It looks like earthly civilizations arrived with non-fossil-fuel energy technology in place. Else the fossil fuel supplies would have been used up already, by developing civilizations before our own. The implication is that our earlier civilization(s) were transplanted on earth rather than native grown.
This assumes that our civilization exploited fossil-fuel based energy sources as the least-cost alternative, as we gradually pulled ourselves out of the disaster(s) that destroyed earlier versions, and that previous civilizations would have faced similar cost/benefit alternatives and would have made the same choices that we did.
Ancient aliens indeed.
I have seen the ETs - they are us (Pogo).