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Anth 207.2 Evolution and the Brain
Transcript of Anth 207.2 Evolution and the Brain
How did evolution produce such an unusual brain?
How does our brain's evolution affect the ways that we think?
why have a brain?
Sea squirt in larval stage
Tunicates including the 'sea squirt'
after fixing head-down to a rock or other anchor, the larva digests its own brain.
animal needs a brain to perceive, react & change behaviour
one of the key points of evolutionary theory:
continuity of all life.
humans & animals not SO different.
some of our cognitive abilities may mislead us about the nature & function of the brain.
More exotic brain functions may be side effect of other brain functions.
evolution can make variation only on the basis of biological mechanisms.
just because a trait would be an 'adaptation' does not mean it arises.
highly 'conserved' genetic mechanisms for bodily structure (HOX genes)
variation can occur by tweeking established structures & processes rather than by creating radical break or novel structure.
developmental constraints matter, especially in area like the brain.
every animal's metabolism imposes energy limits
brain is energy-hungry
brain is heat generating
over evolution, a species may make 'trade-offs' to 'afford' large brain.*
the 'obstetrical dilemma'
what can brains do?
'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 & Baker 2004)
in simple nervous systems, stimuli & reaction tightly linked.
Nikolaas 'Niko' Tinbergen
stimuli 'release' or provoke stereotypical behaviour; that is, response is relatively fixed.
for example, the presence of a rival or potential mate can provoke formulaic aggressive or mating behaviour.
but what about in complex brains, where behaviour appears less stereotyped?
examples in humans of stereotypical response:
response to smells evoking disgust
pain expressions & some facial expressions
'fight or flight' stress response
My own feeling is that I have no problem with either 'evolution' or 'psychology.' Issues with focusing only on a small set of Tinbergen's issues & ignoring very important reservations.
Researchers, often following Noam Chomsky's account of language, have argued that human abilities demonstrate a large number of
innate, domain-specific mechanisms
to solve recurring problems.
Because mechanism so pervasive & important for survival, we can assume (in some cases) an
underlying genetic basis & highly specialized, fixed mechanisms
Instead of a general tool or a 'blank slate,' the mind is like a Swiss Army knife: a
collection of special purpose tools
Because evolution is slow & culture-technology change fast,
can arise between brain & environment: 'Stone Age brain in a Space Age world.'
examples of recurring problems
reading each other's emotions
choosing a mate
detecting social 'cheaters'
detecting predators (snakes)
issues with evolutionary psychology
Are we certain a trait is universal?
Is there a plausible ontogenetic (developmental) mechanism?
Does the account square with what we know of phylogeny?
Is a trait actually an adaptation?
in some cases, evolutionary psychologists seem to focus on accounts of how traits are 'adaptive' but assume that they are from the onset.
In addition, they can leave out the three other questions Tinbergen suggests we should ask.
Is every trait an adaptation?
Lewontin and Gould
Traits that are the side effect of selection for other traits.
Related to Tinbergen's mechanism question: selection may not be able to separate targets for development. Example, in the brain, the whole might have to grow rather than one specific area
Trait that arose or was selected for in one situation is then used in different situation later on.
Is a behaviour 'heritable' or subject to selection?
The more complex, non-stereotypical & varied a behaviour, the harder it is to argue that it is strictly heritable.
Language so complicated there is no way an infant could learn it simply by hearing it.
Acquisition requires a special purpose ‘module’ or mental tool.
Evidence from neural lesions supported (damage to Broca’s area, for example, causes language loss).
Impossible to study language evolution.
hominin brain growth
How then, do you grow a human brain with homologous starting point?
the development of our species
why the increase?
evidence of exaptation?
underlying structural similarity produced by common ancestry
How do brains develop?
(Gerard Edelman) - neurons must 'compete' for survival and connection.
initially, the brain is over-connected but goes through 'pruning'
those neurons that do not get used get pruned.
implications of neural darwinism
neural adaptation (brains can overcome developmental glitches).
profound restructure in cases of deprivation (e.g., echolocation in blind, uni-hemispheric individuals)
phylogentic possibility of developmental 'tinkering'
social production of knowledge
cultural-cognitive niche formation
susceptibility to enculturation & learning
vertical connections (old + new)
So, which parts of the brain are 'primitive'?
Two big differences:
developmental dynamics (it's not the adults)
cumulative cognitive development (it's not the individual)
some of the simplest organisms with a nervous system.
the end of movement & need to perceive or act, the nervous system is superfluous.
although we may be impressed by our ability to 'think,' most cognition is a late arrival evolution.
mammals are especially 'encephalised' or disproportionately 'brainy' in terms of body weight.
although the pattern of encephalisation is general, the actual structure of mammal brains can vary.
= a structural similarity across diverse species because of common descent from a share ancestor
In mammals, homologous structures can vary in proportion, connection or in function.
Humans are disproportionately encephalised, but comparative neuroscientists and biological anthropologists point out that...
all structures in the human brain have homologues in the chimpanzee brain and other closely-related animals.
that is, human distinctiveness does not mean that we have a wholly unprecedented 'special' brain zone or novel neurological structure.
a basic model of natural selection:
Inheritable variation in a species, if that variation affects survival or reproductive success because of selective pressures in the environment, over time, will lead to species change to become more adapted to that selective pressure.
for example, if a part of a species has an inheritable brain trait that makes it more likely for them to pass their genes successfully on to the next generation (that is, to survive and reproduce), we would expect that trait to increase in frequency.
but that's also where it gets more complicated.
so how could variation arise in a complex organ like a brain over a species' evolution?
