Based on known archaeological data, we are the first technological and symbol-using civilisation on Earth (but not the first tool-using species).
This leads to an analogy that fits Fermi’s paradox: Why are we the first civilisation on Earth? For example, flight was invented by evolution independently several times.
We could imagine that on our planet, many civilisations appeared and also became extinct, and based on mediocre principles, we should be somewhere in the middle. For example, if 10 civilisations appeared, we have only a 10 per cent chance of being the first one.
The fact that we are the first such civilisation has strong predictive power about our expected future: it lowers the probability that there will be any other civilisations on Earth, including non-humans or even a restarting of human civilisation from scratch. It is because, if there will be many civiizations, we should not find ourselves to be the first one (It is some form of Doomsday argument, the same logic is used in Bostrom's article “Adam and Eve”).
If we are the only civilisation to exist in the history of the Earth, then we will probably become extinct not in mild way, but rather in a way which will prevent any other civilisation from appearing. There is higher probability of future (man-made) catastrophes which will not only end human civilisation, but also prevent any existence of any other civilisations on Earth.
Such catastrophes would kill most multicellular life. Nuclear war or pandemic is not that type of a catastrophe. The catastrophe must be really huge: such as irreversible global warming, grey goo or black hole in a collider.
Now, I will list possible explanations of the Fermi paradox of human past and corresponding x-risks implications:
1. We are the first civilisation on Earth, because we will prevent the existence of any future civilisations.
If our existence prevents other civilisations from appearing in the future, how could we do it? We will either become extinct in a very catastrophic way, killing all earthly life, or become a super-civilisation, which will prevent other species from becoming sapient. So, if we are really the first, then it means that "mild extinctions" are not typical for human style civilisations. Thus, pandemics, nuclear wars, devolutions and everything reversible are ruled out as main possible methods of human extinction.
If we become a super-civilisation, we will not be interested in preserving biosphera, as it will be able to create new sapient species. Or, it may be that we care about biosphere so strongly, that we will hide very well from new appearing sapient species. It will be like a cosmic zoo. It means that past civilisations on Earth may have existed, but decided to hide all traces of their existence from us, as it would help us to develop independently. So, the fact that we are the first raises the probability of a very large scale catastrophe in the future, like UFAI, or dangerous physical experiments, and reduces chances of mild x-risks such as pandemics or nuclear war. Another explanation is that any first civilisation exhausts all resources which are needed for a technological civilisation restart, such as oil, ores etc. But, in several million years most such resources will be filled again or replaced by new by tectonic movement.
2. We are not the first civilisation.
2.1. We didn't find any traces of a previous technological civilisation, yet based on what we know, there are very strong limitations for their existence. For example, every civilisation makes genetic marks, because it moves animals from one continent to another, just as humans brought dingos to Australia. It also must exhaust several important ores, create artefacts, and create new isotopes. We could be sure that we are the first tech civilisation on Earth in last 10 million years.
But, could we be sure for the past 100 million years? Maybe it was a very long time ago, like 60 million years ago (and killed dinosaurs). Carl Sagan argued that it could not have happened, because we should find traces mostly as exhausted oil reserves. The main counter argument here is that cephalisation, that is the evolutionary development of the brains, was not advanced enough 60 millions ago, to support general intelligence. Dinosaurian brains were very small. But, bird’s brains are more mass effective than mammalians. All these arguments in detail are presented in this excellent article by Brian Trent “Was there ever a dinosaurian civilisation”?
The main x-risks here are that we will find dangerous artefacts from previous civilisation, such as weapons, nanobots, viruses, or AIs. And, if previous civilisations went extinct, it increases the chances that it is typical for civilisations to become extinct. It also means that there was some reason why an extinction occurred, and this killing force may be still active, and we could excavate it. If they existed recently, they were probably hominids, and if they were killed by a virus, it may also affect humans.
2.2. We killed them. Maya civilisation created writing independently, but Spaniards destroy their civilisation. The same is true for Neanderthals and Homo Florentines.
2.3. Myths about gods may be signs of such previous civilisation. Highly improbable.
2.4. They are still here, but they try not to intervene in human history. So, it is similar to Fermi’s Zoo solution.
2.5. They were a non-tech civilisation, and that is why we can’t find their remnants.
2.6 They may be still here, like dolphins and ants, but their intelligence is non-human and they don’t create tech.
2.7 Some groups of humans created advanced tech long before now, but prefer to hide it. Highly improbable as most tech requires large manufacturing and market.
2.8 Previous humanoid civilisation was killed by virus or prion, and our archaeological research could bring it back to life. One hypothesis of Neanderthal extinction is prionic infection because of cannibalism. The fact is - several hominid species went extinct in the last several million years.
3. Civilisations are rare
Millions of species existed on Earth, but only one was able to create technology. So, it is a rare event.Consequences: cyclic civilisations on earth are improbable. So the chances that we will be resurrected by another civilisation on Earth is small.
The chances that we will be able to reconstruct civilisation after a large scale catastrophe, are also small (as such catastrophes are atypical for civilisations and they quickly proceed to total annihilation or singularity).
It also means that technological intelligence is a difficult step in the evolutionary process, so it could be one of the solutions of the main Fermi paradox.
Safety of remains of previous civilisations (if any exist) depends on two things: the time distance from them and their level of intelligence. The greater the distance, the safer they are (as the biggest part of dangerous technology will be destructed by time or will not be dangerous to humans, like species specific viruses).
The risks also depend on the level of intelligence they reached: the higher intelligence the riskier. If anything like their remnants are ever found, strong caution is recommend.
For example, the most dangerous scenario for us will be one similar to the beginning of the book of V. Vinge “A Fire upon the deep.” We could find remnants of a very old, but very sophisticated civilisation, which will include unfriendly AI or its description, or hostile nanobots.
The most likely place for such artefacts to be preserved is on the Moon, in some cavities near the pole. It is the most stable and radiation shielded place near Earth.
I think that based on (no) evidence, estimation of the probability of past tech civilisation should be less than 1 per cent. While it is enough to think that they most likely don’t exist, it is not enough to completely ignore risk of their artefacts, which anyway is less than 0.1 per cent.
Meta: the main idea for this post came to me in a night dream, several years ago.