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Chapter 25: The History of Life on Earth

1. Introduction to the Topic

• Instructor: Dr. Adam Hrincevich

• Course: LSU BIOL 1202 General Biology II

• Focus: Understanding the history of life on Earth.

2. Learning Objectives

• Describe the origin of simple cells from nonliving materials.

• Explain the significance of fossils, their dating, and information they reveal about life's history.

• Identify the origins of unicellular and multicellular organisms and land colonization.

• Discuss the influence of plate tectonics, mass extinctions, and adaptive radiations on Earth's life.

• Describe genetic changes resulting in new body forms and the emergence of complex structures through descent with modification.

• Engage with terminology through crossword puzzles.

3. Conditions for the Origin of Life on Early Earth

Concept 25.1: Stages Leading to Simple Cells

• Stages of life origin:

  • Abiotic synthesis: Formation of small organic molecules.

  • Macromolecule formation: Combination of small molecules into larger complexes.

  • Protocell packaging: Molecules encapsulated within membranes.

  • Self-replicating molecules: Emergence of molecules that can replicate themselves.

4. Synthesis of Organic Compounds

• Earth's Formation: Approximately 4.6 billion years ago. Initial hostile conditions with bombardment from rocks preventing sea formation before 4 billion years ago.

• Early atmosphere: Dominated by water vapor, methane, ammonia, nitrogen oxides, and little oxygen, primarily volcanic gases.

5. Theories on Early Atmosphere and Organic Synthesis

• Oparin & Haldane Hypothesis (1920s): Proposed a reducing environment conducive to organic synthesis.

• Miller-Urey Experiment (1953): Demonstrated abiotic synthesis of organic molecules in a controlled reducing environment; however, evidence suggests early atmosphere conditions may have been more complex.

• Volcanic Origins: First organic compounds formed near volcanic vents where conditions favored molecular stability and complexity.

6. Organic Compounds in Hydrothermal Vents

• Deep-sea hydrothermal vents: Possible environments for organic molecule synthesis; conditions differ from vent formations:

  • Black smokers: Release water at 300-400°C.

  • Alkaline vents: Water with higher pH (9-11) and warmer temperatures (40-90°C).

7. Extraterrestrial Origins of Organic Molecules

• Meteorites: The Murchison meteorite showcases organic complexity including 80+ amino acids and essential biological molecules.

8. Protocells and Their Characteristics

• Protocells: Early life forms characterized by fluid-filled vesicles with membrane-like structures.

• Formation conditions: Lipids and organic molecules spontaneously form vesicles, which could show growth, metabolism, and reproduction.

• Montmorillonite role: Enhances vesicle formation and capability to absorb organic molecules.

9. The Evolution of Genetic Material

• Self-Replicating RNA: RNA likely served as the first genetic material due to its role in catalyzing reactions (ribozymes).

• Stability and replication: More stable RNA molecules with efficient replication would dominate over time, leading to evolutionary success.

• RNA to DNA transition: RNA served as a precursor to more stable double-stranded DNA.

10. The Fossil Record

Concept 25.2: Understanding Life's History through Fossils

• Fossilization Bias: Not all organisms fossilize; those with hard parts and that were abundant are more likely to be preserved.

• Geological timeline: Fossils' layering in rock strata aids in understanding relative ages without precise dating.

11. Dating Fossils

• Radiometric dating: Determines actual ages by measuring the decay of radioactive isotopes.

• Carbon-14 dating: Efficient for fossils younger than 75,000 years, used widely due to its half-life of 5730 years.

• Alternative methods: For dating older fossils, volcanic rock layers above and below fossils are analyzed, using long half-life isotopes.

12. Key Events in Life's History

Concept 25.3: Major Milestones

• Geologic record divisions: The history of life categorized into the Hadean, Archaean, Proterozoic, and Phanerozoic eons.

• Phanerozoic eon divisions:

  • Erers: Paleozoic, Mesozoic, Cenozoic marked by significant extinction events.

13. Early Single-Celled Organisms

• Stromatolites: The oldest known fossils dated at about 3.5 billion years ago, indicating the presence of prokaryotes as Earth's sole life forms for over 1.5 billion years.

14. Photosynthesis and Atmospheric Changes

• Oxygen production: Earliest atmospheric oxygen derived from photosynthesis leading to significant ecological changes and prokaryotic extinctions.

• Oxygen levels: Gradual accumulation of O2 in the atmosphere marked the 'oxygen revolution,' crucially impacting survival strategies in various species.

15. Emergence of Eukaryotes

• First eukaryotic cells: Dated to about 1.8 billion years ago, characterized by complex structures including a nuclear envelope and mitochondria.

• Endosymbiosis theory: Eukaryotes evolved when prokaryotic cells engulfed smaller cells, leading to the establishment of mitochondria and plastids.

16. Evolution of Multicellularity

• Diversification: Multicellularity led to complex forms of life such as algae, plants, fungi, and animals, with fossils of multicellular organisms dating back to 1.2 billion years ago.

17. The Cambrian Explosion

• Significant event: Characterized by rapid diversification of life forms and appearance of various animal phyla around 535-525 million years ago.

• Predatory adaptations: Emergence of larger predators and defense mechanisms evident in the fossil record.

18. Land Colonization

• Timeline: Fungi, plants, and animals began colonizing land around 500 million years ago, leading to advanced adaptations.

• Mutualism: Evidence of early partnerships between plants and fungi for successful terrestrial life.

19. Impact of Plate Tectonics

• Supercontinent cycles: The formation and breaking apart of supercontinents influenced biodiversity and ecological dynamics.

• Continental drift: Geological movements that affect species distribution and adaptation.

20. Mass Extinctions

• Historical perspective: Five major mass extinction events documented, drastically altering life's evolutionary trajectory.

• Factors causing extinctions: Environmental changes, climate shifts, and catastrophic events like meteorite impacts.

21. Current Extinction Rates

• Human impact: Current extinction rates are alarmingly high, posing risks to biodiversity. Factors include habitat destruction and climate change.

22. Recovery from Extinctions

• Timeframe for recovery: Typically spans millions of years, influencing community structure and diversity.

• Adaptive radiations: Occur following events like mass extinctions, allowing new species to diversify quickly.

23. Evolutionary Changes in Body Form

Concept 25.5: Genomic Influence on Evolution

• Developmental genes: Control key developmental processes and body form adaptations.

• Heterochrony: Changes in the timing of developmental events impacting organismal shape and structure.

24. Evolution is Not Goal-Oriented

Concept 25.6: Mechanisms of Evolution

• Natural selection: Operates on existing structures without anticipation for future use, fostering gradual change.

• Evolutionary tinkering: New forms arise from slight modifications of pre-existing structures.