Brain Evolution I: Neuroanatomy - (or Your Brain is Not an Onion!)

Comparative Brain Anatomy

• all vertebrates have a forebrain, midbrain, & hindbrain (same fundamental organization)

  • just have diff. proportions

Maclean’s Triune Brain Theory

1. Reptilian (‘R’) Complex

   1. (amniotes: reptiles, birds & mammals)

   2. shared by reptilian ancestors

   3. Implication: reptiles & birds aren’t that smart

2. Paleo-Mammalian Brain or Limbic System

   1. (all mammals)

   2. shared by all mammls

3. Neo-Mammalian Brain

   1. (primates, other “higher” mammals

   2. aka cerebral cortex

• doesn’t neatly fit into fundamental organization

• additive model

• used to take about functions

The Triune Brain

• brain evolution came in this layering

  • what Maclean was thinking

• Neo-Mammalian “Neocortex”

  • “new”, cerebral cortex

  • evolved recently in primates, and highly developed in humans

  • language, abstract thought, imagination, consciousness

  • “flexible, with almost infinite learning abilities”

• Paleo-Mammalian “Limbic brain”

  • amygdala, hippocampus, thalamus, hypothalamus

    • all mammals evolved this layer

  • all mammals

  • associates emotions (pleasant or threatening) with memories

    • guides behavior

  • “approach or avoidance feelings (often unconscious) that exert strong influence on behavior”

    • "fundamental feelings”

• R-Complex/“Reptilian brain”

  • brainstem, cerebellum, basal ganglia

    • argue thats all reptiles & birds have

  • oldest system found in reptiles, birds, and mammals

  • controlling vital functions

  • “reliable, but rigid, driven by habit and instinct”

    • at core of brain

      • according to theory

  • thought that we have held onto R-brain

Against the Triune Brain (“Your Brain is Not an Onion”)

• was widey disseminated

• not support anymore

• went through many psych textbooks & saw still has model → has been debunked w/ lots of evolution evience

• brain didn’t evolve this way

  • everything evolved

    • nothing is static

    • doesn’t happen in layers

Basic Parts of Vertebrate Forebrains

• Pallium (“cloak”): upper part of forebrain, includes…

  • cerebral cortex in mammals

  • limbic system

  • olfactory bulb

  • shouldn’t see in R-complex

• Striatum (“striped”) (sub-pallium): lower part of forebrain

  • located within basal ganglia

  • in R-complex

• all mammals have this division

• can say that they have different types of neurons in each

• ex: Pigeon brain

  • doesn’t have obvious layering

    • thoughout to have no pallium

Layering in Cerebral Cortex

• only found in mammals

• 6 specific layers → as part of pallium

• each have diff. achitecture & layers

Hox Genes & Vertebrate Brains

• Hox genes → form the fundamental structures - fore-, mid-, hind-brain

• vertebrate brains

  • other animals have pallium, just not organized the same way (w/ layering

• Pallium genes

  • genes mainly expressed in cerebral cortex

• Subpallium genes

  • genes mainly expressed in basal ganglia, striatum

• Mammals

  • pallium became layered

    • in upper upper part

• Birds & Amphibians

  • have pallium genes but no typical layering

Comparing Brains: Hox Genes in Pallim vs Striatum

• human brains are not fundamentally different from other mammals → mammals brains aren’t fundamentally different from reptiles & birds

  • → besides cerebral cortex

• all vertebrate brains are fundamentally similar

• ex: pallium (d), pallium (v), subpallium

  • are expressing genes, just not organized in layers like the cerebral cortex

Transitions: Therapsids, Mammals & Primates

• “Stem” amniotes

  • have amniotic eggs

• Deuterosaurus

  • showing some features that are more mammal-like but still have some reptile ones

• Anicient mammals

  • more true mammals

• Ptilodus & Hadrocrodium

  • relatively small & nocturnal

• w/ intact skull → can recreate brain structure

  • can tell basic architecture

• endocast

  • when animal dies in certain environment, tissues rot/decay slowly

  • sediment gets into creavases → can help “fossilize-like”

  • help determine architecture

Forebrain Expansion: Endocasts of Therapsids & Mammals

• Triassic (~250 mya):

  • early therapsids like Deuterosaurus had “tube-like” forebrains (anterior to posterior), like most reptiles

• Jurassic (~200 mya):

  • early true mammals like Ptilodus show an expanded forebrain (“stretching” in medial to lateral direction)

    • expanding out

    • olfactory driven

• General pattern of brain evolution from reptiles to mammals is characterized by large forebrain expansions → forebrain getting progressively bigger

  • later mammals will show cortical folding (gyri & sulci)

    • not found in reptiles

Forebrain Expansion: Early “Proto” Mammals

• Jurassic: early “true” mammals, Morganucodon & Hadrocodium had brain:body ratios 50% larger than previous therapsids or mammals of their time

• much bigger forebrain

  • little wrinkling

• big brain in proportion to (small) body size

  • there is cost to big brain

    • needs blood, oxygen, nutrients

      • if adaptive → benefits outweigh the costs

Mammalian Brain Evolution

• ex: Canis (dogs)

  • forebrain dominates

    • covers other areas

  • now has folding

• Ex: Hadrocodium → Monodelphis (possums)

  • bigger but not that much difference

• among mammals, some lay (reptilian) eggs (monotrems); eggs develop internally then enter to transfer to pouch (marsupials); use internal placenta (placenta mammals)

Modern Mammalian Brains

• placental mammals

  • characterized by lots of folds

    • but not all species

Gyri & Sulci of Cerebral Cortex: Cephalic Styles

• ex: manatee

  • not a lot fo folding

• ex: dolphin

  • lots of folding

  • massive cerebellum

  • big hemispheres

  • more folded brains → able to pack more neurons into small space (constrained skull)

• can associate, not assume

Layering of Cerebral Cortex

• ex: Dolphin Brain (compared to human)

  • massive cerebellum

  • more wrinkly

  • not direct → more wrinkles ≠ more smart

Cephalic Styles in Mammalian Brains

• Lissencephalic (smoother) vs. Gyrencephalic (more folds/wrinkles) Brains

• ex: common marmoset

  • primate w/ smooth brain → but pretty intelligent

  • compared to gorilla brain that has a lot more wrinkles

• ex A. egg-laying animal

  • brain is more wrinkley

  • thought": wrinkles correspond to sensation in face/tongue

    • mainly processing snesation

• ex B. marsupial

  • brain does not have much folding

• ex C primate but has smooth brain