Seemingly everyone is talking about artifacts and lack thereof.
Maybe we should be looking for something else. Some different kinds of signs.
If -we- wanted something to outlast us, what would we build?
What can preserve information across literally millions, even billions for years?
Life.
The genomes, the bird songs, the designs some fish draw in the sea floor [0], that gland in octopuses that basically cause them to self-destruct within a short lifespan, but removing which allows them to continue living just fine?
Maybe even the growth rings of trees.
Could there be any clues encoded in those patterns? Is there no hint of genetic modification (I hesitate to say "intelligent design" because of its loaded implications related to religion) similar to what we ourselves are doing now?*
* (and have been doing for thousands of years, via selective breeding etc.)
One clue to there not being a civilization like ours in the past, is all the oil that we found in the earth. Such a convenient energy resource would have likely been used up before we arrived, had there been an industrial civilization before.
If a future intelligent species would dig around a hundred million years from now, they'd see traces of us in the form of a radioactive sediment layer.
The paper discusses the longevity of radioactive isotopes over large time scales - basically only a few isotopes have half-lives long enough to last for tens of millions of years, and those can be created naturally as well as artificially.
On the point about oil, the paper points out that oil deposits have been created cyclically over large time scales:
"At least since the Carboniferous (300–350 Ma), there has been sufficient fossil carbon to fuel an industrial civilization comparable with our own"
There were three anoxic events, where the ocean was completely depleted of oxygen, in the Cretaceous period 132, 120 and 93 million years ago.
"these [carbon] releases often triggered episodes of ocean anoxia (via increased nutrient supply) causing a massive burial of organic matter, which eventually became source strata for further fossil fuels.
Thus, the prior industrial activity would have actually given rise to the potential for future industry via their own demise. Large-scale anoxia, in effect, might provide a self-limiting but self-perpetuating feedback of industry on the planet."
Although, as the paper notes:
"Alternatively, it may be just be a part of a long-term episodic natural carbon cycle feedback on tectonically active planets."
> Such a convenient energy resource would have likely been used up before we arrived, had there been an industrial civilization before.
Isn't the earth crust constantly moving, so new petroleum reservoirs are constantly formed and other destroyed as dispersed liquid hydrocarbons move and migrate in reservoirs?
So even if a civilization depleted its reserves, then new reservoirs would be available once the crust has moved enough?
Well the carbon in the atmosphere might build up to a point that it cooks most life on earth. So probably still wise to leave it in the ground as much as possible
Not sure why you're getting downvoted. It's definitely plausible that a prehistoric civilization would have left genetic traces of meddling with other species. Whether they'd have done it deliberately or not is more of a philosophical question.
Regardless, it's another interesting question: if another civilization bred and domesticated, and possible modified, other species 200 million years ago, what genetic traces would be left in the genome of current life?
200 million years is a fair old while in terms of genetic drift, however if you were building DRM or copyright information into your engineered forms, you'd likely want to do it in such a fashion that the form would be nonviable without that set of codons.
So you'd look for things like sudden changes to respiratory complexes that seem to require a large leap, or essential amino acid codings, and try to correlate those changes via genetic clocks with the kind of thermal events described in the article. One thing that came into existence around the PETM was mammals, with all of their novel adaptations.
Still, it could be (is more likely to be) correlative, not causative - or causative in a different way - a changed environment could exert extreme pressure on forms, causing some novel mutations to take hold.
There's another place to store information which isn't in the genes themselves but in the mapping of codons to amino acids. This location would be invariant across generations.
This paper on "biological SETI" made some noise when it came out and then disappeared, literally gone from arxiv, perhaps because the crackpot smell is too strong? I am curious if it merited either debunking or further study but didn't see much of either. If there is a signal there, it seems more plausible that prior inhabitants put it there than ET.
I suspect any evidence short of multiple artifacts from multiple locations would be circumstantial at best, regardless if the traces are in the planet's geological record or in the genetic code of an ancestor of whatever the supposed ancient ones ate for dinner.
