In most hypotheses of early Earth and the formation of life, it is accepted that the atmosphere was a reducing one. This means that there was an abundance of methane, ammonia, and water vapor, but there was an absence of significant amounts of oxygen gas. Based on this assumption, what was likely the first metabolic activity to harness ATP to evolve in primitive cells?
A) Photosynthesis, as plants evolved before animals
B) Cellular respiration, as this generates the most ATP
C) Glycolysis, as this does not require organelles not oxygen
D) Calvin cycle, as glucose was required by all cells for metabolism

It is believed that glycolysis – the first phase of cellular respiration in eukaryotic cells- is considered an ancient biochemical process that early organisms utilized to tap the energy required for cell functions. This is because the process does not require oxygen – because early conditions on earth were anoxic before the great oxygenation event. It is after the great oxygenation event that the Krebs cycle evolved.