lecture 25 Flashcards
What is tree of life?
- idea that we come from common trunk/root - branching out
In what way do vertebrate brains differ?
- all things being equal, smaller animals have proportionately larger brain
- georges cuvier 1769 - 1832
How best to measure brain size?
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
In what way have brains changed (or at least in what way do they differ) regarding overall size?
- overall size
- birds, mammals and cartilaginous fish have increased relative brain size
- bony fish, reptiles small brains
- jawless fish have tiny brains
In what way have brains changed (or at least in what way do they differ) regarding relative regional size increases?
- 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
In what way have brains changed (or at least in what way do they differ) regarding brain architecture?
- 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
What’s the deal with primates?
- 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
What’s the deal with hominids?
- 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.)
What has the most recent discovery in human evolution been?
- 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
What patterns are currently seen when looking at human ancestors?
- homo sapiens and ancestors have larger brains/lie on different relative brain size
- absolute brain size larger in homo sapiens
What are ideas about the evolution of brain size?
- 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
Has the recent development of “high culture” been associated with any evolutionary change in the brain?
- 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)
What drove the evolution of big brains?
- 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
How have brains become less “ape-like”?
- 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?)
What are other constraints on having a big brain?
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
