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How and why is the human brain
different?
• The human species is psychologically and
behaviourally unique.
• At the level of behavioural ecology, humans
are a special species simply by not being
confined to a single ecological niche (Bingham,
1999)
• Beyond that there is a widespread consensus
that the human species has unusual
psychological capacities, even though there
are wide differences in how these should be
described or defined.
Neuropsychology Option Week 5, 2006
• Assuming unique psychological capacities of
some kind, the neuropsychological question is
then how these depend on special
characteristics of the human brain (see e.g.
Deacon, 1997a&b).
• Broadly speaking, the main answer to this
question is “we don’t know”
• but there is a range of attempted answers
which are worth exploring.
• The why? and how? questions are in fact
difficult to separate, but the standard answer
to why? would be because human evolution
has made the human brain different
(Bradshaw, 1997; Moll et al., 2005).
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This standard evolutionary answer does not
give us very much to go on, but
“evolutionary psychology” of various kinds
is becoming more popular, and there is a
fair degree of consensus that present and
future genetics will be able to add a great
deal of detail to our knowledge of human
brain evolution.
(See, e.g. Ramus, 2006 and Sikela, 2006 on
page 7 of the handout)
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Millions bottom
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Millions top
“Lucy”
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Mary leakey
footprints
Fossilized footprints,
discovered by Mary Leakey
in Laetoli, Tanzania.
They are dated at 3.5 Mbp,
and only the “Lucy” species
is known from that time, but
the imprints look very like
modern human imprints.
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Family Tree
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Alemseged, Z., et al. (2006). A juvenile early hominin skeleton
from Dikika, Ethiopia. Nature, 443(7109), 296-301
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Alemseged, Z., et al. (2006). A juvenile early hominin
skeleton from Dikika, Ethiopia. Nature, 443(7109), 296-301
Dikika is only 4km from where ‘Lucy’ was
found (Australopithecus afarensis )
The Dikika specimen, from 3.3m yrs ago was
about 3yrs old and probably female.
The legs were human-like for bi-pedal
walking, but the arms and hands ape-like.
The hyoid bone (for the larynx) was also
ape-like
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Napier (1980)
[not on handout
UCL Library]
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Napier, ape
hands
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Napier –
Screwtop
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Brain Size
• This gives one simple answer to the how?
question: the human brain is outstandingly
big, given our body size.
• However it is clearly not the whole answer,
since relating brain-size to body weight is
complicated (Deacon, 1997b) and the human
gain in behavioural terms is more extensive
than would be predicted on size alone.
• Genes which may be responsible for primate
and human brain expansion are currently
being investigated (Ponting & Jackson, 2005;
Evans et al., 2006; Pollard et al, 2006; Sikela,
2006; Tang, 2006)
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Brain Size
• To what extent are human capacities what we
would expect from a 1,500 primate, or 1,500
mammalian (cf dolphins, elephants) brain?
• Or are there special ingredients or “magic
bullets” (Elston et al., 2006; Allman et al.,
2005) which produce uniquely human
capacities?
• Or are both the above correct?
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Number of neurons in the nervous system
1,000,000,000,000
350,000,000,000
100,000,000,000
500,000,000
300,000,000
50,000,000
850,000
250,000
20,000
381
302
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Homo sapiens (maybe 1014)
Chimpanzee
Rhesus monkey
Mouse
Octopus
Stickleback
Honey bee
Fruitfly
Sea slug
Thread worm male
Thread worm
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Neuropsychology Option Week 5, 2006
In Striedter, G. F. (2005). Principles of brain evolution. and
Striedter, G. F. (2006). Precis of Principles of Brain Evolution. Behavioral and
Brain Sciences, 29(1), 1-+. [not on paper handout, see intranet]
Striedter brain szie
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Ponting and Jackson, 2005
• …….recent advances from the cloning of two human
genes promise to make inroads in the area .. of brain
size evolution.
• Microcephalin (MCPH1) and Abnormal spindle-like
microcephaly associated (ASPM) are genes mutated in
primary microcephaly, in which, the brain is of a size
comparable with that of early hominids.
• It has been proposed that these genes evolved
adaptively with increasing primate brain size.
Subsequent studies have lent weight to this hypothesis
by showing that both genes have undergone positive
selection during great ape evolution.
