# How long would it take to remove all trace of a civilisation, with malice and intent?

If you look at the history of the earth today, we've got quite a long timeline - about 4.5 billion years.

We've got a few hundred years of 'good' historical record, and it gets progressively more sketchy the further back we go. There's not much sign of human civilisation before 10,000 years ago. We have a few relics from 'a few million years' of the stone age. But then there's another 4 billion years.

Is it at all plausible that there could have been a fairly advanced civilisation that occurred earlier in the planetary lifespan, but enough time has lapsed - and thus erosion, geology etc. have obliterated all trace?

And if not, what would be the telltale signs?

(Let us assume for the sake of this discussion that somehow sufficiently advanced brains were possible, and our understanding of the fossil record and genesis/evolution of life might be flawed.)

The concept of Ancient Astronauts indeed claims that there were 'visitors' of highly advanced civilizations. Their claim is that many of the architectural constructions of the Ancient High Cultures (like the Pyramids for the Mayan Godkings and the Egyptian Pharaos) could indeed have only been built with higher technical sophistication and therefore evident of higher technological culture present on the planet at the time.

Going back further in time, if a civilization of intelligent living beings had existed for example during the time of the dinosaurs, by now one could reasonably assume that due to the geological changes in particular, all traces obtainable with our current available means would be destroyed.

Telltale Signs of Real Early Advanced Civilizations

The most telling marker of an higher developed culture present on our planet would probably be finding traces of artificial radioactive elements (such as Uranium-235 in higher than its natural distribution) that would give away beings akin with nuclear technology.

However, the presence of a higher concentration of such elements is not limited to civilizational activity, it can under certain circumstances as well occur in nature:

Further Reference: http://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor

The biggest paradox encountered when looking for signs of Early Advanced Civilizations on the Planet would be the absolute unfamiliarity with their culture, and hence their cultural symbols.

If intelligent life has developed before the advent of the Homo Sapiens, its cultural relics might not be perceivable for us due to our inherent anthrophocentrism.

While we might attribute a cultural meaning to a well preserved stick (i.e. Wizard staff) we might be totally oblivious to the artifact value of for example a perfect sphere of stone, which we might consider a freak of nature, while its attribtues might have served as the whole underpinning of spirituality for a civilization that consists of Myriads of Myelin cells interactinv to form a 'fungal consciousness' on a planet.

Point given though, the presence of a giant intelligent fungus on our planet would have probably left traces, but my point is, if there was such a civilization before us and they had artifacts, we might not be able to distinguish them from natural objects due to our limitation to our human heritage.

• +1 Even if we found a "tell-tale" sign like U-235, we wouldn't know it to be a tell-tale sign, since we found it "occurring in nature", so it would just seem natural to us. – Geobits Nov 24 '14 at 19:38
• Yes, but there could be clues. Imagine we found a bicycle completely intact with frame and wheels in an iron deposit digging into the earth. Now that would amount to a miracle, like a bunch of leaves falling from a tree forming the letters of a sentence. But what if your letters look different and you have a different pedality than the bipedal human? We would not be able to see those and thats the sob-part of the story. – Robert Boettcher Nov 24 '14 at 21:26
• I want to +1 @Geobits comment. How do we know that the current distribution of U-235 is 'natural' vs. 'artificial'? We just know what it is currently, and that we're changing it. Earth's distribution of elements is not the same as Mars' distribution, IIRC. – user3082 Nov 24 '14 at 22:53
• @Geobits they don't just appear randomly. The natural reactors all formed under plate boundaries - it's the tectonic movement that focused the material there. If we were to find one in the middle of a tectonic plate however... – Baldrickk Jun 20 '17 at 8:12

We actually have fossil records of biological life at least 2.7 billion old, possibly 3.4 billion. And if something as fragile as bacteria leaves traces over that timespan, it's rather unrealistic to believe that a civilzation wouldn't, even if all obvious traces would be wiped out after a few dozen millenia.

However, it's a game of numbers. It takes a lot of coincidences for something to be preserved that long, and then to be found. So the more artifacts a civilization produces (should depend mainly on area/time covered and population density) and the more people are looking, the more likely it is that something will be found.

