Tetrapod Brain Evolution

Question 1

What do brains do?

Answer 1

sensory information -> information processing -> bodily response

Question 2

What animals do not have a brain?

Answer 2

Porifera, echinoderms, and cnidaria

Question 3

Why evolve a brain? Not needed to process info - example?

Answer 3

Maze-solving in slime moulds (no brain, no nervous system)

Question 4

Why evolve a brain?

Answer 4

Centralized control of nervous system

Question 5

Common structures in vertebrate brains

Answer 5

• cerebral hemispheres
• cerebellum - motor control
• olfactory bulb
• optic tectum - vision in most lineages except mammals
• medulla - brainstem - autonomic functions

Question 6

Variation in mammalian absolute brain size

Answer 6

mammalian brains vary in size across several orders of magnitude - e.g. shrew, human, sperm whale

Question 7

Costs of evolving a larger brain

Answer 7

• processing info with electrical and chemical signals - need ATP conversion
• brains = metabolically expensive
• humans - brains = 2% of body mass but ~20% resting energy
• Chimpanzees brains consume 13% of resting energy, mice it is 9.5%
• per unit weight, brains consume ca. 10x more energy than other somatic tissues

Question 8

Benefits of evolving a larger brain

Answer 8

• larger brained species = more intelligent?
• greater behavioural flexibility, better able to exploit their environment?
• respond to changing environment, where optimal environment is not stable all the time
• e.g. grazing algae off rock surface - does not vary much - not intelligence needed
• e.g. big brains help to exploit resources
• hard to make intelligence comparable between species
• along with technological evolution

Question 9

Mitigating costs of big brains - adaptations?

Answer 9

large brains may enable adaptations that mitigate costs - reduced storage of adipose fat? reduced gut size? improved foraging efficiency. Identifying social cheats. Behavioural innovations that increase energy extraction

Question 10

Mitigating the costs of big brains
The Expensive Brain Framework (Isler & van Schaik, 2009)

Answer 10

Paying for brain enlargement
-> energy turnover increase
-> allocation -> maintenance decrease (->guts decrease, muscles decrease)
-> allocation -> production decrease (litter size decrease, birth interval decrease)

but subject to constraint:
large brained adults must produce large offspring

Question 11

Is a large brain necessary?

Answer 11

complex behaviour is not limited to large-brained species

Question 12

Challenges in measuring cognitive abilities in animals

Answer 12

• what is intelligence, and can it be compared across species?
• How can we measure cognitive abilities across species?

Question 13

Consequences of increased brain size

Answer 13

1. number of neurons
2. connectivity
3. composition

Question 14

1. Number of neurons

Answer 14

larger brains = more neurons = greater ‘processing power’?

Question 15

3. Composition

Answer 15

“Neocorticalisation” in mammals

• outer cortical layers disproportionately increase
• diverse cognitive, sensory & motor functions
• increases in ‘higher’ cognitive functions?

grey matter increases from 25% in insectivores to 50% in humans

Question 16

Consequences of increased brain size
Summary
Generally, bigger brains = …

Answer 16

1. more neurons - greater ‘processing power’?
2. more modularity - greater functional specialisation?
3. proportionately larger neocortex (mammals) - ‘higher’ functions?

Question 17

Changing the size of brain regions

Answer 17

brain regions change size due to specific selection pressures

Question 18

Changing the size of brain regions
The social brain hypothesis

Answer 18

Neocortical enlargement driven by selection for ‘social intelligence’?

Question 19

Changing the size of brain regions
Ecological selection hypothesis

Answer 19

Neocortex enlargement for visual specialisation?

• Neocortex is relatively large in diurnal than nocturnal primates
• social and ecological explanations not mutually exclusive

Question 20

Brain evolution vs neuroplasticity

Answer 20

brain evolution:
- change in vol/structure of brain regions over evolutionary time

neuroplasticity:
- change in vol/structure of brain regions over individuals’ lifetime
- example: human ‘echolocation’ - visual areas used for processing sound
- example - changes in the hippocampus with expertise in spatial navigation - London taxi drivers

Question 21

Overall Summary
How do brains change in evolutionary time?
Why do brains change in evolutionary time?

Answer 21

How?
- whole brain: absolute size, relative size
- changes in size lead to changes in organisation, larger brains = more neurons, more modularity, greater % neocortex (in mammals)
- smaller brain regions can change due to specific selection pressures

Why?
- costs of evolving a larger brain: energetic & developmental
- benefits of evolving a larger brain: ‘processing power’, behavioural flexibility, novel functions?
- enlargement of primate neocortex: social & ecological pressures
- change in evolutionary time vs neuroplasticity