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

Cyanobacteria: Bacteria that can carry out photosynthesis 1. Depositing minerals in the form of stromatolites Changed Earth by 2. Producing oxygen as a by-product of photosynthesis

Fossil stromatolites provide evidence of early colonies of life.

Endosymbiosis: A relationship in which one organism lives within the body of another & both benefit from the relationship 1. Early mitochondria & chloroplasts were once simple prokaryotic cells that were taken up by larger prokaryotes Sketch:

Endosymbiosis: 2. Instead of being digested, some of the larger prokaryotes may have survived inside the larger ones Sketch:

Endosymbiosis: 3. The smaller prokaryote could have given the larger cell energy or surgars, while the larger cell could have provided a stable environment & nutrients to the smaller cells Sketch:

Wrap-up (Nothing for notes here…) 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.