If an index fossil whose absolute age is used as a reference for other fossils, it was determined only by radioactive dating or another absolute method.
But roughly 2.45 billion years ago, the isotopic ratio of sulfur transformed, indicating that for the first time oxygen was becoming a significant component of Earth's atmosphere, according to a <span>2000 paper in Science</span>. At roughly the same time (and for eons thereafter), oxidized iron began to appear in ancient soils and bands of iron were deposited on the seafloor, a product of reactions with oxygen in the seawater."What it looks like is that oxygen was first produced somewhere around 2.7 billion to 2.8 billon years ago."It looks as if there's a significant time interval between the appearance of oxygen-producing organisms and the actual oxygenation of the atmosphere."
Producing gametes I’m pretty sure