• the evolutionary patterns of all four presently
known primary microcephaly genes are consistent
with the hypothesis that genes regulating brain size
during development might also play a role in brain
evolution in primates and especially humans (Evans,
2006; Tang 2006)
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Lateralization
• One additional factor is lateralization.
• Although there are some who suspect that
human brain lateralization is a development of
primate asymmetries (Corballis, 2003), the
data on population handedness suggests a
very sharp distinction between the degree of
handedness observed in other primate
populations
• (which is zero according to the meta-analysis
by Papademetriou et al., 2005)
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Lateralization
Lateralization figure
- old
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Lateralization - new
• the human figure is 90% right handedness
• which does not correlate completely with
language lateralization
• but they may be related
• And the link between physical lateralization
and human specializations may be
strengthened
• By changing techniques (Buchel et al., 2004;
Hutsler, 2003; Sun & Watson, 2006; Sun et
al., 2006; Luders et al., 2006)
• and theoretical models (Monaghan & Shillcock,
2004)
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They found that the left
arcuate fasculus was
larger in right handers
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Luders, E., et al. (2006). Hemispheric
asymmetries in cortical thickness. Cerebral
Cortex, 16(8), 1232-1238.
• Used MRI on 60 healthy adults.
• cortex in the left hemisphere was
generally thicker.
• the precentral gyrus, middle frontal, anterior
temporal and superior parietal lobes were
significantly thicker on the left
• but the inferior posterior temporal lobe and
inferior frontal lobe were thicker on the right.
• Asymmetry profiles were similar in both sexes
Neuropsychology Option Week 5, 2006
Sun, T., et al. (2006). Genomic and
evolutionary analyses of asymmetrically
expressed genes in human fetal left and right
cerebral cortex. Cerebral Cortex, 16, I18-I25.
• they compared gene expression levels in the
perisylvian regions of human left-right cortex
at fetal weeks 12, 14, and 19
• identified dozens of genes
• “identified a subset of genes with human
asymmetry humans and altered expression
levels between chimps and humans.”
• “Our results identify candidate genes
involved in the evolution of human
cerebral cortical asymmetry.”
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Sun, T., & Walsh, C. A. (2006). Molecular approaches
to brain asymmetry and handedness. Nature
Reviews Neuroscience, 7(8), 655-662
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Sun, T., & Walsh, C. A. (2006). Molecular approaches
to brain asymmetry and handedness.
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Hutsler, 2003
7 autopsies:
50-97 yrs of
age
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Monaghan, P., & Shillcock, R. (2004). Hemispheric
asymmetries in cognitive modeling: Connectionist
modeling of unilateral visual neglect. Psychological
Review, 111(2), 283-308
They claimed that a hemispheric distinction between
coarse- coding in the RH and fine- coding in the LH exists
at the neuronal level as different sized receptive fields.
Simulations with connectionist models with these
properties were successful in modeling various tests of
unilateral visual neglect
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nottebohm
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Brain re-organization: expansion
of the frontal lobes
• Apart from lateralization, other kinds of reorganization with-in the brain might have been
possible within the time-scale of human
evolution.
• The most popular hypothesis for many
decades has been that the human frontal
lobes, presumed to be the main site for
planning and self-control, have either
generally expanded or undergone some more
detailed change (Deacon, 1997a&b;
Schoenemann et al., 2005).
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• However, careful MRI scanning of different
great ape species and comparison with human
scans has led to the conclusion that there has
been no disproportionate expansion of the
human frontal lobes
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semendeferi1
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The Semendeferi et al., (2002) table
Semendeferi table
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Brain re-organization: expansion of the
frontal lobes
The sherwood 2005
• Schoenemann et al.(2005) recently suggested
that prefrontal white matter is
disproportionately larger in humans than in
other primates
• but Sherwood et al. (2005) countered that a)
the boundary between prefrontal and other
cortex is not well defined; and b) that in any
case, although the data showed humans
having more white matter than the
average primate, they did not show a
difference between humans and great
apes.
Neuropsychology Option Week 5, 2006
Brain re-organization: expansion of the
frontal lobes?
Preuss (2004) suggests that what has
happened is that, while the primary motor
and sensory areas in the human brain are
roughly the same size as those in apes,
secondary areas (“association cortex”)
has greatly expanded in all the lobes of
the human brain.
