What was the earths atmosphere like a long time ago?

Within the last few thousand years it's been pretty consistent but with changes mainly in CO2 (higher now than in the recent past).

Within the last billion years, the oxygen content has varied from almost nothing to over 25% (currently about 20%), which gave insects a big boost in the game of survival.

Prior to about a billion years ago, it becomes more of a guessing game, but we can assume that the nitrogen level has stayed fairly constant, that there was no detectable (or free) O2 before the blue-green algae, that CO2 might well have been very high (the sun was cooler in those days, but the Earth still had liquid seas), and that significant free methane might have been around (also some weird sulfur compounds).