The Advanced Civilization That Might Have Ruled Earth Millions Of Years Ago

We know that there was life on Earth well before we humans arrived. That evidence is in the form of dinosaur fossils, among other things, per National Geographic.  But what if there was a civilization before humans earlier on in the planet's lifespan? It might be hard to find since things decay and erode over the course of millions of years, so there would be scant evidence that it even existed. How would we know what to look for? Is it possible that we could be missing something big? Could they have left some sort of sign confirming their existence?

Okay, let's hold up a bit. Before we get into conspiracy theories, let's underline that this is something that scientists are using to look for possible advanced life on other planets. As of today, they do not believe there were any advanced civilizations before us, but they have an idea of what markers to be on the lookout for. 

The principle that would be used is called the Silurian Hypothesis, which is named after aliens from the show "Dr. Who" (via Discover Magazine). The thinking behind it is that any signs of an ancient civilization that was around millions of years ago would have been lost to time. It's not that the scientists think that we were preceded by anything as advanced as us — they want to use that information to look to other planets. 

The thing that researchers would target is whether there is a high level of carbon in the atmosphere. Why? Well, we have a larger amount of it in our atmosphere, so it makes sense that another civilization might have as well. The deciding factor would be whether it was in a long burst or a shorter burst. A longer burst might indicate that it was something that was just happening in the planet's development. After all, there was a period well before we came about when the Earth also had a lot of carbon dioxide  But a short burst might be something that comes about when a civilization reaches a certain level of technology and creates carbon dioxide.

Interestingly, the carbon dioxide search might be something that another civilization has to do for us if we go extinct and they arrive a couple hundred million years later, when all traces of our society have disappeared.