Neuropsychology Option Week 5, 2006
Others have suggested that certain
neuronal features of primate and human
frontal lobes hold the key to understanding
human intelligence:
pyramidal cells in prefrontal cortex
(Elston et al., 2006)
or certain spindle cells in anterior
cingulate and fronto-insular cortex
(Allman et al. 2005; see back of handout,
and p. 7 of handout for abstracts).
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Fig. 4. Neurolucida tracings of pyramidal (left) and von Economo
(right) neurons from Fronto-Insular (a) and Anterior Cingulate Cortex
(b). Notice the vertical symmetry and relative sparseness of the VEN
dendritic tree. [back of handout; Allman et al., 2005]
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[Back of handout:
Allman et al., 2005]
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Allman, J. M., Watson, K. K., Tetreault, N. A., & Hakeem, A. Y.
(2005). Intuition and autism: a possible role for Von Economo
neurons. Trends in Cognitive Sciences, 9(8), 367-373. [p 7 of
handout]
Von Economo neurons (VENs) are a recently evolved cell type
which may be involved in the fast intuitive assessment of
complex situations. As such, they could be part of the circuitry
supporting human social networks.
We propose that the VENs relay an output of fronto-insular
and anterior cingulate cortex to the parts of frontal and
temporal cortex associated with theory-of-mind, where fast
intuitions are melded with slower, deliberative judgments.
The VENs emerge mainly after birth and increase in number
until age 4 yrs. We propose that in autism spectrum
disorders the VENs fail to develop normally, and that this
failure might be partially responsible for the associated
social disabilities that result from faulty intuition.
Neuropsychology Option Week 5, 2006
Elston, G. N., et al. (2006). Specializations of the granular
prefrontal cortex of primates: Implications for cognitive
processing. Anatomical Record Part a-Discoveries in Molecular
Cellular and Evolutionary Biology, 288A(1), 26-35. [Abstract on p.
7 of handout]
The biological underpinnings of human intelligence
remain enigmatic.
….we demonstrate that the basic neuronal building block
of the cerebral cortex, the pyramidal cell, is characterized
by marked differences in structure among primate
species…….
…..pyramidal cells in the granular prefrontal cortex of
humans had a disproportionately high number of spines…
…. the highly branched, spinous neurons in the human
granular prefrontal cortex (gPFC) may be a key
component of human intelligence.
Neuropsychology Option Week 5, 2006
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Kaas, J. H. (2005). From mice
to men: the evolution of the
large, complex human brain.
Journal of Biosciences, 30(2),
155-165. [not on handout]
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Elston et al 2006,
abstract on p. 7 of
handout. Note the
absence of
comparisons either
with apes or with
association cortex in
other lobes.
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Granular prefrontal
cortex is represented
by stipple. bar 2 cm
for human and 1 cm
for other species.
(another paper from
the Elston Lab)
However, since even
the galago, a
prosimian, has got
some prefrontal
cortex, it is another
case where it looks as
though the human
brain is what would
be expected in a
typical primate brain
expanded to 1500 ccs
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The galago or bushbaby,
a noctural cat-sized tree
dweller, has brain
organisation which is
recognizable in terms of
the larger rhesus
monkey version.
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An elephant brain = 3,886.7 cc
Hakeem, et al. (2005)
Above is the elephant
hippocampus, in pink
C is a bottle nose dolphin
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Plotnik, J. M., et al. (2006). Self-recognition in an Asian elephant.
PNAS, online, October 30, 2006.
Movie1,
movie2
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movie3
Plotnik, J. M., et al. (2006). Self-recognition in an Asian elephant.
PNAS, online, October 30, 2006.
‘we report a successful MSR (mirror self-recognition)
elephant study and report striking parallels in the
progression of responses to mirrors among apes,
dolphins, and elephants. These parallels suggest
convergent cognitive evolution most likely related to
complex sociality and cooperation
Movie1,
movie2
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movie3
A brain larger than ours, killer whale brain =~5,000 cc
Marino, L., Sherwood, C. C., et al. (2004). Neuroanatomy of the killer whale
(Orcinus orca) from magnetic resonance images. Anatomical Record Part a-Discoveries in
Molecular Cellular and Evolutionary Biology, 281A(2), 1256-1263. (not on handout)
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• There still remains the difficulty that
superficially the human brain is more similar
to that of a chimpanzee than we might expect
by comparing the behaviours of the two
species. Thomas Huxley, a champion of
Darwin’s put it this way in a supplement to
Darwin’s book The Descent of Man —
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End of lecture 1
• Start of lecture 2
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