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Language, complexity, and brain evolution
The role of relaxation of selectionTerrence W. Deacon - Montreal, June 2010
It is a species-unique, highly divergent capacity that undoubtedly reflects extensive evolution, and yet a huge fraction of the constitution of this highly complex adaptive behavior is contributed by social transmissionIt is ubiquitous in all activities throughout the lifetime (e.g. not just for courtship) used for diverse purposes, and is not sexually dimorphicIt is perennial paradigmatic case cited to challenge the ability of natural selection to explain complexity (e.g. Wallace to ID)
The oddity of language evolution
Language functions are complex, cognitively demanding, highly robust, and thoroughly integrated into human cognition, suggesting long evolution and intense selection on brain functionLanguage utilizes neurological substrates that are quite distinct and independent of those underlying any mammalian vocal callsIt requires a highly synergistic, phyletically atypical, involvement of diverse brain systemsand these critical brain structures are not human-specific, but originally evolved for other functions — not for vocal communication.
The oddity of language evolution
Synergy among structures evolved for other purposes
Language functions are not merely localized to discrete cortical areas. As more aspects of language are considered we are finding that many cortical and subcortical brain regions are involved.
Language processing thus involves complex synergies between multiple brain systems.
“The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me feel sick!”
The extravagant complexity problem
Darwin postulated that selection with respect to sex can explain many extravagances, even those which decrease the health and survival of their bearer.
He argued that language evolution might be compared to the evolution of complex displays, like some bird songs, and that it evolved from a courtship display—song.
— Darwin in a letter toAsa Gray, April 1860
Wallace’s doubts about complexity
Alfred Russell Wallace
"... natural selection could only have endowed the savage with a brain a little superior to that of an ape."— Wallace: Quarterly Review, April, 1869, p. 392.
Wallace also denied the validity of sexual selection because it seemed to import aesthetic psychological factors into natural selection.He was wrong about sexual selection, but its role in language evolution seems unlikely because it tends to produce divergent and complementary male / female traits, whereas both human sexes have similar language abilities.
Moving beyond lucky accidentsBut the appeal to extreme accident e.g. “hopeful monsters” to explain language is the biological equivalent of appealing to miracles.
Did a lucky genetic accident make language possible, or did language make some genetic accidents more valuable than others?
"Natural Selection is too often treated as a positive agency. It is not a positive agency; it is entirely negative ”
— J. M. Baldwin (1896)It is a subtractive process, like sculpting. It requires a generative process to produce the living forms from which to chisel away the less-well fitted and preserve the rest.The generative process is epigenesis — its mechanisms contribute constraints and biases that indirectly influence natural selection.
A complement to natural selection?
Duplication in development & evolution
1. Reproduction ... including development & learning
2. Divergence, drift, recombination3. Environment-correlated preservation
= niche complementation
1. Duplication of gene or function... including epigenetic accommodation
2. De-differentiation or degradation3. Synergy-correlated preservation
= redistribution & complexification
Inter
Inter-
orga
nism
Intra-
orga
nism
gene duplication
functional variations
different variants evolve in different lineages
reduced selection
mutations are not eliminated
= relaxed selectionIdealized single gene selection circuit
Intra-organismic replication-duplication
Hemoglobin gene duplication synergy
Spontaneous duplication of the hemoglobin gene allowed one to accumulate mutations, while function was maintained by the other. Drift in form increased the probability of “discovering” a complementary shape.
α β
Further βduplications
α and β evolve to fit
hemoglobin gene duplication
ancestral hemoglobin gene ev
olut
iona
ry ti
me
Duplication of beta hemoglobin genes (above)Expression during mammalian gestation (below)
Duplication ... functional drift ... synergySpontaneous degradation of duplicate genes produced beta- hemoglobin variants with different oxygen affinities, as well as some pseudo-genes.The problem of transferring oxygen from maternal to fetal hemoglobin offered “functional niches” that certain variant hemoglobins could fit
Duplication of genes that control the expression of suites of other genes
Evodevo: Multiple duplications of fly Homeobox genes (1) produce body segmentation. The entire Hox gene family is duplicated in vertebrates (2).
1
2 2
Functional redundancy of duplicate body parts relaxes selection on others, in which accumulated mutations produce variants of structure and function. Variant forms will tend to dedifferentiate but may also come to complement the functions of others,thus initiating selection for their synergistic effects.
= duplication of body structures
divergence complementation
• Most mammals synthesize vitamin C endogenously but not anthropoid primates, fruit bats, and many birds.
• Monkeys and apes must regularly acquire vitamin C from their diet.
When duplication is external
Human non-functional vitamin C gene
A non-functional human gene for the enzyme essential for endogenous synthesis of vitamin C was discovered by using a probe gene from rat.
The human LGO gene has accumulated many random mutations and is no longer tran-scribed (pseudogene).
Extrinsic masking factor masked function
reduced selection & degeneration
... is analogous to gene duplication in influence, but it can lead to very different consequences.
Extrinsic functional duplication
=> fragmentation & recruitment of other loci.
