ago
  > 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.
  - samplatt 3y ago
    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.
civilitty 3y ago
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.

deleted 3y ago

[deleted]

crazygringo 3y ago

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.

shevis 3y ago
> 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.
- crazygringo 3y ago
  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 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.
  - civilitty 3y ago
    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.

wheelerof4te 3y ago

"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.

timmaxw 3y ago

> 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.

overgard 3y ago
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
- timmaxw 3y ago
  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.
tsimionescu 3y ago
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.