What sequence describes the evolution after the Great Oxidation Event?

• A. Multicellular organisms evolving first
• B. Prokaryotes becoming eukaryotes
• C. Unicellular organisms evolved into eukaryotes, then complex multicellular organisms
• D. Eukaryotes regressed to unicellular forms

Answer: (C)

Oxygen rising after the Great Oxidation Event boosted energy availability for life, making it possible for cells to become more complex. Early life was mostly single-celled prokaryotes, and with more energy from aerobic respiration, cells that developed a nucleus and internal organelles—eukaryotes—could evolve. From there, organisms began to form coordinated, multicellular bodies, leading to complex life. That sequence—unicellular organisms giving rise to eukaryotes, then to multicellular organisms—best fits what we know about the history of life after the GOE. The other options don’t match the established timeline: multicellular life did not come first, prokaryotes didn’t directly become eukaryotes in that straightforward way, and there’s no widespread evidence of eukaryotes regressing to unicellular forms.