During the Precambrian Era, single-celled organisms, bacteria, microbes and some multi-cellular organisms existed. The Precambrian Era is split into the Hadean, Archaean and Proterozoic eons.
During the Hadean time, the environment was so volatile that scientists believe it's unlikely that any life could have existed. The next era was the Archaean, in which life began in the seas. The Eubacteria was the first form of life; it was a prokaryotic, or single-celled, primitive bacteria. Bacterial microfossils are the earliest known fossils. There were also stromatolites, or microbial growth structures possibly created by eukaryotes, which may have begun to develop during this era.
The Proterozoic time brought even more bacteria and microbes to life, such as the eukaryotic cyanobacteria, or blue-green algae. There were also Cnidarians, or early jellyfish. Early eukaryotes, or multi-cellular organisms, are believed to be the ancestors of all plants, fungi and animals.