What Was Early Life Like?
Earth's atmosphere in early life was composed of carbon dioxide with little to no oxygen. The atmosphere would have also contained small amounts of water vapor, ammonia, and methane. When the Earth cooled down it would cause most of the water vapor to condense. That would then form the oceans we've come to know today. The atmosphere still didn't contain much oxygen so it would be every difficult for organisms who can't handle extreme conditions to survive. That would then allow things like bacteria to flourish, and things like mammals to not even exist.
History in Rocks
Fossils are the remains or impression of a prehistoric organism preserved in petrified form or as a mold or cast in rock.
Fossils are usually made when a plant or animal dies in a watery environment and is buried in mud and silt. The soft tissues then decompose leaving hard material or bones behind. Over time sediment builds over the top and hardens into rock.