Extrinsic functional duplication
selection shifts to any other gene
loci that fractionally
contribute to the reliability of obtaining the extrinsic
factor
in the absence of selection mutations
accumulate producing increased individual
variants and loss of
function allowing
selection to shift loci
variants and variants and
evolutionary sequence
2. dietary 2. dietary substitution
3. dependence & 3. dependence & re-adaptation
1. Ascorbic acid not in diet = stabilizing
selection for endogenous synthesis.
frui
t acq
uisit
ion
mutationLGO genead
apta
tions
for
pseudogene
Loss of vitamin C synthesis
rhodopsin gene duplications
X
gene duplications
Color vision for ripeness detection?
gene duplications
Duplication of retinal color pigments as a vitamin C adaptation
Domestication as a source of extensive external redundancy
White Rump Munia
Bengalese Finch
wild domesticated250 years of breeding for color without the effects of natural or sexual selection resulted in more complex song, social song learning, and involvement of multiple brain regions
Increased song complexity in the absence of selection
from the laboratory of Kazou Okanoya
Primary song elements
Transition probabilities
Wild
Domesticated
a1 a2 a3 b1 b2 b3 c d e f
a1 a2 a3 b1 b2 b3 c d e f
a b c d e f
a b c d e f a b c d e f
a b c d e f
a b c d e f a b c d e f
a b c d e f
a b c d e f a b c d e f a b c d e f a b c d e f a b c d e f a b c d e f a b c d e f a b c d e f a b c d e f a b c d e f a b c d e f a b c d e f a b c d e f
HVC HVC
NCM L1 L2L3
OVMld
Cochlearnuclei
mMANiMAN
X
Av
DMDLM
RA
nXII
RAcup
cochleasyrinx
redrawn fromJarvis & Nottebohm 1997
PNAS 94,4097-4102
cHV
VOCAL MOTOR AUDITORY
HVC HVC
NCM L1 L2L3
OVMld
Cochlearnuclei
mMANiMAN
X
Av
DMDLM
RA
nXII
RAcup
cochleasyrinx
redrawn fromJarvis & Nottebohm 1997
PNAS 94,4097-4102
cHV
VOCAL MOTOR AUDITORY
used byboth
used onlyby Finch
Nuclei & connections functioning in
song
connections
Simulation of relaxation effect
reduced linearity
degraded filter
increased learning role
increased variation
modified from Ritche & Kirby ’05
Is there inter-action between song structure and learning biases?
stabilizing selectiondirectional selection no selection
“mutational” noise introduced in each
generation
Innate song production involves an auditory template because
deafening results in degraded song
produce subsonglisten to self singcompare to auditory biaslearn differencemodify subsong
adult song
Innate vocal-motor biasInnate auditorysong template
Song-acquisition program
Off-loading to social transmission occurs because degradation of the
auditory template increases the influence of auditory experience
produce subsonglisten to self singcompare to auditory biaslearn differencemodify subsong
adult song
Weak vocal-motor biasDegradedsound bias
Next generation earlyauditoryexperience
Degradation = susceptibility to influence
By removing the stabilizing effects of sexual selection, constraints on song generation are degraded to the point that other neural influences can now affect song. Since auditory bias is modified by experience song structure becomes increasingly subject to social influence.
degraded control
Wild
Domestic
Offloading control to epigenetic processes increasingly opens the door to social transmission
innate-song transmitted-song
epigenesis
learning
degradedgenetics
epigenesisepigenesisepigenesisepigenesis
learninglearninglearninglearning
epigenesisepigenesisepigenesisepigenesisepigenesis
learninglearninglearninglearning
1. Reduced arousal-coupling of vocal behavior (babbling)
2. Equalization of phonological-transition biases
3. Reduction, simplification, and re-use of innate call features in speech prosody
4. Increased role of auditory learning in vocalization
5. Neurologically distributed synergistic organization
6. Increased social-cognitive regulation
Finch analogues in language?
innate call system
cortical language system
Could humans be a self-domesticated species; i.e. a degenerate ape?
The Finch analogy suggests that genetic de-differentiation affecting the nervous system may have contributed to functional complexity in human language evolution. Though, as in the case of ascorbic acid synthesis, de-differentiation is only relevant for opening the door to higher-order synergies, these can eventually come under the influence of selection because of their synergistic effects.
wild
domesticwild
domestic
3. Unmasked selection for new functional synergies drives anatomical reorganization
Relaxed selection and co-evolution both contributed
of primate limbic-midbrain control over vocal emotional communications
cross-talk involvingmany cerebral cortical systems
2. Relaxed selection allows
1. Initial state
31
Language structures selected for learnability
and ease of use
Brain functions selected for the special learning for the special learning & production demands & production demands
of language
selection pressure on language
rapid historical
change
selection pressure
on brains
slow evolutionary
change
Brains and language co-evolved
Once language-like behavior became critical to hominid life
it effectively became an artificial niche to which hominid
brains had to adapt.
... like beavers have adapted to the aquatic niche they create.
The human neural adaptation to language is analogous to beaver
adaptation to its aquatic niche
languageadaptation
symbolicculture
aquaticadaptation
dambuilding
Competitive shifts in connectivity?
Embryological divergence of brain/body proportions should affect axonal competition, favoring connections from relatively enlarged structures.
= reduced peripheral representations (left), cortical recruitment of visceral motor targets (1), prefrontal dominance (2, 4), greater cortico-cerebellar connectivity (3).
Brain size increase as duplication?
Expansion of cerebral cortex itself may have contributed to a form of neurological relaxation effect, allowing regions to partially reduce functional demand on one another, and thus analogously increase regional diversity and the probability of randomly diverging into complementary synergistic relationships. This was exaggerated by disproportional expansion of cerebral cortex with respect to thalamic, striatal, and spinal structures.
*
atg c c gatg c c gatg c c ga#c#Genetics
Epigenesis
Acquisition
Transmission
Nicheconstruction
g
constructionNiche
construction
EnvironmentFunction
Naturalselection
Distributed causality of language evolution