Rocks containing layers of iron oxide formed before oxygen was released into the atmosphere by living things.

The oxygen that entered Earth's atmosphere was produced by

Stromatolites are piles of cyanobacteria that have been secreting a lime substance and hardening over time so that they resemble rocks?

Iron oxide is also known as rust?

an organism that grows without air, or requires oxygen-free conditions to live.

a microorganism that grows in the presence of air or requires oxygen for growth.

Single-celled organisms without a nucleus are called a

an organism consisting of a cell or cells in which the genetic material is DNA in the form of chromosomes contained within a distinct nucleus.

Oxidation is the process where a substance combines with oxygen.