It turns out that asking the question, "Have there been previous human civilizations on Earth" is not just crackpot goofiness.
If you are a diligent scientist, it forces you to ask some interesting questions, and look at human history in a different way.
I love this. Science done right.
The article they link to in the Atlantic is a good read that delves deeper into what we might look for and what the evidence we leave behind might look like far into the future.