Key Concepts in Earth’s History and Microbiology

Posted on May 27, 2025 in Biology

Earth’s Ancient History and Life’s Emergence

Major Evolutionary Milestones

  • Solar System Forms: Approximately 14 billion years ago (bya)
  • Earth Forms: Approximately 4.6 bya
  • Formation of Planet Earth: 4.5 bya
  • Prokaryotes First Appear: 3.5 bya
  • Eukaryotes First Appear: 1.8 bya
  • Multicellular Organisms Appear: 1.5 bya
  • Animals Appear in Fossil Record: 600 million years ago (mya)
  • Fungi and Plants Colonize Land: 490 mya
  • Early Humans Appear (Not Modern Man): 2.5 mya

Early Earth Conditions

  • Initially fiery and chaotic.
  • Atmosphere lacked oxygen, with an abundance of poisonous gases.
  • As the atmosphere formed, temperatures dropped.
  • Water condensed, carbon dioxide dissolved, and ammonia deconstructed, providing building blocks for proteins, nucleic acids, carbohydrates, and lipids.

First Life and Metabolic Evolution

  • Early metabolism was anaerobic, utilizing the glycolytic pathway.
  • Photosynthesis evolved, likely using H2S to fix CO2.
  • Photosynthesis using H2O led to oxygen production (the Great Oxygenation Event).
  • This changing atmosphere created significant selective pressure, driving further evolution.

Understanding Prokaryotes

Prokaryote Characteristics

  • Prokaryotes are found in two domains: Bacteria and Archaea.
  • They are typically small in size and lack a nucleus.
  • Most have a cell wall; many possess a capsule and flagella; some form endospores.
  • They typically reproduce via binary fission.

Common Bacterial Shapes

The three common shapes of bacteria are:

  • Cocci: Spherical
  • Bacilli: Rod-shaped
  • Spirilla: Spiral-shaped

Extremophiles: Life in Extreme Environments

Archaebacteria, ancestral extremophiles, include:

  • Methanogens: Anaerobic methane producers.
  • Halophiles: Salt-loving organisms thriving in high salt concentrations.
  • Thermophiles: Heat-loving microbes adapted to extremely hot temperatures.

Prokaryote Structure: Key Features

Prokaryotes generally have a cell wall, flagella, and a capsule. They can also form endospores under harsh conditions.

They lack membrane-bound organelles such as a nucleus, mitochondria, or chloroplasts.

Pathogens: Disease-Causing Microorganisms

A pathogen is a microorganism that can cause disease in its host.

Beneficial Roles of Bacteria

Bacteria can be beneficial to humans in various ways, including:

  • Aiding in digestion.
  • Producing essential vitamins.
  • Being used in food production (e.g., yogurt, cheese).

Bacterial DNA Transfer Mechanisms

Bacteria share DNA through three primary mechanisms, not associated with reproduction:

  1. Transformation: Bacteria take up free DNA from their environment and incorporate it into their genome.
  2. Conjugation: DNA is transferred directly between two bacterial cells through a physical connection.
  3. Transduction: DNA is transferred between bacteria using a viral vector (bacteriophage).

Comparing DNA Transfer Methods

  1. Transformation involves naked DNA, while conjugation and transduction involve DNA protected within cells or viruses.
  2. Conjugation requires direct cell-to-cell contact, while transformation and transduction do not.
  3. Transduction is dependent on viruses, while the other two methods are not.
  4. Transformation can potentially incorporate DNA from any source, while conjugation is limited to bacterial DNA, and transduction typically transfers bacterial DNA but can occasionally transfer host DNA.

Insights into Viruses

General Structure of Viruses

The general structure of viruses includes a capsid (protein coat), nucleic acid (DNA or RNA), and often recognition spikes.

Are Viruses Living Organisms?

The question of whether viruses are alive is debated. They lack many characteristics of living things (e.g., cellular structure and independent metabolism) but can replicate by utilizing host cells.