[tabtab]

Being that animals did not even appear until about 600 million years ago, It's quite unlikely industrial beings evolved before that. And any industrial civilization after 600m would leave sufficient fossils/evidence. If there were civilization before 600m, it would likely have to be aliens.

[CalRobert]

What evidence of us will exist in 600 million years? Some of the combusion products of engines are distinct as I understand it, perhaps some materials are stable that long (and if so, they could pose large pollution problems)?

[tabtab]

I'm sure there's plenty of junk in landfills that will make for interesting fossils. The material doesn't have to be "stable", it just has to make an imprint. Most existing fossils don't comprise the original material, but are rather "mineral shadows" of what was there before.