Life in the Precambrian
The first living beings to appear were anaerobic prokaryotes, of which there is evidence of their existence from about 3,500 my ago. These prokaryotes were heterotrophic, and they fed on organic molecules from archaic seas.
When organic molecules began to be scarce, the first photosynthetic autotrophic prokaryotes, cyanobacteria, emerged 2000 my ago, which due to their photosynthetic activity, reduced the amount of CO2 and increased the amount of O2 existing in the atmosphere. During the Proterozoic, colonies of bacteria formed rocks called stromatolites.
The appearance of oxygen in the atmosphere caused the first aerobic respiration organisms to appear.
Then, about 1500 my ago, the first eukaryotic cells appeared, and at the end of the Proterozoic, the first multicellular organisms. They were flat-bodied organisms, such as sponges, coelenterates, etc., which left few fossils. The few fossils that have been found are known as the Ediacara fauna, to be discovered in this Australian locality.
By Daderot (Daderot) [CC0 or CC0], via Wikimedia Commons
The Ediacara fauna represents the first great explosion of life on Earth, made up of marine invertebrates that became extinct at the end of the Proterozoic.
By Ryan Somma (Life in the Ediacaran SeaUploaded by FunkMonk) [CC BY-SA 2.0], via Wikimedia Commons