Lecture 01 Evolution

Evolution

Overview

This lecture explores the evolution, structure, and function of the human brain, emphasizing evolutionary and ecological perspectives. Key topics include the biological fitness of brains, hypotheses for brain evolution, scaling laws, and comparative approaches using examples from other species.

Key Topics

1. Biological Fitness and the Evolution of the Brain

  • Biological fitness: Defined as reproductive success, including survival long enough to reproduce and ensure offspring survival.
  • Brains enhance fitness by:
    • Coordinating adaptive movement (e.g., finding food, avoiding predators, and finding mates).
    • Supporting complex behaviors for social cohesion and survival.

Examples:

  • Amoeba: No brain but responds adaptively to stimuli via its membrane.
  • Sea Squirt: Larval stage uses a simple brain for movement; adult absorbs its brain after anchoring.

2. Hypotheses for Brain Evolution

a. Ecological/Dietary Foraging Hypothesis

  • Larger brains evolved to solve ecological challenges, such as:
    • Finding food.
    • Manipulating objects.
    • Remembering spatial locations.
  • Example: Spider monkeys (frugivores) have larger brains than howler monkeys (folivores) due to their reliance on higher-quality diets.

b. Cognitive Buffer Hypothesis

  • Larger brains act as a reserve to cope with unpredictable environmental challenges.
  • Study Example: Bumpus (1899) found sparrows with larger skulls (and presumably brains) were more likely to survive severe storms.

c. Sexual Selection

  • Mates may prefer individuals with traits indicative of intelligence, driving brain expansion.
  • Example: Displays of intelligence in courtship behaviors.

d. Social Brain/Machiavellian Brain Hypotheses

  • Social pressures (e.g., tracking relationships, group coordination) require larger brains.
  • Nicholas Humphrey: Social intelligence involves planning, deception, and theory of mind.
  • Robin Dunbar: Clique size correlates with neocortex size in primates, suggesting social complexity drove brain size.

Study Designs:

  • Correlations between neocortex size and social group size in primates.
  • Comparisons between caching vs. non-caching birds (e.g., corvids).

3. Scaling and Allometry

  • Brain size correlates with body size but not linearly.
  • Allometry:
    • Studies scaling laws for anatomical structures across species.
    • Isometry: Proportional scaling (slope = 1).
    • Positive allometry: Brain grows faster than body size.
  • Encephalization Quotient (EQ):
    • Developed by Harry Jerison.
    • Measures brain size relative to body size, adjusted for expected allometric relationships.

Examples:

  • Humans have high EQ due to disproportionately large brains for their body size.
  • Mammals generally have larger brains than reptiles of the same size.

4. Energy Costs of Large Brains

  • The human brain is ~2% of body mass but consumes ~20% of total energy.
  • Trade-offs:
    • Expensive Tissue Hypothesis: Brain size increases are balanced by reductions in gut size and improvements in diet quality (e.g., cooked food, starchy diets).
    • Infants use 60% of their basal metabolism for brain development.

Study Example:

  • Susana Herculano-Houzel’s work quantifying energy costs based on neuron counts.

5. Evolutionary Constraints on Brain Size

  • Energy limitations: Larger brains require more calories.
  • Gestational constraints: Obstetrical dilemma limits brain size at birth.
  • Connectivity: Larger brains face challenges in maintaining efficient neural communication.

6. Adaptationism and Epiphenomena

  • Caution against adaptationism: Not all traits are adaptations (e.g., Gould and Lewontin’s “Spandrels of San Marco”).
  • Example: Brain size may sometimes increase epiphenomenally with body size.

7. Comparative Approaches

  • Cross-species comparisons reveal general principles of brain evolution:
    • Birds: Corvids and titmice that cache food have larger hippocampi.
    • Primates: Neocortex size scales with group size.
    • Mammals: Folding of cortical surface increases surface area in larger brains.

Key Takeaways

  • The brain evolved primarily to enhance survival and reproductive success through adaptive behavior.
  • Hypotheses for brain evolution often overlap and are tested through comparative and allometric studies.
  • Energy and gestational constraints are critical in understanding the limits of brain size.
  • Avoid overinterpreting evolutionary “just-so” stories; brain traits may not always be adaptive.

Applications and Open Questions

  • How do hypotheses like the social brain hold up against modern comparative data?
  • What are the precise trade-offs between brain size, energy consumption, and reproductive success?
  • How can we better quantify intelligence across species using ecological and evolutionary approaches?

Lecture by Gregory McCarthy, highlighting the interplay between evolution, energy, and intelligence in shaping the human brain.

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