• Yes, there's a few things that seem fairly persistent. We've the long now clock that's intended to be a 10,000 year artifact. longnow.org/clock Fossils and the like are the result of auspicious circumstances mostly it seems. – Sobrique Nov 24 '14 at 21:57

Honestly...not much survives, especially on a geological time scale. If all humans were to suddenly die, we'd leave a fossil record of about 10 very incomplete bodies (and if history is right, if we did discover these bodies, we probably would have assembled them wrong on our first few attempts). The number of dinosaur fossils (and trilobites) is very much a testament to how long they dominated the planet for.

The oldest rocks found on the planet are (oddly enough) not actually from earth. Several meteorites that originated for the moon (ejected by asteroid impact and captured by Earths Gravity) date to the 4.3-4.5 billion year range, while the oldest discovered terrestrial rocks come from the Canadian shield and date about 3.6 billion years. I believe there's some ancient rocks in Australia as well...but the majority of the Earth surface is significantly younger than that. To give an idea of what these rocks had to survive:

2 billion years ago, the super continent Nuna was the planets primary landmass (and kinda tiny compared to the current landmass). It broke up and by about 1.1 billion years ago, the super continent of Rodinia formed. Rodinia broke up, separated itself with oceans and became 3-4 proto-continents, which eventually reformed to Pangaea. Pangaea formed when the two super continents of Laurasia and Gondwana collided about 200 million years ago. Most of this is exists only as theory as there is little evidence of pre-Pangaea...but a lot changed, what is now North America was equatorial at this time. 250 million years from now, the landmass will likely be Pangaea Ultima as North America squishes Japan into Asia.

The reason I bring this up is to show how volatile earths surface is across this timeline. Each of these formations, separations, and reformations of continents/super continents came with incredibly slow violence...masses of land pushed under the surface and new ones brought up. There is pretty much zero chance that any relic dating back to then exists now.

Terrestrial life took a good long time to develop...The first appearance is somewhere in the range of 4 - 3.6 GA, however it took nearly 3 billion years for this basic life to become multi-cellular (life existed, but did very little other than exist). Prior to Pangaea, Earth wouldn't have been much more than a barren desolate rock anyway. If there was a civilization prior to 500 million years ago, it most certainly developed else where and came here. Why they would is a bit beyond me.

Lets try this from a "what would we leave behind". Most of our garbage and easily seen traces breakdown in well under 1000 years through a 3 pronged attack that we actually fight on a daily basis: Plant life (and animal life), Erosion (including rust/oxidization), and short term geological processes such as volcanic and earth quakes. It's possible things like the floating garbage island in the pacific might have a slightly longer lifespan, but nothing that would last into the 10k years timeline.

In the slightly longer time scale (10k - 100k) glaciation becomes the challenge. Glaciers weigh an incredible amount and 'flows' works as a giant scrub brush crushing the land and any artifact from a previous civilization underneath it. In a 'snowball earth' scenario where the majority of the landmass is covered in ice, there is very little chance of anything not in the fossil record surviving for us to discover. Edit to add: Note that a single glacial cycle will leave relics behind to some degree...it's the constant cycle between glacial periods that crush, bury, melt and redistribute, and then crush and bury again. Glacials grind up sediment from the rocks below them, then melt off allowing water to take this sediment downstream and bury what's in it's path (pending on which theories you follow, this melt event can be semi-catastrophic...lakes form ontop of the melting glacier as large as some of the great lakes, until the ice walls holding them in break away and release the entire lake in a single event)

Add in - cataclysmic events, though very rare in a single lifetime, definitely occur in a drawn out timeline. Volcanic super eruptions for example have two impacts...Not only does it cover much of the surrounding land in dust/rock (pending the size of these eruptions this 'surrounding area' can be continental), but they also cause the on-set of 'nuclear winter', bringing on an ice age and inciting the glacial cycle once again

Our longest surviving gift to the earth is likely Cesium -138 with a half life of 2.3 million years...though hard to say if detecting this is 'proof of life' as natural processes could develop it to some degree.

But our longest surviving remnants of civilization won't be found on earth. Objects that we have left orbiting earth, including those on the moon, will be our longest living relics as they are not exposed to the forces of Earth. And it's also where I'd suggest searching...if an intelligent civilization existed on Earth, proof of it will be most likely in orbit of earth, not actually on earth. If we were to disappear today, it's possible we'd even leave a legacy that outlives our sun in the Voyager 1 probe which exited our solar system and into interstellar space in 2013.