After biologists began sequencing the genomes of diverse animals, they were surprised to find that the number of 'genes' or areas coding for proteins, was smaller than expected, and that these 'genes' were quite similar across different species.
what does that mean?!
the toolbox to build bodies was small; quite similar tools got used to build a whole range of different body types. The key was how they were put together, connected together & in what proportion.
species change partially through altered developmental processes, not entirely unprecedented genes.
One of the most dramatic examples biologists discovered were HOX genes, the genes that seemed to 'oversee' the formation of body sequence in bilateral animals. HOX genes seemed to tell cells what segment of a body they were in – head, neck, limb.
HOX genes were remarkably similar across a wide range of species; that is, they were strongly 'conserved' even as evolutionary processes produced a wide variety of different organisms.
the diagram shows how HOX genes are conserved (colour coding) but also how body segments like repeating vertebrae can be built by carrying multiple copies of that gene.
also, development matters a lot. Genes are less like a blueprint and more like a pattern book. A gene can be used to produce more or less of a protein, and in diferent ways.
the combination of evolutionary theory and developmental biology, especially in light of new genetic evidence.
but also, the rapid increase in human brain size suggests that both intense directional selection and
(a selective pressure getting relaxed) may be factors.
we have to understand the biological trade-offs (the costs & dangers of a trait) that affect the size and complexity of the brain.
for example, 'energy' is a constraint on the brain.
*please note: this is a metaphor. No animal actually chooses to evolve or makes any choices about the matter.
in humans, brain growth is subject to...
for a bipedal species with increasingly narrow hips (because narrow hips are better for balance), a big-brained infant can cause problems for childbirth.
the bigger a brain, the longer it takes for neurons in networks to send signals.
Longer neurons (to get across bigger brain) have to have thicker connections to keep speed up.
Brain gets crowded with more and more thick connections (white matter is like cable & we have proportionally more of it).
the bigger a brain, the more specialised zones it has, but the smaller proportion of theses zones any one area is connected to.
a big, specialised brain can have a harder time getting signals among all its zones.
in many cases, it's not clear that a specific brain area can be enlarged or altered alone.
developmental processes may only be able to influence the scale of the whole organ or sections of it.
That is, variation may only be able to take certain forms & not be too specific.
one argument is that the jaw & the dome of the skull are linked; growing the dome has required shrinking the jaw.
Of course, this can make getting enough energy to run that brain even more of a challenge.
Pointed out that we really have four questions about the origin of a trait or behaviour in evolution.
For example, for any brain capacity, we could ask:
causation: what is the neural mechanism for the function?
development: how does that function arise as the organism matures?
evolution: when and how did the mechanism emerge over time?
function: what purpose or adaptive function does a trait serve?
These responses are quite uniform once they are initiated (or 'released') across individuals. That is, responses like sexual arousal are easily recognised, even though in humans they can be initiated by varied stimuli.
the error of assuming every trait must be an adaptation.
That is, evolutionary psychology can focus (but not always) almost exclusively on the 'function' question, much less on the 'evolution' issue, and virtually not at all on 'causation' or 'development.'
Just claiming a trait is an 'adaptation' is not a robust evolutionary account.
Brains are built be a kind of developmental unfolding, not by simply following a 'blueprint.'
For one thing, not enough genetic information for 'blueprint' given how big and complicated brain is.
This developmental process is 'emergent' in that it needs feedback - neurons must communicate & compete for the neurons all to get connected up or pruned.
DNA is crucial as resource, but NOT solely 'in charge'; DNA can be switched off & on by context & other processes.
even weirder: 'chimera' brains; tissue implanted, even from other species, can sometimes successfully 'wire into place'
the 'triclops' or three-eyed frog
implant of a third eye into a frog during development can be successfully integrated due to developmental processes (even gets 'stripes' of alternate eye reception)
The point is that the distinctly human brain and cognitive capacities can be created, not by the introduction of entirely new traits or brain regions, but instead by:
changing the maturation process (prolonged, proportions shifted, slowed such as extended immaturity)
altering the metabolic processes in homologous brain regions
strengthening or weakening connections among brain regions (balance of power among areas) –
re-purposing of brain regions to subserve novel functions or 'neural recycling' (exaptation).
How do we compare to other mammals & to extinct primates, especially other hominins?
For example, the olfactory sense, once dominant in our mammalian ancestors, has become weakened in relation to vision in primates.
One key change in the evolution of species is the way that sensory systems can become specialised.
loosely homologous brain regions co-opted by difference senses depending upon species sensory specialisation (platypus, raccoon, and human).
brains of mammals in general scale allometrically (that is, in predictable pattern with overall body size).
primate brains skew higher in brain-body allometry (the plot of the ratio is higher).
humans are disproportionately 'encephalised,' although size alone does not explain human cognition.
If size alone determined intelligence, elephants and dolphins would be smarter than humans.
If percentage of body weight in brain determined intelligence, small-bodied mammals like pocket mice have highest %.
But BOTH do matter....
the 'metabolic ceiling'
brain growth in utero so fast that it taxes mother's metabolism — reason for relatively early birth for humans?
Bottom line: adaptation is not an instant and unconstrained process.
We need to recognise the brain's limits.
Often, adaptations are sub-optimal.
How do we understand big brains with lots of varied behaviour?
Nb: until about 2mya, brain size of our ancestors close to consistent with other great apes.
first, alter the developmental pathways
approximation of growth trajectories in mammals
Is natural selection targeting particular brain regions?
Mosaic evolution v. late-equals-large
We'll come back to th is question next week because of the issue of emotions, but the bottom line is that even pervasive structures shared by many species (that is, very old) are crucial for modern functions.
A misnomer, in some ways, to talk about 'primitive' parts because of exaptation and recycling.
variable geological period
ability to pass on designs
symmetrical sex selection?