On the other hand, multiple correlating indicators could give a pointer about where to look. Alas, it's likely such a place either deep underneath the ocean floor or that it's buried under kilometres of rock.
> that gland in octopuses that basically cause them to self-destruct within a short lifespan, but removing which allows them to continue living just fine
Wait what? has this been tried? Can we make long living octopodes by just doing a small operation?
> Octopus reproductive organs mature due to the hormonal influence of the optic gland but result in the inactivation of their digestive glands, typically causing the octopus to die from starvation. Experimental removal of both optic glands after spawning was found to result in the cessation of broodiness, the resumption of feeding, increased growth, and greatly extended lifespans. [0][1]
what we've learned until now is that, if such an information processing device would have existed then any sufficiently advanced civilization would have been using it to mine crypto currency.
If 'Dinomen' existed, we may see it in the other dinos or plants via domestication. If they were carnivores, we may see dinos bred to be fatter or larger. If they were herbivores, then we'd see certain plants that were bred for food, and maybe dinos bred for labor or as pets. One of the issues with domestication is controlling the breeding/birthing cycles of animals. Chickens and pigeons tend to be easier to do this with because they are dinosaurs and lay eggs.
We see this in our own mammalian species like cows. Their bone structure is (slightly) different from wild cattle and certain breeds have massive horns because us humans just like it. Dogs too, we've made a lot of breeds of them that all look very different. I'd imagine that a lot of the world should be covered in cow and dog bones of a wide variety of shapes and sizes.
Come to think of it, the ceratopsids (like triceratops) have a large variety of head crests, there were tons of them about, and they were pretty fat and muscular. Hmmm ...
We might be able to detect genetic engineering quite simply...
By comparing genomes, you can construct a philogenetic tree of known animsls, and thier hypothetical ancestors the instances where a mutation popped into being making a species and it's defendants distinct.
You can slot genomes from near extinct species and amber trapped insects into this tree and confirm where they sat.
So genetic engineering would look like - fish genes suddenly appearing in the dog family tree or butterfly genes turning up in a plant.
This would be pretty detectable with just a computer and open data. Has someone done it?
Keep refining/miniaturizing the technology. from mainframes, PCs, tablets, mobile watches to tiny chip in brain. In energy from coal, oil, electric, solar to zero-power. Eventually the grossness will disappear to a great extent. leaving little to know artifacts. The remaining artifacts including the dwelling structures get recycled into their raw sources given sufficient time; especially if remain under water for good amount of time. Continents may fall into ocean destroying the evidence more quickly.
This is only when you are expecting a civilization as current one. How about those who weren't dependent on technology ever - they may have chosen a route to discover about the nature, the universe and themselves drastically different from that of ours and hence we, enamored by a false sense of superiority, fail to even contemplate of such possibility let alone any chance of discovering an evidence of their existence.
I have a different idea. There are numerous historical accounts of basic resources available directly on the surface. These were cleaned up in preindustrial times when we had to start to dig for them. We have already removed almost everything that was in easy reach and now the resources we are digging up are either difficult to reach or require a lot of technology to concentrate and clean up. I am convinced there were no previous industrial civilizations because we would not see so many resources available in so clean form and so close to surface.
The problem is to insert something into the genetic code that survives evolutionary pressures over hundreds of millions of years. There would of course be great irony in this point; a civilization knows how to modify an animal in a way that is more adaptable for millions of years, yet it that civilization itself went extinct.
Maybe we should be looking for something else. Some different kinds of signs.
If -we- wanted something to outlast us, what would we build?
What can preserve information across literally millions, even billions for years?
Life.
The genomes, the bird songs, the designs some fish draw in the sea floor [0], that gland in octopuses that basically cause them to self-destruct within a short lifespan, but removing which allows them to continue living just fine?
Maybe even the growth rings of trees.
Could there be any clues encoded in those patterns? Is there no hint of genetic modification (I hesitate to say "intelligent design" because of its loaded implications related to religion) similar to what we ourselves are doing now?*
* (and have been doing for thousands of years, via selective breeding etc.)
[0] https://www.youtube.com/watch?v=hpdlQae5wP8