Answer:
The answer is the Archean Eon.
Step-by-step explanation:
This period of history occured 4,000 to 2,5000 million years ago. Regarding its atmosphere, it is believed to have been almost completely oxigen-free. The Sun presented approximately 3/4 of its current luminosity, but the heat levels were nearly those of modern Earth.
It is believed that is was during this period that life began to exist on Earth, due to the Earth's crust gradual decrease in temperature.