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Week 3 10/11/07
Chapter 45 Human Brain Evolution
3 Main Ways to learn about how different brains evolved:
1) ______________________
-Skull cavities reflect size and shape of brain as well as location of major fissures
-Learn by studying changes in proportions of brain parts
 Essen’s Theory: densely interconnected regions tend to resist
separation during brain growth and form bulges that limit the
separation distance, whereas poorly interconnected regions are free to
fold and form fissures that would increase the separation distance
2) ______________________
-Examining brains of suitable ranges of extant species
-determine what features they share and whether these features are shared because
they were inherited from a common ancestor or because they evolved separately
3) _______________________
-understand mechanisms and modes of development as well as constraints
Cladistics; comparative method of reconstructing phylogenetic relationships
 homology;_______________________________________________________________
________________________________________________________________________
 analogy;_________________________________________________________________
________________________________________________________________________
3 Misconceptions about Brain Evolution:
i) Misconception that theories of evolutionary change are highly speculative
ii) Misconception that evolution has a single goal or direction
iii) Misconception that ontogeny recapitulates phylogeny
Early Stages of Brain Evolution
-Did Vertebrates Turn Their Brains Upside Down?
Origin of the Neocortex:
 hallmark of evolution of mammalian brains from reptilian ancestors was the emergence
of the neocortex.
 In mammals, Cortex is generally divided into three parts:
-Two phylogenetically older:
1) ______________ or olfactory piriform
2)_______________ or hippocampus and subiculum
-“Newer”
3)_______________ imp. for broad range of behaviors and abilities
 Laminar Organization of neocortex appears to similar in most mammals
Layer:
1)____________
2-3)____________
4)____________
5)____________
6)____________
 The neocortex has changed from its ancestral design by; diversifying its neuron types,
changing its overall size, changing size of individual cortical areas and adding cortical
areas, and diverging areas into specialized modular processing units (columns)
Brains of Early Mammals
 Neocortex subdivided into small number of functionally distinct areas 10-20
 Early mammals brains were dominated by a few sensory and motor representations of
areas. Small brains with little neocortex (association area)
 Nearly all existing mammals have a v1, v2 and a more lateral zone of visual cortex and
this pattern likely emerged with the first mammals
 Although the brain structures throughout mammals are quantitatively similar, some
mammals have adapted to their environment by quantitatively changing the functions of
brain structures i.e. Duck billed platypus, moles, etc.
Evolution of Primates
 Three main lines;
o Prosimians (lorises, lemurs, bushbabies)
o Tarsiers
o Anthropoids (New World monkeys, Old World monkeys, and the ape-human
group)
 Early primates emphasized ________and ____________reaching and grasping
 60 mya haplorhine primates emerged and shifted from nocturnal to diurnal life
o Shift to diurnality meant enhancement of the visual system and enlarged body size
o Olfactory apparatus reduced in size and eyes enlarged and brought close together
o Larger social groupings enhanced protection from predators
 5-6 mya, a line of apes diverged into modern common chimpanzees and bonobos
and hominids:
o Bipedal apes that includes modern humans
 Oldest known ________________(4mya)
 Bipedal but still could climb trees
 H.habilis 2 mya used stone tools increased cranial capacity
 Homo erectus 1.7mya larger body, larger brain
 Homo sapiens emerged from 250,00 to 300,00 years ago
 Modern Homo Sapiens emerged 35,000 years ago
Evolution of Hominid Brains
 Humans only surviving hominid
 Cortical asymmetry: Brain specializations evolved to support the ability for language
such as Wernickes and Brocas area
Why is Brain Size Important?
 All organs and systems of the body confront design problems and limits as they become
larger or smaller
 2 major ways in which larger brains can be modified to reduce design problems:
1) Brain becomes more modular such that connections between neurons are local
a. Done by increasing number of processing areas so areas are smaller
b. Confining connections within an area
2) Connections that require thick, long axons should be reduced
a. Functionally related areas grouped/ increased degree of specialization
w/in hemis
-cannot change size of dendrites or lengths of axons too much b/c impacts the
computational power of neurons