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PS2061: The Evolving Brain: Is the Human Brain Special - Coggle Diagram
PS2061: The Evolving Brain: Is the Human Brain Special
Evolution
Fundamental Qs
Science seeks out rational, evidence-based explanations
For human origin & nature
Qs
Where did we come from
Who are we now
Intelligence?
Does it make us different from other species?
What is its basis in brain?
Evolution
Whether bio can explain human origins & intelligence
On the Origin of Species (Darwin, 1859)
Evidence many extant species descendants of ancestral species
Proposed mechanism for evolutionary process
Natural Selection
Traits (now understood in terms of genetics) vary randomly & can be inherited
Evolution a change in genetic composition of population over time
Gradual accumulation of inherited characteristics
Enhance organism's ability to survive & reproduce in specific environment
Process
Traits vary randomly
Can be inherited
Organism's environment applies selectional pressure
Organism's w. advantageous traits survive & reproduce
Heritable traits/genes passed on
Tree of Life
Evolutionary tree
How species split off from common ancestor
Relation between extant & extinct species
As to have as many species in same genus as is there must be extinction
Evolutionary Tree
About 2 million known species
Many millions more not known about
Process of Evolution
Mutation creates variation
Unfavourable mutations hamper reproduction
Are selected out
Adaptive mutations favoured
Are spread through population
Key Words
Homology
Genetics
Homology
Common ancestry between species
Revealed by common physical traits shared by populations
Limited by convergent evolution
Convergent Evolution
Similar environments can induce two relatively independent specieis to evolve similar homology
Two genetically dissimilar species can have very similar homology
Mammals
All mammalian forelimbs have commonly inherited characteristics
Genetics
Similarities between species independently of homology
Through advances in scientific techniques
Humans & Chimpanzee
Human's closest living relative
Share 99% directly comparable sequences DNA
Share 96% sequences DNA when account for deletions & mutations
Types (Streidter, 2005)
Mosaic
Concerted
Mosaic Evolution
Individual neural structures evolved independently of each other
Concerted Evolution
Evolutionary pressures act on whole functional systems
Not just individual brain structures
Compromise several interconnected parts of the brain
Evolution
Individual brain areas not evolve independently
Systems interconnected brain areas evolve together & similarly
Origin of Humans
Descent of Man (Darwin, 1871)
Man descended from less highly organised form
Co-descended w. other mammals of common progenitor
Evolution of Humans
Ancestral primates
Early hominoids
Divergence of hominoids & ape
Hominoids
Modern humans
Machiavellian Brain
Selection pressures that led to evolutionary expansion of brain
Social Brain Hypothesis
Need bigger brains to interact w. bigger social groups
Info processing demands should increase w. no. social relationships
Supporting Neocortex Scales w. Group Size (Barton, 1996)
Relationship between size of neocortex & size of groups typically lived in
After adjusting for overall brain size
Limitation (Dunbar, 1992)
Complexity of social relationship that matters
Not size of social group per se
Human Social Networks (Dunbar et al., 1992)
Neocortex volume by volume of rest of brain
In simian & hominid
Ratio used to predict mean group size
Can be used for extinct hominids
Human Social Networks (Hill & Dunbar, 2001)
Social network size in comtemporary Western society
Based on exchange of Christmas cards
Max network size averages 153.5 individuals
Mean = 124.9 for those explicitly contacted
Values close to group size of 150 predicted for humans on basis of size of neocortex
Grooming (Kudo & Dunbar, 2001)
Primates regularly groom each other
Important social activity that promotes bonding between group members
Grooming cliques vary w. species
Acorss range of species neocortex size varied w. size of grooming cliques
When corrected for overall brain size
Tactical Deception (Byrn & Corp, 2004)
Social primates often deliberately mislead members of group
To achieve goals
Specific example of cog ability related to social brain hypothesis
Frequency across diff species scaled w. neocortical size
Special About Humans
Research Question
