2.90 billion years ago: The emergence of photosynthetic life. 2.70 billion years ago: Mantle overturn. The first photosynthetic bacteria were anaerobic microbes, which produced no oxygen. Through time, life developed the mechanism to utilize oxygen to produce more energy, recorded in the appearance of cyanobacteria.　Around 2.7 billion years ago, due to the cooling of the Earth’s interior, the old slab of the primordial crust, which had been transferred to the bottom of the upper mantle by plate tectonics where it had stagnantly rested, collapsed to the lower mantle. Ensuring abundant mantle plumes ascended from the lower mantle to the upper mantle, a distinct event known as mantle overturn.