Atmosphere

At the beginning of the Archean, the atmosphere was completely toxic to life today. It was mostly methane, CO2, ammonia, hydrogen, carbon monoxide, and other greenhouse gases. While these gases formed a greenhouse and kept the Earth warm, they weren't very breathable. Even though volcanic eruptions were extremely common and released lots of water vapor (among other things) into the air, there was little to no free oxygen in the atmosphere.

This is one of the reasons inter-eon time travel is generally a bad idea.

The story of how oxygen came to be in Earth's atmosphere is as much a story about life as it is about air. While the first life forms were centered around methane and other sulfates and simply lived out their lives, along came cyanobacteria, otherwise known as blue-green algae or some of the first photosynthesizing life. These cyanobacteria, living in settlements called stromatolites, produced oxygen through photosynthesis. While this did not dramatically increase atmospheric oxygen at first, it steadily rose for millions of years until the Great Oxygenization Event.

The Great Oxygenization Event occurred 2.5 billion years ago for unclear reasons. While we are not sure of the specifics, cyanobacteria started pumping large amounts of oxygen into the air. In fact, there was more oxygen in the air 2.5 billion years ago than there is in the air today! This actually poisoned life in the early Proterozoic, because they were not used to all that oxygen! 

Evidence of the Great Oxygenization event can be found in the isotopic ratio of sulfur in Archean rocks, as well as oxidized bands of iron on the sea floor and in the ground.