Biota - Lifeforms

The first life on Earth reproduced most easily in shallow continental margins. This stable continental shelf environment was rare in the Archean eon, but began to thrive 2.5 billion years ago, fostering the growth of photosynthetic organisms and allowing oxygen production.

Oxygen concentrations during this period coincided with the peak of the accumulation of banded iron formations. Later, the concentration of oxygen in the atmosphere increased due to continued biological activity.

Eukaryotic organisms became established in the environment and atmospheric oxygen levels rose from low values to about 10% of their present levels.

• "Megascopic" eukaryotes first appeared about 2.3 billion years ago and became widespread about 1.8 billion years ago. Eukaryotes adopted respiration and oxidative forms of metabolism, had a central nucleus, and for the first time were able to differentiate into sex cells that allowed them to pass on a complex and variable genetic code to younger generations.
  

Oxygen Boom
Red sandstones first appeared on the continental margin as evidence of a rapid increase in oxygen levels. This color is due to quartz grains coated with hematite.

Another piece of evidence is the presence of a layer of hematite-rich fossil soil from about 2.5 billion years ago. This layer is consistent with a rapid rise in oxygen concentration to 0.1 atm (100 millibars) between 2.2 billion and 2 billion years ago.

The discovery of small spongy fossils in 890 million-year-old rocks suggests that the first metazoans appeared after the mid-Proterozoic, but arguably the oldest metazoans appeared much later: 600 million to 543 million years ago, the multicellular ediacaran fauna appeared.