What special about humans & their cog abilities
Tech advances allowed to gather unprecedented knowledge about neural basis of human cognition
Despite this know little about what abilities specific to humans
Cannot understand specialisations of human cog & its neural basis by simply studying brain
Also have to compare human brain & abilities w. those of other species
Specific to Human Species (Preuss, 2000)
Human Brain Size
Human brain 3x larger than hypothetical primate of same body size
Tells us there is something special about human brain compared w. other species
But don't know what is special about human brain
Absolute Brain Size (Falk, 2006)
Indication of brain volume is cranial capacity
Space inside skull normally occupied by brain
Humans have largest cranial capacity
Compared to diff species of primates
Endocast
Can apply same cranial capacity measure as extanct species to extinct species
Same type of data collected means can directly compare
But does not reveal sucal & gyral anatomy of brain
Fossil record showed hominid brain volume increase dramatically over last 4 million years
Relative Brain Size (Jerison, 1973)
Proposed brain size should be corrected for body size
Brain size known to scale w. body size
Human brain still much larger than would expect
Encephalisation Quotient
Ratio of species actual brain size to size expected given its body weight
Human's have largest EQ relative to other primates
Size Matters
More of the same
New brain organisation
More of the Same (Jerison)
Brain size & cog capacity must be related
Brains organised according to common plan are just getting bigger
New Brain Organisation (Holloway)
Evolutionary change takes place through brain reorganisation
Not simply by expansion of same areas
Suggests brain of modern humans is qual different from other primates
Not just quant bigger
EQs cannot be useful predictors of general cog ability
Selective Evolution of Separate Brain Areas (Finlay & Darlington, 1995)
Sizes of 10 measured brain subdivisions from 131 species plotted as function of total brain size
Cerebral cortex evolved more rapidly than other brain areas
The Cortex
Encephalisation vs Reorganisation (Preuss, 2000;2001)
Problem discussed in multiple terms
Micro architecture
Macro architecture
Micro Architecture
Organisation of cortex on small scale
Neurons, networks, cortical column organisation
Theory of Micro Architecture
Concept of cortical columns suggested doctrine of uniformity (Mountcastle, 1957)
Cortical columns organised in same way throughout brain in every species
Some report columns highly conserved
Almost same no. neurones in every column in nearly all species
Input/output organisation of cortical layers also remained consistent
Others reported cortical column properties & cytoarchitecture differ significantly across species
Macro Architecture
Organisation of cortex on larger scale
Size of existing/new cytoarchitecture/Brodmann areas
Theory of Macro Architecture
Selective evolution of specific cortical areas & interconnectins
Some suggested primates tend to have same no. cortical areas (Petrides, Pandya)
Others emphasise evolution of new cortical areas (Allman, Kaas)
Frontal Cortex
Good functional grounds for suggesting frontal lobes evolved selectively
Language
Memo
Theory of mind
Prefrontal Cortex
Frontal lobes consist of many areas w. widely differing cytoarchitecture
In humans most constitute the prefrontal cortex
Frontal Lobe Evolution
Gross morphology
Cytoarchitecture
Connectivity
Cytoarchitecture
Frontal lobe occupies same proportion of cortex in most species (Semendeferi, 1997)
Nothing special about human frontal cortex
Frontal lobe composed of prefrontal, premotor & primary motor cortex
Is prefrontal cortex that has selectively evolved
Motor areas not evolved as much
Connectivity
Support concerted evolution theory
If prefrontal cortex evolved could be related to expansion in connectivity
More white matter that connects prefrontal cortex to target areas
Support (Schoenemann et al., 2005)
46 structural MRI scans of 46 primates of 11 species
Excellent grey/white matter contrast
Controversy (Passingham, 2002)
MRI scans not reveal cytoarchitecture boundaries
Prefrontal boundaries in Schoenemann seem to be arbitrary
Language (Rilling et al., 2008)
Comparative DTI
Compared connectivity in brain in humans & chimpanzees
Revealed evolution of arcuate fasciculus
Prominent temporal lobe projection that is smaller/absent in non-human primates