ANTH151 Lecture Notes - Lecture 5: Intraspecific Competition, Deep Homology, Homo Ergaster
1 Jun 2018
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Week 5 - Brains
How did evolution produce such an unusual brain?
• Encephalisation → getting a bigger brain, process through which a species
devotes more of its body weight, energy etc. to its brain
• Developmental dynamics → how the brain grows and develops from a small
cluster of neurons
Constraints and ‘relaxing’ of selective pressure:
• Why doesn’t every animal have one?
• Constraints led to a rapid increase in brain size
Why have a brain?
• Tunicate or ‘sea squirt’ (larval stage)
• Adult possess no ‘brain’ or neuron cluster; immature larva does
• Once immobilized and a passive feeder, the need for the brain is no longer
present (eats its brain)
• What our brains can do should not confuse us about what brains are ‘for’ in
evolutionary sense
Simple brains:
• With simple brains, stimuli and response are closely linked
• A stimulus ‘releases’ or provokes a stereotyped (fixed) response (recoiling
from pain, moving towards food, presence of rival provokes threat display)
• Humans have traits like this e.g. fixed response from a surprised scream
• “Throughout the animal kingdom, the innate nature of basic behaviour
routines suggests that the underlying neuronal substrates necessary for their
execution are genetically determined and developmentally programmed” –
Manoli and Baker 2004
Humans?
• Disgust responses to certain smells
• Some facial expressions (not all) – when something embarrassing happens in
Indonesia, they smile
• Sexual arousal
• Pain response
• Stress response – fight or flight
• The responses themselves may be stereotypical and universal, but cuing them
can be a wide range of stimuli
→ The more complex brain, the less automatic and stereotypical responses
• Although ‘triune brain’ idea is misleading, the layered emergence (both in
evolution and development) means that slow-growing, large brains have more
overlaying control and variation
Encephalisation – Evolutionary Trends:
• Absolute brain sizes
o Strepsirhines – 12.6cc
o New world Monkey – 34.1cc
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find more resources at oneclass.com
o Old World Monkey – 89 cc
o Lesser apes – 97.5cc
o Great apes – 316.7cc
o Human – 1251.8cc
• Most formulae based on body size suggest humans should be 600cc (so
double)
• Encephalisation Quotient – ‘expected’
o Lower and higher vertebrates
o Outliers – male gorilla, chimpanzee, australopithecus, porpoise
• Encephalisation indices;
o Humans EI = 4.8 (unusual brain)
o Non-human anthropoids EI = 1.9
• When did the brain get so big?
o Encephalisation among hominins – homo erectus has the largest
timespan
• Brain growth over evolution (look at graphs)
o Hominid does not just get bigger, it spikes upwards in the last 2
million years
Why is this interesting?
• Intense directional selection?
• Released constraint? – plateaus of other species show constraint
• New niche? Or a new behaviour pattern
• Intra-specific competition e.g. sexual selection?
How big does a brain have to be? 750cc
Homo habilis (Australopithecus habilis?);
• ‘Habilis’ because of ‘handy man’ discovered 1950
• Remains 2.3-1.6 mya → overlaps Australopithecenes and Paranthropus
(competitive exclusion?)
• Skeletal traits are variable (H. rudolfensis for robust variant)
• 600-700 cc. brain – is the big jump with H. IA habilis or with H erectus?
• May have made stone tools
Homo ergaster (African variant):
• ‘Work man’ 1976
• 1.8 mya to .6 mya
• Larger body than earlier hominins with modern proportions (savanna
populations)
• Human-like traits:
o Left Africa for Eurasia
o Diet included meat (cooking?)
o Tool use
o Brain size variable, around 800cc
• Used to be called H. erectus, but now name is reserved for East Asian remains
• Oldest spread of Homo to Eurasia
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find more resources at oneclass.com
Variety in brain architecture:
• Comparative neurology –
o Human brain not simply quantitatively different (bigger)
o Qualitative differences are crucial
o Terrence Deacon: searching for special ‘language’ part of human
brain
• Genetic evidence shows deep homologies and yet functional variation
(amphibian, fish, reptile, mammal) – conserved structure
o Conserved and derived traits: using similarity and difference in brains
▪ Deep homologies help us to see and understand the deriving of
specific variants
▪ Comparison amongst different orders of mammals and
amongst primates – growth of ‘associative cortex’
• Allometric scaling
o As the brain size increases, not all areas increase equally
o Disproportionate increase of some areas shifts cerebral ‘balance of
power’
• Relative brain sizes:
o Cerebral cortex much larger
o Rat’s cortex = postage stamp
o Monkeys = post card
o Human’s = four pages
o Chimpanzees = page of printer paper
• Neocortex shape (sulchi, fissures, gyri)
o Chimpanzee compared to man
Brain areas that grew:
• Frontal lobe associated with synthesizing information from other areas, and
inhibiting action (downward processes)
• Volume of white matter, brain interconnections, grows faster than neocortex,
eventually constituting 34% of human brain BUT interconnection not
proportional (becomes less integrated); specialization
• Differentiation or specialization of tissue (but only through development)
Key concepts –
• Neuroplasticity = developmentally based malleability
• Displacement = as brain areas grow in relative size, they send out extra
connections
• Cortical specialization = outer layers less interconnected
• Executive control = brain internally directed more than externally reactive
Evolutionary psychology:
• Human abilities are innate and ‘domain specific’ i.e. highly specialized
• ‘Stone Age’ mind in Space Age world because evolution slow (culture and
technology fast)
• Humans today are a good proxy for our ancestors (as mind is innate)
• Examples;
o Learning language
o Social bonding
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Constraints and relaxing" of selective pressure: why doesn"t every animal have one, constraints led to a rapid increase in brain size. Humans: disgust responses to certain smells, some facial expressions (not all) when something embarrassing happens in. Indonesia, they smile: sexual arousal, pain response, stress response fight or flight, the responses themselves may be stereotypical and universal, but cuing them can be a wide range of stimuli. The more complex brain, the less automatic and stereotypical responses: although triune brain" idea is misleading, the layered emergence (both in evolution and development) means that slow-growing, large brains have more overlaying control and variation. Evolutionary psychology: human abilities are innate and domain specific" i. e. highly specialized. Behavioural genetics is quite subtle and emerging science (biases behaviour but does not seem to determine it: adaptationism" is every trait an adaptation, inconsistent with how the brain develops.
