1. 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?

2. Two goals of current review:
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

3. Social brain hypothesis
best known hypo regarding evolution of big brains in primates.
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 strategizing.

4. Problem: not found in other mammals or birds.
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 arrangement.
Unlike most mammals and birds, primates are rather unique in forming friendships for fitness purposes.

5. Social Brain Hypo
Despite success of social brain hypo, it does have limitations:
Lemurs have as complex a social world as cercopithecoid primates (old world monkeys, rhesus macaques, baboons, etc.) but have much smaller brains.
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 brain.

6. 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 % of adult human metabolism spent on brain; 60% in infants.

7. 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 systems.
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 possible.
Evidence: primates with better diets have higher BMR and primates with higher BMR have bigger brains.

8. 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.

9. 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 mates).

10. 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.