Final answer:
Oxygen accumulated in the Proterozoic ion atmosphere due to photosynthesis by cyanobacteria, supporting the development of eukaryotes and multicellular organisms.
Step-by-step explanation:
In the Proterozoic ion, the first eukaryotes and multicellular organisms have the oxygen accumulated in the atmosphere that supported them. During the first two billion years of Earth's existence, the atmosphere was anoxic, with no molecular oxygen, so only anaerobic organisms could survive. However, autotrophic organisms known as phototrophs appeared within one billion years of Earth's formation, leading to the evolution of cyanobacteria, which were responsible for the initial oxygenation of the atmosphere through photosynthesis. This increase in atmospheric oxygen allowed the development of more efficient O2-utilizing catabolic pathways and increased colonization of the land, as some of the oxygen was converted into ozone (O3), which absorbs harmful ultraviolet light. Consequently, the accumulation of oxygen laid the foundation for the emergence of eukaryotic life and multicellular organisms.