Geological and Biological Events of Earth's History

Geological Records

• Earth's history is recorded with the oldest layers at the bottom and the youngest at the top.

Biological Events

• Major evolutionary events, such as the emergence of new organism groups, define the geologic record.
• The Cambrian Explosion, about 541 million years ago, marked a rapid diversification of life.
• Many animal phyla appeared at this time.

Mass Extinctions

• Mass extinctions are significant events where a large percentage of species go extinct in a relatively short time.
• There have been 5 major extinction events in Earth's history.

The Big 5 Mass Extinctions

• Ordovician-Silurian Extinction occurred approximately 443 million years ago.
  • It was likely caused by a combination of climate change and a drop in sea levels.
  • About 85% of species, including many trilobites and brachiopods, went extinct.
• Late Devonian Extinction occurred approximately 375-360 million years ago.
  • It may have been caused by changes in sea level, climate shifts, and asteroid impacts.
  • Around 75% of species were affected, especially marine life, including fish and reef-building organisms.
• Permian-Triassic Extinction occurred approximately 252 million years ago.
  • Known as the "Great Dying," it was the most severe extinction event.
  • Around 90-96% of all species went extinct.
  • Causes included massive volcanic eruptions, climate change, and ocean anoxia.
  • Significantly affected marine invertebrates and terrestrial vertebrates.
• Triassic-Jurassic Extinction occurred approximately 201 million years ago.
  • Thought to have been caused by volcanic activity, climate change, and possibly asteroid impacts.
  • About 70-75% of species, including many reptiles and amphibians, went extinct.
  • Paved the way for the dominance of dinosaurs in the Jurassic period.
• Cretaceous-Paleogene (K-Pg) Extinction happened approximately 66 million years ago.
  • Associated with the extinction of non-avian dinosaurs.
  • Likely caused by a massive asteroid impact (Chicxulub impactor) and volcanic activity.
  • Around 75% of species, including many marine reptiles and ammonites, went extinct.
  • Led to the rise of mammals and birds.

Uses of the Fossil Record

• Fossils provide evidence of past life.
• Fossils help scientists understand how organisms have evolved over time.
• Archaeopteryx fossils shows the transition from dinosaurs to birds.

Limitations of the Fossil Record

• The fossil record is incomplete.
• Not all organisms fossilize well, especially soft-bodied ones.
• Jellyfish rarely leave fossils.

Impact of Permian Extinction

• Caused by volcanic eruptions and climate change, leading to the extinction of ~60% of all biological families.
• Many marine invertebrates, like trilobites, did not survive this extinction.

Impact of Cretaceous Extinction

• Linked to the Chicxulub asteroid impact, which blocked sunlight and disrupted the climate.
• Resulted in the extinction of ~50% of marine species and many terrestrial species, including non-avian dinosaurs.
• The Tyrannosaurus rex and many other dinosaur species became extinct.

Events Leading to Adaptive Radiation

• After mass extinctions, surviving species often undergo adaptive radiation.
• Surviving species rapidly evolve to fill ecological niches left vacant by extinct species.
• After the Cretaceous extinction, mammals diversified into various forms to occupy niches previously held by dinosaurs.
  • Examples include rodents, primates, and whales.

Hypothesized Sequence of Events Leading to Life

• Life likely originated through chemical reactions that formed simple organic molecules.
• Led to protocells and eventually prokaryotic cells.

Key Steps in the Origin of Life

• Abiotic synthesis of small organic molecules:
  • Simple organic compounds, like amino acids, formed from inorganic precursors.
• Polymerization of these molecules:
  • Small organic molecules joined to form larger polymers, such as proteins and nucleic acids.
• Formation of protocells:
  • Polymers became enclosed in lipid membranes, forming simple cell-like structures.
• Emergence of self-replicating molecules:
  • RNA molecules capable of replication emerged, leading to the first living organisms.

Prokaryotic Adaptations

• Prokaryotes have several adaptations that help them thrive:
  • Cell Wall: Provides structure and protection.
  • Capsules: Help bacteria stick to surfaces and evade the immune system.
    • Streptococcus pneumoniae has a capsule that helps it evade the immune system.
  • Endospores: Allow survival in harsh conditions by entering a dormant state.
    • Bacillus anthracis can form endospores that survive extreme conditions.

Genetic Diversity in Prokaryotes

• Genetic diversity arises from horizontal gene transfer and mutations.
  • Horizontal Gene Transfer: Prokaryotes can exchange genetic material through transformation, transduction, and conjugation.
    • Escherichia coli can acquire antibiotic resistance genes through these methods.
  • Mutations: Rapid reproduction leads to the accumulation of mutations, contributing to genetic variation.

