Thinking about this some more, if there was an industrial civilisation at the PETM I think it would be most likely marine based. (Maybe cephalopods?)
I previously asked myself what evidence we would see if there was a prior industrial civilisation on Earth and I came up with 1) transfer of biological species to other continents as per my previous comments 2) depletion of fossil fuels (I don’t remotely know enough geology to begin to answer the question of whether we are ‘missing’ fossil fuels that ought to be there) and 3) technofossils especially plastics. I only commented about 1) because that’s the one I thought the most compelling argument.
But actually, a marine civilisation accounts for all three arguments. The oceans were and are connected, so it’s not a surprise to find the same species in both the Pacific and the Atlantic. And the oceans are severely under-studied so we’d be much less likely to notice any oddities. We’re also much less likely to notice missing fossil fuels under water (yes our civilisation drills in the sea if it’s shallow enough but it’s less explored and less understood than the land). And the odds of us finding fossils under the seabed are practically zero. (Unless there’s a good area where what was seafloor at the PETM is now uplifted into a landmass?)
We still have a complete absence of evidence that any such civilisation existed, but I might join you in giving it a 5% possibility.
And now my mind goes down more speculative routes—what does the tech tree of a marine civilisation look like? They couldn’t have fire, therefore no metalworking or pottery. Is there some other basic technology, as hard-to-imagine for us terrestrials as fire would be to a stone-age squid? (Maybe: can you exploit large changes in water pressure to change the properties of wood or other materials? Could a marine civ drop or lower objects a couple of kilometres down, leave them for a while, and then retrieve them, and would that produce useful changes?) How would a civilisation without fire invent combustion engines or steam turbines or other ways to get energy out of fossil fuels? Hmm. I will stop now before I spend all day going down this rabbit hole.
Thinking about this some more, if there was an industrial civilisation at the PETM I think it would be most likely marine based. (Maybe cephalopods?)
I previously asked myself what evidence we would see if there was a prior industrial civilisation on Earth and I came up with 1) transfer of biological species to other continents as per my previous comments 2) depletion of fossil fuels (I don’t remotely know enough geology to begin to answer the question of whether we are ‘missing’ fossil fuels that ought to be there) and 3) technofossils especially plastics. I only commented about 1) because that’s the one I thought the most compelling argument.
But actually, a marine civilisation accounts for all three arguments. The oceans were and are connected, so it’s not a surprise to find the same species in both the Pacific and the Atlantic. And the oceans are severely under-studied so we’d be much less likely to notice any oddities. We’re also much less likely to notice missing fossil fuels under water (yes our civilisation drills in the sea if it’s shallow enough but it’s less explored and less understood than the land). And the odds of us finding fossils under the seabed are practically zero. (Unless there’s a good area where what was seafloor at the PETM is now uplifted into a landmass?)
We still have a complete absence of evidence that any such civilisation existed, but I might join you in giving it a 5% possibility.
And now my mind goes down more speculative routes—what does the tech tree of a marine civilisation look like? They couldn’t have fire, therefore no metalworking or pottery. Is there some other basic technology, as hard-to-imagine for us terrestrials as fire would be to a stone-age squid? (Maybe: can you exploit large changes in water pressure to change the properties of wood or other materials? Could a marine civ drop or lower objects a couple of kilometres down, leave them for a while, and then retrieve them, and would that produce useful changes?) How would a civilisation without fire invent combustion engines or steam turbines or other ways to get energy out of fossil fuels? Hmm. I will stop now before I spend all day going down this rabbit hole.