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.