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 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. 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. 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. • 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 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. 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 mates). 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.