Evolving a big brain

Evolving a big brain
Evolutionary dilemma: big brains can confer fitness
advantage through greater intelligence, innovation,
problem-solving ability all of which can be put to use in
acquiring resources (better predation skills, extractive
foraging, social skills, etc.)
But big brains can be a disadvantage because they are
energetically expensive. For big brains to evolve organism
must find a way of minimizing energetic costs while
maximizing benefits. How is this done?
Evolving a big brain
• Two goals of current review:
• Integrate cost-benefit approaches to create a
predictive model of evolution of big brains
• Apply this model to human evolution, how, when,
and why did we evolved big brains
Social brain hypothesis
• best known hypo regarding
evolution of big brains in
• Big brain was result of larger
more complex social groups.
• Increased cooperative
behavior conferred fitness
benefits, but big brain was
needed to keep track of
multiple and varied social
relationships and for social
Social Brain Hypo
• Evidence: increased
neocortex size found in
primates with larger
social groups.
• Problem: not found in
other mammals or birds.
• Revision: in some
mammals and birds,
pair-bonding is most
complex social
• Unlike most mammals
and birds, primates are
rather unique in forming
friendships for fitness
Social Brain Hypo
• Despite success of social
brain hypo, it does have
• Lemurs have as complex a
social world as
cercopithecoid primates
(old world monkeys, rhesus
macaques, baboons, etc.)
but have much smaller
• Why? More complex
ecological challenges in
monkeys (lager territories,
more varied diets, etc.)
• Evidence: survival rates,
innovation, adaptation to
new environments all
correlated with bigger
• Question: If bigger brains are better both socially and
ecologically, why doesn’t everyone have a big brain?
• Brains are costly. All brain benefits can be easily wiped
out by the increased metabolic energy necessary to
grow and maintain a larger brain. 20-25% of adult
human metabolism spent on brain; 60% in infants.
Expensive Brain Framework
• the idea that to afford a big brain
and organism must utilize better
energy sources or it must reduce
energy expenditure on other
• Put another way – pay for the brain
by getting better fuel or by taking
fuel from other systems and
reallocating it to the brain.
• Better fuel (higher quality diet) can
increase basal metabolic rate BMR,
which can make a bigger brain
• Evidence: primates with better
diets have higher BMR and
primates with higher BMR have
bigger brains.
Resource allocation trade-offs
• Most famous is the Expensive
tissue or gut-for-brain
hypothesis. One can afford a
bigger brain by reducing the size
of the digestive tract. Once quite
popular, but has weakened
recently because more
comprehensive studies have not
supported it.
• Authors have recently proposed
brains-for-brawn hypothesis,
reduced muscle tissue in big
brained animals. Support comes
from the fact that primates are
under-muscled compared to
other mammals.
Reproduction trade-offs
• pay for brain by producing fewer, larger, better quality
(more likely to survive) offspring.
• Even greater energetic savings can be realized through
cooperative breeding, where mother offloads some
proportion of the energetic costs of offspring on to
others (allomothers – non-reproductive females; male
Human Evolution
• 1.8 mya: Suite of traits in H. ergaster all pointing toward reducing
cost of brain ecologically (getting better fuel):
• committed bipedalism: more efficient movement, more effective
predation; cooperative hunting, scavenging;
• use of weapons, tools for acquiring and processing resources.
• Bottom line: spending less energy getting better food.
• Sometime later, not sure exactly when; more extensive cooperative
breeding; pair-bonding, grandmothering, etc.

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