1. KEY CONCEPT Single-celled organisms existed 3.8 billion years ago.

2. Microbes have changed the physical and chemical composition of Earth.
The oldest known fossils are a group of marine cyanobacteria.
prokaryotic cells
added oxygen to atmosphere
deposited minerals

3. Fossil stromatolites provide evidence of early colonies of life.

4. Eukaryotic cells may have evolved through endosymbiosis.
Endosymbiosis is a relationship in which one organism lives within the body of another.
Mitochondria and chloroplasts may have developed through endosymbiosis.

5. The evolution of sexual reproduction led to increased diversity.
Genetic variation is an advantage of sexual reproduction.
Sexual reproduction may have led to the evolution of multicellular life.