Imagine you are the first microbial cells, new to the scene of earth as a habitat billions of years ago. Describe your surroundings. What does the air contain? what’s the temperature you’re experiencing? what else?
The primary atmosphere air did not contain oxygen. As a result, the first bacteria were anaerobes and did not use oxygen for oxidation reactions. Earth's original atmosphere was rich in methane, ammonia, water vapor, and the noble gas neon. The lack of ozone layer resulted in the high amount of UV solar radiation on the Earth. So, the rate of mutations of the bacteria was very high. The microbes probably lived in oceans with a relatively high temperature (50-80 °C).