text: how and why was earth's atmosphere changed throughout the first 2 billion years of its existence.?   the first 2 billion years of earth's existence saw some pretty drastic changes, mostly the result of cooling from a molten mass. the remainder of the 4.55 billion years has seen this process continue, and the evolution of life, which likely started soon after the temperatures dropped enough for liquid water to form, has played an important role. very recently, human activities like agriculture and manufacturing have had some effect.   the how and why is nothing more than laws of thermodynamics.