PSYB64 Lecture 6-7

Evolution of Animal Life

  • The Earth is approximately 5 billion years old.

  • Organic molecules appeared a couple of hundred million years later.

  • Bacteria emerged over 1 billion years later.

  • Unicellular organisms appeared around 3 billion years ago.

  • Multicellular animals, vertebrates, and mammals emerged about 4.5 billion years after Earth's formation.

  • Simple neural networks appeared about 700 million years ago.

  • Nervous systems are relatively recent in evolutionary history.

Study of Brain Evolution

  • Problem: Soft tissues like brains leave no fossil record.

  • Solution: Comparative biologists study early animal phyla descendants to infer brain evolution.

Sea Sponge

  • Animals without neurons or a nervous system.

  • Contains independent effector cells (myocytes) capable of sensation and contractility.

  • Contract in response to stimuli.

  • Essential for feeding by pushing water out after nutrient absorption.

The First Nervous Systems

  • Animals: Jellyfish, corals, sea anemones, hydras possess the simplest nervous systems.

  • Hydra: One-layer nervous system; able to feed and locomote.

  • Contains sensorimotor neurons that activate muscular effectors.

  • Neural Concepts:

    • Divergence: One neuron activating multiple effectors.

    • Convergence: Multiple neurons activating one effector.

  • Layered Nervous Systems:

    • Two-layer: Separate sensory and motor functions.

    • Three-layer: Introduces interneurons for complex processing.

Nervous System Centralization

  • Centralized nervous systems improve efficiency and response time.

Evolution of Human Brains

  • Lucy (Australopithecus): 4 million years ago, first bipedal ancestors, simple tool use.

  • Homo Erectus: 2 million years ago, coexisted with modern humans, used fire and tools.

  • Modern Humans: Emerged 250,000 years ago; complex technologies developed about 65,000 years ago.

  • Cognitive capacities evolved under social and environmental pressures.

Theories on Brain Evolution

  • Social Brain Hypothesis: Larger groups necessitate complex social interactions.

  • Mating Mind Hypothesis: Want for mate attraction led to advanced cognitive capabilities.

  • Dunbar's hypothesis links neocortex size to social group size and complexity.

Genetics and Brain Structure

  • Homologous structures share genetic commonality; analogous structures evolve independently.

  • Genes influence brain structure and function.

Genetic Terminology

  • Genotype: Genetic composition.

  • Phenotype: Observable traits affected by genotype expression.

  • Genes determine phenotype and hereditary traits.

Gene Expression and Development

  • Genes encode for proteins; segments of DNA form chromosomes.

  • Transcription and translation processes lead to amino acid assembly and protein formation.

  • Genetic variations (alleles) can affect phenotype.

Sex-Linked Characteristics

  • Dominant versus recessive alleles influence traits.

  • Sex chromosomes (X and Y) demonstrate differences in genetic expression, e.g. hemophilia, color blindness.

Epigenetics

  • DNA Methylation: Changes gene expression potential through chemical modifications.

  • Maltreatment may alter gene expression patterns in individuals.

Stress Response

  • Cortisol: A stress hormone with potential neurotoxic effects at high levels.

  • Associated with brain responses to threats.

Heritability and Personality Traits

  • Genes shape environments; influences on traits are both shared and unique among siblings.

  • Non-shared environment accounts for more variation than shared environment in traits.

Prenatal Development Stages

  1. Zygote Formation: Conception to 2 weeks.

  2. Embryonic Stage: Weeks 2-8; organ differentiation begins.

  3. Fetal Stage: Week 8 to birth; specialized development occurs.

Neurogenesis and Growth Stages

  1. Neurogenesis: New neuron creation from ventricular zone.

  2. Cell Migration: Cells travel along radial glia paths.

  3. Differentiation: Stem cells specialize into distinct neuron types.

  4. Axon/Dendrite Growth: Formation of neural connections.

  5. Apoptosis: Programmed cell death for fine-tuning.

  6. Synaptic Rearrangement: Pruning of unnecessary connections during development.

Aging and Brain Structure

  • Myelination and brain physical structure changes with age.

  • Neurogenesis continues but generally slows into adulthood.

  • Healthy aging is linked to physical activity and mental engagement.