Proterozoic eon

Proterozoic is the third period of the Earth and is the longest period in the history of the Earth and the events that occurred were: Earth glaciation, oxygen catastrophe, and the emergence of multicellular organisms.

life
there were no multicellular organisms in the early Proterozoic. multicellular organisms appeared at the end of the Proterozoic. the first eukaryotes were formed during this period

oxygen disaster
at the beginning of the Proterozoic, an event called the oxygen catastrophe began, which caused the rocks to  oxidize

the fact that a lot of sulfates got into the ocean waters, which in turn caused the appearance of a large amount of sulfur bacteria.

glaciation
after the oxygen catastrophe, the Earth glaciated. a lot of photosynthetic algae died, some survived at volcanoes. later the Earth unfroze. and the cycle repeated itself. The Earth froze and thawed again