When did algae first start to produce oxygen on Earth?

The correct answer is that algae first started to produce oxygen on Earth around 2.7 billion years ago. This period is significant because it aligns with the Great Oxidation Event, a time when photosynthetic organisms, particularly cyanobacteria (which are a type of algae), began to release oxygen as a by-product of photosynthesis into the atmosphere and oceans.

This increase in atmospheric oxygen had a profound impact on Earth's environment, leading to the oxidation of iron and the eventual formation of banded iron formations, as well as paving the way for more complex life forms to evolve in the future. The development of oxygenic photosynthesis marked a transformative shift in the planet's atmosphere and chemistry, ultimately allowing for the evolution of aerobic organisms.

Earlier time frames, like 3.5 billion or 4.1 billion years ago, are associated with the existence of simpler life forms and the formation of the Earth's crust, but they do not reflect the onset of significant oxygen production. Thus, 2.7 billion years ago marks the more accurate timing for when algae began producing oxygen on a scale large enough to have a lasting impact on the atmosphere.