You're not likely to have much evidence to show of a civilization a billion years ago if all you have is excess carbon dioxide that might be due to any number o

You're not likely to have much evidence to show of a civilization a billion years ago if all you have is excess carbon dioxide that might be due to any number of natural phenomena: forest fires due to overgrowth followed by drought, asteroid strikes thawing the tundra or whatever. Good luck claiming ancient civilizations if all you have is "a lot of organic material returned the CO2 to the atmosphere" but with no artifacts or anything else to show. Now, I'm just using a billion years as a huge stretch to make it harder to find the artifacts I'm talking about. Anything within a 100M would be leaving fossils of all sorts of junk. A fossilized piece of an obviously artificial concrete building or bridge would be solid evidence and clearly Plan A compared to arguments about the cause of ancient CO2 fluctuations if you are purely interested in evidence of ancient civ.