In several posts, it was mentioned that an alien civilization could become extinct. There were a variety of causes for this. One was simply that they did not desire to continue to exist, the category A4 civilization. This could come to pass by them simply not continuing to produce young aliens. Other reasons included running out of resources, the planet meeting some sort of peril, either planetary, stellar, or galactic, or even that they left no one behind when the migrated elsewhere. They could descend into idiocracy and not be able to maintain their civilization or even their continued existence. Certainly there could be other reasons, such as a large world-wide war with advanced weapons, or a very devastating minefield that they encountered before reaching asymptotic technology, such as terrorism with advanced capabilities for destruction.
Consider what happens afterward. Here is a planet which originated life, which then proceeded through all the evolutionary steps to reach intelligence and then developed a civilization, which for one of very many reasons, disappeared off the face of the planet. What does the rest of the life do, now that the capstone creatures are no longer there? It is back to evolution, perhaps with a far different genetic content if the alien civilization passed the genetic grand transition and modified the ecology. Since evolutionary timescales are of the order of ten thousand years to a million years, something happens fairly soon. The fauna on the planet keep changing. Sooner or later, another species develops intelligence, as the environment is suitable for it, and there is nothing left behind from the original alien civilization which lasts forever to block it. With luck, or after several tries, another alien civilization arises on the same planet. They have their chance at survival. They have their chance at building a civilization which might leave the planet and travel to other planets or to other solar systems.
Can they tell they are number two? Would there be any signs of the previous civilization that the second civilization could find?
If the first civilization disappeared, but did not really disappear, but just broke down into a repeat of the past, such as hunter-gatherers, and then recovered after ten thousand years of evolution straightening out the errors of the first civilization, there would certainly be relics remaining. Large stone structures do not disappear in that length of time, and may not be covered with airborne dust and dirt by that length of time. There might even be non-physical artifacts, such as some lingering oral tradition that talks about un-understandable events and phenomena. According to some archeologist, Earth went through something similar to that, in that there are remains of small cities from about ten thousand years ago, then a period of some millennia when there were none, and then they started again and grew larger than the maximum found in the first epoch. This may be instead an observational fillip, the result of incomplete investigations or the result of some chance finds. But on our hypothetical alien planet, something similar could be found.
If the period between civilizations was a hundred thousand years, it would be expected that even stone monuments would disappear. Archeological remains would be the only remaining memorial to the first alien civilization. In wet dirt, these would disappear over a hundred millennia, but if somehow fossil remains were created of the alien creatures themselves, with some extreme luck on the part of the second civilization’s citizens.
Recall how fossils come to exist. An animal is caught in some trap or dies in some area which is quickly covered and then covered again, until some type of rock such as sandstone is formed. The bones remain embedded and are turned into rock as well, by normal chemical processes in ground water, replacing the bony material with some sort of mineral in the shape of the bones.
Would aliens likely undergo such sudden burial, if they lived in cities? Perhaps not, but would there be other objects from an alien city which would be fossilized if buried in dust and dirt or mud? Would a gold object, if buried accidentally in a desert climate, remain that long? Gold nuggets last for very long times when formed naturally, so it is likely. Would gold objects be lost in a civilization? Certainly, assuming the planet had gold in sufficient quantities. Thus, instead of fossils remaining to be discovered, gold objects might be buried and await excavation by second civilization citizens. Other precious metals, silver, platinum, or others might also last for this period if buried in the right conditions.
Aside from precious metals, could anything else be fossilized? Could other materials, such as non-precious metals, undergo the same transformation as bones, so they would form fossil fragments? The key phenomena is that the object is buried and is not changed until some later period after the buried, during which time it does not lose its form. Some metals, such as iron, would oxidize and dissolve too fast to form a mold for the fossil mineralization to form, but perhaps if the alien civilization used other metals with a slower removal rate this could happen. Once mineralization happens, the fossil can endure for millions of years, as the migration of rock minerals is virtually non-existent. Otherwise we would not have minable veins.
It might be extremely unlikely that a fossil relic of the first civilization would be dug up by the second. Many fossils are found because some geologic process exposes one, and then archeologists dig deeper in the same areas, under the assumption that the burial site probably caught more than one animal. To find a single object, when there was no indication that something like this existed, is even more unlikely than the finding of a skeleton, unless an entire city was somehow buried, and it happened to have objects which could become fossils. At this point, nothing like this is known.
Only a mass burial of aliens, in the proper situation where fossilization can occur, is likely to exist for the second civilization to find, and such mass burials would not be likely accidentally. A mass grave or large gravesite could become fossilized. Thus, a cemetery might be the only way for the second civilization to learn there was one that came before them.
It is hard to imagine what would be the psychological effect on an alien civilization if they suddenly found out that there had been an alien civilization inhabiting the same planet long before, perhaps ten million years before or at any period long enough in the past for fossilization to occur. Finding precious metal objects from a hundred thousand years before might have the same startling effect. A civilization-wide understanding that everything they had built could simply disappear and only exist as fossils or relics changes how a civilization views itself. It might be taken as a warning, and some actions taken as a result of the realization. The fear of obliteration could actually lead to the adoption of a meme that would work to preserve the second alien civilization, perhaps even to the extent of advocating star travel as soon as possible.
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