Domains and Kingdoms of Life

The Three Domains of Life

Modern biological classification divides all living organisms into three major domains based on genetic and cellular differences.

• Bacteria: Single-celled prokaryotes with no membrane-bound nucleus.

• Archaea: Single-celled prokaryotes, often found in extreme environments, genetically distinct from bacteria.

• Eukarya: Organisms with eukaryotic cells, which possess a true nucleus and membrane-bound organelles.

Major Kingdoms within Eukarya

Eukaryotes are further classified into several kingdoms based on their characteristics.

• Plantae: Multicellular, photosynthetic organisms (plants).

• Animalia: Multicellular, heterotrophic organisms (animals).

• Fungi: Mostly multicellular, heterotrophic organisms that absorb nutrients (fungi).

• Protista: Diverse group of mostly unicellular eukaryotes that do not fit into the other kingdoms.

Additional info: Some classification systems now split protists into multiple kingdoms due to their diversity.

Cell Types: Prokaryotic vs. Eukaryotic

Basic Differences

Cells are classified as either prokaryotic or eukaryotic based on their structure.

• Prokaryotic Cells:

  • No membrane-bound nucleus; DNA is in the nucleoid region.

  • Lack membrane-bound organelles.

• Eukaryotic Cells:

  • Have a true nucleus enclosed by a nuclear membrane.

  • Contain membrane-bound organelles (e.g., mitochondria, chloroplasts).

Cellular Organelles: Mitochondria and Chloroplasts

Mitochondria

Mitochondria are organelles responsible for producing energy through cellular respiration.

• Function: Convert chemical energy from food into ATP via aerobic respiration.

• Organisms containing mitochondria: All eukaryotic cells (plants, animals, fungi, protists).

Additional info: Mitochondria are believed to have originated from free-living aerobic bacteria through endosymbiosis.

Chloroplasts

Chloroplasts are organelles that carry out photosynthesis in certain eukaryotes.

• Function: Convert light energy into chemical energy (glucose) via photosynthesis.

• Organisms containing chloroplasts: Plants and algae.

Additional info: Chloroplasts are thought to have originated from photosynthetic cyanobacteria via endosymbiosis.

Origin of Life and Major Eras

When Did Life Originate?

Life is believed to have originated on Earth approximately 3.5 to 4 billion years ago during the Precambrian era.

Major Geological Eras

The history of life on Earth is divided into several major eras, each characterized by dominant life forms and significant evolutionary events.

Endosymbiosis and Organelle Origins

Origin of Mitochondria and Chloroplasts

• Mitochondria: Arose from the engulfment of aerobic bacteria by ancestral eukaryotic cells (endosymbiosis).

• Chloroplasts: Arose from the engulfment of photosynthetic cyanobacteria by ancestral eukaryotic cells (endosymbiosis).

Oxygen and Photosynthesis

Origin of Oxygen in the Atmosphere

Oxygen began to accumulate in Earth's atmosphere due to photosynthetic activity by cyanobacteria (also called blue-green algae).

• Photosynthetic Bacteria: Used sunlight to convert carbon dioxide and water into glucose and oxygen.

• Impact: Led to the Great Oxygenation Event, enabling aerobic respiration and the evolution of complex life.

Timeline of Pre-Cambrian Life

Sequence of Major Events

The following timeline outlines the major evolutionary milestones from earliest to most recent:

1. Anaerobic Bacteria (first life forms, no oxygen required)

2. Photosynthetic Bacteria (Cyanobacteria; began producing oxygen)

3. Increase of Oxygen (Great Oxygenation Event)

4. Aerobic Bacteria (adapted to use oxygen for respiration)

5. Endosymbiosis of Mitochondria (aerobic bacteria engulfed by ancestral eukaryotes)

6. Endosymbiosis of Chloroplasts (cyanobacteria engulfed by ancestral eukaryotes)

7. Multi-cellular Eukaryotic Cells (complex life forms)

Mass Extinctions

Paleozoic-Mesozoic Extinction

The mass extinction between the Paleozoic and Mesozoic eras is known as the Permian-Triassic extinction event.

• Cause: Believed to be due to massive volcanic eruptions, climate change, and ocean anoxia.

• Impact: Largest extinction event in Earth's history; up to 96% of marine species and 70% of terrestrial vertebrate species went extinct.

Mesozoic-Cenozoic Extinction

The mass extinction between the Mesozoic and Cenozoic eras is known as the Cretaceous-Paleogene (K-Pg) extinction event.

• Cause: Most likely triggered by a large asteroid impact and subsequent environmental changes.

• Impact: Led to the extinction of non-avian dinosaurs and allowed mammals to become the dominant land animals.