Creative Speculation

Also, some of the signs we're seeing left behind could be misinterpreted. I'm thinking of plastic, that in 50+ million years might be seen as the result of some "natural" process.

What does seem a bit more likely though was that there was a species, or even class of species, that swarmed the earth at some time in the past and caused major shifts. Not necessarily "advanced", but so overwhelming, to a planet that may have been "out of balance" anyway, so as to trigger major changes. Such an event might not leave any obvious traces of their existence, or their affect.