Approximately when did algae first start to produce oxygen on Earth?

The timing of when algae first began to produce oxygen on Earth aligns with significant geological and biological events in the planet's early history. Algae, particularly cyanobacteria, are believed to have emerged around 2.7 billion years ago, which corresponds with the Great Oxidation Event.

During this period, photosynthetic organisms, including cyanobacteria, started to release oxygen as a byproduct of photosynthesis. This increase in atmospheric oxygen drastically changed Earth's environment, leading to the development of aerobic life forms and the eventual formation of the ozone layer, which protected living organisms from harmful solar radiation.

The other choices reflect significant time periods in Earth’s history but do not accurately represent the time algae began oxygen production. Approximately 1.5 billion years ago, more complex life forms were emerging, while 3.5 billion years ago and 4.0 billion years ago correlate with the early existence of microbial life, but not necessarily with the effective production of oxygen in significant amounts. Therefore, the choice of around 2.7 billion years ago is corroborated by geological evidence and the study of ancient sediments that indicate oxygen levels began to rise significantly around that time.