I guess a bit more abstract..Hitler might be an odd leftover of our civilization. I'm not sure on the validity of it, but I've heard that nazi Germany shooting radio waves off into space might be the first contact an alien will have with messages from earth. More realistically, there was a concerted effort to send a message to a star system (messier 13 I think?) in 1974ish. These radio waves into space might be an enduring symbol of our existence as a species and may be the optimal place to look for signs of others.

• That's marvelous. Just the sort of thing I was looking for. – Sobrique Nov 24 '14 at 21:54
• Re the Nazi messages - was that inspired by Contact? – HDE 226868 Nov 24 '14 at 22:00
• You forgot e.g. figures, tools or machine parts made of noble metals. These will not go away as fast as 10k years. What with nuclear garbage, with containers/barrells designed to last very, very long? What with battle tanks made of uranium, like the Abrams? Even when covered by flora then, a bushfire may suddenly uncover them again. And don't forget that traces of the oldest, known cities on the world are as old as about 10k years (e.g. en.wikipedia.org/wiki/…). Even with glaciers, we found a lot of evidence far older than 10000 years. – phresnel Nov 25 '14 at 9:56
• @phresnel: I think you missed the point that at the billion-year timescale, a lot of artifacts will get covered not by flora, but by continents. – Michael Borgwardt Nov 25 '14 at 11:33
• @apaul34208 - fossilization and preservation are very separate topics. Remember we are talking millions of years, unless the fossilization process (stone replacing bone) occurs, no preservation will work on these time scales. – Twelfth Jun 5 '17 at 21:43

Beware that this debate is somewhat political, but there's at least an argument that an identifiable geological age can be marked by the presence of our industry:

http://www.theguardian.com/science/2014/oct/16/-sp-scientists-gather-talks-rename-human-age-anthropocene-holocene

Regardless of whether you think it's worth a new name or not, the geological markers under discussion (specifically, planet-wide geological layers of radioisotopes, or polyaromatic hydrocarbons, or possibly other things) could be identifiable in the geological record almost indefinitely.

1) Would "a fairly advanced civilisation" inevitably generate such identifiable geological markers? (probably "no", since we're talking about things that happened in the mid-20th and late 19th centuries respectively, it's a question of what you mean by "advanced"). Furthermore you can speculate an intellectually advanced civilisation without industry of a form that affects the composition of the atmosphere. There might be nothing inevitable about burning hydrocarbons, for all that it's easy energy once you know how.

2) Would we acknowledge them as indicating a civilisation? (possibly not, but we don't know for sure since we haven't found anything exactly like our layers. If the K-T boundary is the result of intelligent action, we've yet to deduce that!).

As far as artifacts and fossils from a pre-industrial civilization are concerned, anything lasting, say, 10k years, longer ago than the last ice-age or other surface-scouring geological event, in a limited area, could I think easily leave no trace that we'd expect necessarily to find. Geologically speaking that's a very small target. Consider that we may have as few as 3 complete Stegosaurus skulls, plus 30 partial skulls, from a group of species that lasted as much as 5 million years. IIRC there is at least one biological phylum containing only a single known species. That's a heck of a lot of missing links with unknown properties, albeit probably they were all small soft blobs milling around on the Cambrian sea floor ;-)

We really see only a small proportion of historical organisms, so I think a sufficiently small "civilization" could easily leave no trace. Suppose (as Lovecraft did) that it was located on Antarctica before it was mostly ice-covered. We've closely examined approximately 0% of that continent's geology and so we would not expect to see it. You'll have to decide for yourself how small and primitive something can be for you still to call it a "civilization", though, and that might give you some notion how long ago it would have to be to plausibly leave no trace.

What's much harder is to set a time period beyond which you think a civilisation is guaranteed to leave no trace. All it takes is one fossil dude together with a recognisable fossil tool and the gig is up.

Also consider the tech level of the people looking for it. You sort of imply present-day, but for much of European history, we (by which I mean, my ancestors) haven't really recognised the existence of anything pre-Classical in Europe.

• Sadly I suspect that if we found a layer that suggested use of hydrocarbons as a fuel source it would be reduced to evidence against global warming... – apaul Jun 3 '17 at 8:23
• @apaul34208: well, I've heard that atmospheric CO2 probably doubled in the course of the Triassic-Jurassic extinction event. But that's usually pinned on volcanoes, not so much on some civilisation burning the previous global supply of fossil fuels ;-) – Steve Jessop Jun 5 '17 at 12:34

Interestingly there was a paper recently published on Arxiv: The Silurian Hypothesis: Would it be possible to detect an industrial civilization in the geological record? which covers this very question and is indicated to have been published in the International Journal of Astrobiology

If an industrial civilization had existed on Earth many millions of years prior to our own era, what traces would it have left and would they be detectable today? We summarize the likely geological fingerprint of the Anthropocene, and demonstrate that while clear, it will not differ greatly in many respects from other known events in the geological record. We then propose tests that could plausibly distinguish an industrial cause from an otherwise naturally occurring climate event.

