Precambrian

Eukaryotes first appeared during the Proterozoic Eon. Eukaryotes are more advanced than prokaryotes because they have a nucleus enclosed in a nuclear membrane. They also have organelles, specialized organs for functions such as respiration, photosynthesis, and food storage. Most important, eukaryotes are able to reproduce sexually, which means that early eukaryotic organisms were able to achieve genetic diversity and adapt to and survive environmental changes. Eukaryotes are found in the form of advanced algae and bacteria from about 2 billion years before present. By 1.7 billion years before present, eukaryotes were widespread, as indicated by the microfossil record in Proterozoic cherts. By the middle Proterozoic Eon, stromatolites deposited by cyanobacteria occurred in greater numbers than at any other time. By 1 billion years before present, cyanobacteria and a great variety of protozoa (single-celled animals) were the dominant organisms on the Earth.

Between 1 billion and 800 million years ago, the oxygen levels in the oceans and the atmosphere were high enough for multicellular organisms to evolve. Because these organisms were soft-bodied, they were not well preserved as fossils; they are found only as molds or casts in rocks. The first Proterozoic fossils of multicellular organisms discovered by scientists were the Ediacaran fauna, originally found in shallow-water marine sediments in the Ediacara Hills of southern Australia. Since this discovery, these fossils have been found on all continents, indicating that they were quite widespread during the Proterozoic Eon.