Which eon is characterized by the presence of free oxygen and multicellular organisms?

The Proterozoic Eon is notable for significant developments in Earth's atmosphere and biosphere, particularly the accumulation of free oxygen in the atmosphere, a process largely driven by photosynthetic organisms, such as cyanobacteria. This period witnesses the "Great Oxygenation Event," which dramatically altered the composition of the atmosphere and allowed for more complex life forms to evolve.

During the Proterozoic Eon, multicellular organisms began to emerge, marking a transition from primarily unicellular life forms that dominated earlier eons. The rise of multicellularity set the stage for the vast diversity of life that would flourish in later eons, particularly in the subsequent Phanerozoic Eon.

This Eon played a crucial role in Earth's evolutionary history, as the increase in atmospheric oxygen allowed for the development of more complex metabolic processes and ecosystems. The presence of multicellular life during this time also indicates an evolutionary advancement that was dependent on the enriched oxygen conditions.