origins and SPEND
exist because your big brain was "hot" as a child. Simply,
your 20W adult brain used 30W of energy between four
nine years of age. This is due to SPEND (Synaptic Prolonged Expensive Neurodevelopment).
Paleoanthropologists ignore this neuroscience, and its key role in
making us smart. Here the jig-saw puzzle of your origins is
You know that.
Why your human brain is unique
It is big—particularly its
cerebral hemispheres—for the
size of your body.
Some background details.
|What you do not is that it under went
when you were a child, SPEND
(Synaptic prolonged expensive neurodevelopment)
Brain size--important--but not the complete story
||It has to do with big brains in a fairly
small body. Elephants
and whales and dolphins can have larger brains than humans
but their brains
make up only a tiny part of their massive bodies.
For a literature review and explanation, "Human
evolution expanded brains to increase expertise capacity, not IQ",
Psycoloquy: 10(002) 1999, pdf
is not just relative size. Some small monkeys and rodents in percentage
terms have as large brains
relative to their bodies. The pygmy marmoset has a body of 140 g and a
brain of 4.5 g (3.2%) while a typical human might have a body of 79 kg
and a brain of 1.5 kg (2.1%).
these high brain size to body size brains in small animals fail to take
into account allometrics--the fact larger animals generally have
smaller organs relative to their body size. Scientists use allometric
exponential calculations to take this into account to create an
encephalization index. So adjusted, humans do have large brains for our
more importantly, and intuitive is that the large brains relative to
bodies or small animals are more subcortical than cortical.
In contrast, human brains are mostly cortical--higher brain.
|But even this does not explain our unique
smartness. Brain size variation in humans correlates only
intelligence as measured by IQ. Indeed, the clinical literature
cases of humans (microcephaly, hemispherectomy) with normal levels of
IQ with brains around half that of most people.
size helps but it SPEND
that makes us smart. SPEND is the missing piece in the human origin
jig-saw puzzle. This is because SPEND allows the integration in an
empirical context of diverse previously unexplained facets of human
existence such as why we have childhood, why we are smart, why
hunter-gatherers pool food and why Neanderthals went extinct.
It is a hub fact that brings together previously unlinked issues in
neurodevelopment, cognitive development, child exercise physiology,
human evolution and
Only humans have childhoods
Other animals have a juvenile stage but
only humans have childhood—the period of slow growth in our lives
between infancy and puberty.
Body growth during childhood is not
slowest amongst mammals but is so retarded that it matches that of cold
Young Neanderthals lacked
this extreme period of slow growth. This makes its existence unique to
our particular type of human species: modern anatomical humans (Homo
gives rise to your human mind
Childhood from kindergarten to high
is a period of very rapid mental growth. During this period, we acquire
species unique cognitive abilities such as language. This growth in
mental ability is due the refined wiring up of
synaptic links between neurons that make up the neural circuits in our
Key to this writing of neural circuits
is starting big with a two-fold excess in the number of synapses and
winnowing them down so they join neurons into effective information
processing circuits. It is important to note
that the neurons and axons of the adult brain already are in
place by infancy—the
development in childhood is thus critically concerned with this network
refinement by elimination of synapses.
(Synapse Prolonged Expensive Neurodevelopment) is the story
75% of the brain's
energy is burnt by synapses.
Their doubling in the childhood brain causes childhood cerebral gray
consume twice that of adult cerebral gray matter.
As a result, the childhood brain in
of energy compared to 20 W in an adult. In adolescence, this declines
to adult levels.
High energy consumption shifts across
cortical mantle with age. This can be seen when MRI snaps shots of
the brain are compared because changing
local energy use causes a
changing thickening/thinning of the cerebral cortex.
consumption, for example, requires extra capillary vasculization and
support cells and this expands the nearby cortex.
This cortical thickness/thinning has a
pattern of change across the brain as children grow older and mentally
mature. Significantly, the
and the more complex pattern of change, the more intelligent the
that is acquired.
child's body is small due to SPEND
The high energy burn of the childhood
makes it more sensitive to metabolic disruption. Such
disruption occurs when there is reduced blood levels of glucose and
oxygen; dehydration; increased body temperature, and the build up of
waste products from protein breakdown. These happen when the body is
exercised for long periods both for adults and children but are much
greater for the latter.
Young humans have been selected to miminize such metabolic disruption upon the childhood brain
by limiting body size as this reduces the amount of skeletal muscle
that produces this disruption. This is the adapative reason why
children's bodies have been selected to grow slowly when their brains
are undergoing their most
intense synapse remodeling of neural circuits.
answers the question why humans are so smart
The lack of such a slow growth period
Neanderthals—and so lack
of a proper period of small body size childhood—strongly
suggests that SPEND—Synapse Prolonged Expensive Neurodevelopment—is
unique to modern humans.
Anothe jig-saw puzzle fits in here. Hunter-gatherers—the form of
in which humans evolved—pool high-energy foods between nonkin. This
was a critical innovation since it supports the prolonged childhood
period of energy dependence caused by
SPEND. Further, anthropologists find that nonkin food pooling links to
for example, the
social sanctions made possible by gossip. This suggests the existence
of a self-sustaining loop that makes our species possible:
SPEND, that enables human language, that enables nonkin food pooling.
Neanderthals have been shown to have similar hunting skills to early
modern humans and it is this new cooperation that came with language
that advantaged our species over them. Thus, the key to understanding
our biological uniqueness is a
cycle between energy, neurodevelopment, cognitive abilities
and cooperative social behavior .
Where to read the story in greater detail
Full long review article: "Human metabolic
adaptations and prolonged expensive neurodevelopment: A
45,567 words, 10 figs, 445 refs, 9 appendixes pdf, online pdf, comments subject to review here
London School of Economics, Department of Anthropology,
Series on Culture and Cognition, 30 Jan 2008, 78 slides Powerpoint
from brain size and SPEND do other important traits exist that
might effect the intelligence capabilities of the human brain?
in "Human metabolic
adaptations and prolonged expensive neurodevelopment: A
review", I discuss some of them in section 6 and appendix 3.
These including possibly genetically
adult synapse numbers in the prefrontal cortices (but not visual ones),
enhanced developmental heterogeneity in synaptic neurodevelopmental
timing across the cortical mantle (see section 6); changed
connections, cerebello-cerebral reorganization, late developing neuron
types, late developing dendritic arbor re-sculpturing, altered
dendritic arbor geometry and the presence of novel internal stimulation
as a result of language (appendix 3).