lecture 25

Question 1

What is tree of life?

Answer 1

• idea that we come from common trunk/root - branching out

Question 2

In what way do vertebrate brains differ?

Answer 2

• all things being equal, smaller animals have proportionately larger brain
• • georges cuvier 1769 - 1832

Question 3

How best to measure brain size?

Answer 3

total brain weight (g)

• shrew 0.25
• mouse 0.5
• sheep 100
• chimpanzee
• human 1400
• elephant 5000

brain weight as a percentage of body weight (%)

• elephant 0.20
• sheep 0.25
• chimp 0.95
• mouse 2.08
• human 2.33
• shrew 3.33

encephalisation factor (body weight/(body weight)^0.69

• mouse/shrew 0.06
• sheep 0.07
• elephant 0.19
• chimp 0.26
• human 0.71
• something about the brain in terms of capacity

Question 4

In what way have brains changed (or at least in what way do they differ) regarding overall size?

Answer 4

1. overall size
   - birds, mammals and cartilaginous fish have increased relative brain size
   - bony fish, reptiles small brains
   - jawless fish have tiny brains

Question 5

In what way have brains changed (or at least in what way do they differ) regarding relative regional size increases?

Answer 5

2. relative regional size increases - esp. neocortical scaling
   - within mammals, brains are incredibly similar in regards to the bits that make them up
   - things like brain stem/cerebellum look remarkably similar amongs vertebrates

Question 6

In what way have brains changed (or at least in what way do they differ) regarding brain architecture?

Answer 6

3. brain architecture design (lamination, migration, segregation, mappings) esp. neocortical parcellation
   - simple organisms: like a pocket watch vs a grandfather clock, everything is there it is just smaller
   - are bigger brains more sophisticated brains? whale brains perhaps simpler despite being larger, don’t seem to have such clear stratification
   - primate brains show different levels of gyrification: bigger brains = more folded
   - thickness of cortex has remained basically the same
   - folding the cortical sheet allows more cortex in a smaller volume
   - thinner cortex allows tighter folding

Question 7

What’s the deal with primates?

Answer 7

• bigger brains (in relation to body size)
• more neocortex
• more distinct architectures in the neocortex

primate brains arranged by absolute brain size

• galago almost no gyrufication
• marmoset
• macaque
• gibbon
• human

Question 8

What’s the deal with hominids?

Answer 8

• australopithecine - Africa (only) for 2 million years
• homo hablis 2-1.5 mya - lineage to erectus? (only africa)
• homo erectus 1.5 mya found on three continents - but how recent was the migration?
• australopithecus
• hablis
• erectus
• sapien (neand.)

Question 9

What has the most recent discovery in human evolution been?

Answer 9

• discovery of very intact skulls in georgia
• 1.8 million year old hominin skull
• these features had not been observed together in an early Homo fossil until now
• long face: similar to the more recent homo erectus
• a small brain: similar to the older homo habilis
• large teeth: similar to older homo rudolfensis
• the blend of features suggests early humans were one species that had diverse facial and cranial characteristics

Question 10

What patterns are currently seen when looking at human ancestors?

Answer 10

• homo sapiens and ancestors have larger brains/lie on different relative brain size
• absolute brain size larger in homo sapiens

Question 11

What are ideas about the evolution of brain size?

Answer 11

• the evolution of grassland - the plain - has been considered a driver of evolutionary changes towards intelligence (i.e. freeing the forelimbs to allow hands to make tools etc)
• but, brain size has increased well after bipedalism: bipeds existed for two million, many several million years before brains got bigger than chimp size
• freed hands wasn’t a trigger for cerebral expansion

Question 12

Has the recent development of “high culture” been associated with any evolutionary change in the brain?

Answer 12

• bipedal: doesn’t change brain size much
• hands: “
• teeth: duration of childhood? maybe explains change between hablis and erectus
• cerebral volume:
• material culture: crude stone tools (not much), use of fire (kinda yeah), early art, agriculture, cities, writing (yes)

Question 13

What drove the evolution of big brains?

Answer 13

• some say that rapid climate changes forced adaptation
• pulsing climate change
• great lakes appearing and disappearing over 1000 years in the rift valley
• the period of high climate variability (in Africa) laste from about 2mya to 0.5 mya
• the “variability selection hypothesis” proposes that relatively rapid environmental change replaced habitat specific adaptations and favours adaptations that increased the ability to respond and accommodate this change (such as increased intelligence and greater social complexity)
• problem solvers survive

Question 14

How have brains become less “ape-like”?

Answer 14

• australopithicine apes may have had longer childhoods
• it has been theorised that the lunate slcus shows a difference between chimps and australopithecine apes
• the lunate s. divides visual from other neocortex, and that australopitheicus endocats show lunate sulcus positions suggestive of bigger proportion of non-visual cortex
• which includes association cortex (higher functions e.g. social cognition?)

Question 15

What are other constraints on having a big brain?

Answer 15

birth

• human brains can’t really get bigger unless born more immature
• if you look at brain development much more brain growth occurs after birth in humans than chimps

tooth development
- if they develop quickly it’s probably because they spend less time with their mothers

Question 16

How did homo sapiens migrate?

Answer 16

• hypothesised east African source area of modern human, 80,000 to 60,000 years ago
• hypothesised west asian source area of modern northern Eurasians annd north africans, 50,000 to 45,000 years ago
• late migration across the Bering straight to the america
• early spread of modern humans from east african source 60,000 to 40,000 years ago
• later spread of modern humans from west asian source 45,000 to 35,000 years ago
• even later migration across the Bering strait, 20,000 to 15,000 years ago
• islands of indonesia
• floresiensis
• 13,000 years ago
• erectus
• normal healthy people
• particular species of hominin
• nothing like it in recent times
• quite possible that homo sapiens were interacting with this species not all that long ago

Question 17

What are the features of a bigger brain (in our lineage and near relatives)?

Answer 17

• more cells, bigger cells?
• differentiation of cell types?
• • doesn’t seem to be any sort of new cell that arose in human brains
• connectivity (patterns, speed, dynamics)?
• • very different, hard to look at
• molecular patterns dictating connectivity/plasticity?
• • anything with neurons seems to have plasticity
• micro-architecture (lamination, migration, segregation, mappings)?
• • doesn’t seem to be all that different
• parcellation?
• • in terms of parcellations there are some differences between human and non-primates
• • but quite similar to other primates
• • nothing unique to humans
• rather than genomic homology, patterns of gene expression might be more indicative of evolutionary changes in brain v. other organs
• for blood and liver cells, humans and chimps are more similar to each other than either species is to rhesus monkey
• in the brain however, the patterns of gene expression in humans and chimpanzees are very different

Question 18

What made brains get bigger?

Answer 18

pressure for bigger brains

• benefits of social organisation
• survival enhanced by being smarter (tools, clothes, hunting/gathering)

high cost of big brain

• big brain need long gestation and long parental care - huge burden
• big head - difficult birth
• complexity: more genes, more mutations
• large energy expenditure/heat production

Question 19

How do brains get bigger?

Answer 19

• faster growth?

- longer growth?