Final answer:
The oxygen in Earth's atmosphere, crucial for life, was generated by cyanobacteria during the Archaean and Proterozoic eras. The correct completion for the sentences is 1) oxygen, life. Cyanobacteria's photosynthetic process was significant in evolving aerobic life and shaping Earth's conditions for the development of diverse life forms.
Step-by-step explanation:
Complete the sentences to explain how photosynthetic organisms changed our lives on Earth. The oxygen in the atmosphere that all living beings depend on was generated by cyanobacteria during the Archaean and Proterozoic eras. The presence of this substance is what makes life possible on Earth.
During the early stages of Earth's history, the atmosphere was anoxic, which means there was no molecular oxygen, and anaerobic organisms were the only life forms able to survive. The emergence of cyanobacteria, which evolved from early phototrophic organisms, was a pivotal moment in Earth’s evolution. These organisms began the process of oxygenation of the atmosphere, a by-product of their photosynthetic activity. Their ability to convert solar energy to chemical energy, using carbon dioxide and water, and subsequently releasing oxygen, facilitated the development of aerobic life forms and shaped our planet's environmental conditions, including the protection afforded by the ozone layer, which absorbs harmful ultraviolet light.