Prokaryotic Taxa by Nutritional Modes

• Prokaryotes can be classified based on how they obtain energy and carbon:
  • Autotrophs: Produce their own food.
    • Photosynthetic bacteria like Cyanobacteria.
  • Heterotrophs: Obtain food from other organisms.
    • Decomposers like Bacillus species.
  • Phototrophs: Obtain energy from light.
    • Rhodobacter.
  • Chemotrophs: Obtain energy from chemical compounds.
    • Nitrosomonas, which oxidizes ammonia.

Prokaryote Taxon Groups

• Major groups include:
  • Bacteria: Diverse metabolic pathways and ecological roles.
    • Proteobacteria include many pathogens and nitrogen-fixing bacteria.
  • Archaea: Often extremophiles, adapted to harsh environments.
    • Halobacteria thrive in high-salinity environments.

Ecological Interactions Involving Prokaryotes

• Prokaryotes engage in various ecological interactions:
  • Mutualism: Both species benefit.
    • Gut bacteria in humans help with digestion.
  • Commensalism: One benefits, the other is unaffected.
    • Skin bacteria.
  • Parasitism: One benefits at the expense of the other.
    • Staphylococcus aureus can cause infections.

Differences Between Eukaryotes and Prokaryotes

• Eukaryotes:
  • Have membrane-bound organelles, including a nucleus.
  • Larger and more complex.
  • Animal cells, plant cells, and fungal cells.
• Prokaryotes:
  • Lack membrane-bound organelles.
  • Generally smaller and simpler.
  • Bacterial cells.

Endosymbiont Theory

• Eukaryotic cells originated from a symbiotic relationship between different prokaryotic cells.
• Mitochondria and chloroplasts are thought to have evolved from engulfed bacteria.
• Mitochondria are similar to alpha-proteobacteria, while chloroplasts are similar to cyanobacteria.

Primary vs. Secondary Endosymbiosis

• Primary Endosymbiosis:
  • Involves the engulfing of a prokaryote by a eukaryotic cell.
  • The origin of mitochondria.
• Secondary Endosymbiosis:
  • Involves a eukaryotic cell engulfing another eukaryotic cell.
  • Some algae.
  • The chloroplasts in some protists originated from red or green algae through secondary endosymbiosis.

Factors Contributing to Protist Diversity

• Protist diversity is influenced by:
  • Habitat Diversity: Protists inhabit various environments (freshwater, marine, soil).
  • Nutritional Modes: Autotrophic, heterotrophic, and mixotrophic lifestyles contribute to diversity.
  • Reproductive Strategies: Protists can reproduce asexually or sexually, leading to genetic variation.

Protist Groups

• Protist diversity is influenced by:
  • Excavata: diplomonads (Giardia intestinalis) and parabasalids (Trichomonas vaginalis).
  • SAR (Stramenopiles, Alveolates, Rhizarians): diatoms (Thalassiosira), dinoflagellates (Pfiesteria), and foraminifera (Globigerina).
  • Archaeplastida: red algae (Porphyra) and green algae (Chlamydomonas).
  • Unikonta: amoebas (Amoeba proteus) and fungi (Saccharomyces cerevisiae).

Protist Classification

• Protists can be classified based on characteristics such as:
  • Cell Structure: Presence of cell walls, flagella, and organelles.
  • Reproductive Strategies: Asexual vs. sexual reproduction.
  • Nutritional Modes: Autotrophic, heterotrophic, or mixotrophic.

Protist Relationships

• Protists have sister taxa relationships with:
  • Animals: Choanoflagellates.
  • Plants: Green algae.
  • Fungi: Nucleariids.

Key Characteristics of Protists

• Eukaryotic Cells: Protists have cells with a nucleus and membrane-bound organelles.
• Diversity: Protists can be autotrophic (photosynthetic), heterotrophic (feeding on organic matter), or mixotrophic (capable of both).
• Reproduction: They can reproduce asexually or sexually.
• Habitat: Protists inhabit a wide range of environments and can be free-living or symbiotic.
• Examples of Protists:
  • Algae: Photosynthetic protists (Chlamydomonas and Porphyra).
  • Protozoa: Heterotrophic protists (Amoeba proteus and Paramecium).
  • Slime Molds: Fungus-like protists that can exist as single cells or form multicellular structures.