Prokaryotes: Biology for Majors

Introduction to Prokaryotes

• Domains: Two of the three domains of life are prokaryotic: Bacteria and Archaea.

• Appearance: Prokaryotes were the first life forms on Earth, emerging approximately 3.5 to 3.8 billion years ago.

• Distribution: They are abundant and ubiquitous, found in all environments, including moderate and extreme conditions.

Early Forms of Life: Microbial Mats and Stromatolites

• Microbial Mats: These are multi-layered sheets of prokaryotes (primarily bacteria, but also archaea) that represent potentially the earliest forms of life, dating back about 3.5 billion years.

  • The first microbial mats likely formed at hydrothermal vents.

• Stromatolites: These are fossilized microbial mats and serve as the earliest physical record of life on Earth.

  • They are sedimentary structures formed by the precipitation of minerals out of water by prokaryotes within a microbial mat.

Earth's Early Atmosphere and Evolution of Life

• Anoxic Atmosphere: For the first two billion years of Earth's existence, the atmosphere was anoxic, meaning it lacked molecular oxygen (O_2).

  • During this period, only anaerobic organisms (those that grow without oxygen) could survive.

• Phototrophs: Autotrophic organisms capable of converting solar energy into chemical energy appeared within one billion years of Earth's formation.

• Cyanobacteria: These