As a civilisation, we have created a LOT of non natural exotic materials, which we have blasted all over the solar system and spread all over and under our planet. It is going to be very, very difficult to remove all of it. I mean, consider the fact that a fleck of paint floating through space technically counts as a trace of civilisation.

So if you consider an advanced civilisation as 'one that can produce non natural materials' then it would be pretty hard to remove all trace of it. That said, it might already be impossible to detect an ancient civilisation because of all the stuff we have spread about.

The other answers have focused on natural erosion, reforestation and geological shifts as the primary methods by which a planet can erase a civilization. I'd like to cover a few less natural forces which might come to bear against an extremely advanced civilization.

Conquest When the Romans sacked Carthage, they attempted to leave no stone upon another, literally disassembling the entire city. We still don't know who carved the Sphynx or what its head originally looked like. Winners make lousy historians.

Religion Similar to conquest but often more diligent in its execution, Religious leaders often outlawed the original belief systems of newly evangelized lands.

Note that neither of these forces require that the surviving civilization be more advanced than the one being erased.

New Construction Modern cities are often built right over the top of previous cities, taking advantage of the same natural resources, rivers and trade routes which made the location desirable thousands of years earlier.

Looking forward a bit from our current place in technological development, several more civilization erasing technologies are just around the next corner.

Nanites Imagine a war being fought between two countries that each employ nanite disassemblers as weapons. Ordering the microbots to "Take apart everything made of plastic, metal or concrete" would be a very effective attack against an enemies infrastructure. If taken to extremes, it might also foil future historians for the rest of time.

Non-biologically Targetted Diseases Same as nanites, but easier to build. A bacterium that consumes all rubber and petroleum would leave most of our tools useless and rusting in pretty short order.

Expatriation There is a good chance that someday, we earthbound humans will leap out to the stars. When we do, how thoroughly we clean up our newly vacant planet may depend on many factors. How plentiful are the raw materials which we use to make our tools and toys, on whatever planet we are moving too. What religious significance or emotional attachment do we grant to every relic of our home world. ...and if we are running from hostile invaders, how thoroughly do we cover our tracks, cleaning up all our old space junk and orbitally visible artifacts before we make our escape.

Eviction We know that our planet has extreme ice ages, naturally occurring frozen over ages when all but the most primitive life gets scrubbed away. Perhaps, in response to an infestation of water-fouling, air-polluting vermin (us), our planet also has acid ages. Billowing clouds with sulfuric rain which dissolve us out of the history books. I seem to remember that many of the early ages of our planet's evolution are referred to as too toxic for life. Maybe we are just seeing the closing days and after-effects of our planet's previous infestors. We may not be the first "intelligent" life form to piss off our wonderful host, Gia.

Geology. There would be obvious clues of mining in a few places that could not be caused by natural processes.

We are currently mining metal deposits that were formed a very long time ago. For example, the Red Lake gold deposits in Ontario are dated from 2.7 billion years ago. What this means is that there are places where we are mining now which have held valuable commodities for a very long time and which would be a target for mining for any civilization that had preceded us. So what would we find if that had happened? We'd find mineral showings where the richest part of the ore was missing and all that was left was the low-grade material that no one would bother mining. That type of oddity would be accompanied by significant anomalies. Consider an open pit mine and what would happen to it; you'd have that low-grade host rock, but right where you'd expect the high grade material would be, the host rock is missing and you'd find a deposit of sedimentary rock, as if there was a hole where the high-grade material had been removed and that hole had been filled in over time.

Mines are not located at random; we know where we should be looking for them, and that's in the same places someone a century ago would be looking for them, someone a millennia ago would be looking for them, and someone millions of years ago would be looking for them.

For the sake of this answer, I will call this potential civilization before us the Precursors.

You say, "There's not much sign of human civilisation before 10,000 years ago," but there is. Just look in the dirt beneath you and you will find a recorded history billions of years long.

Many less obvious signs of civilization exist in the ground. Dig up fossils and you might find fossils along with signs of someone burying their dead. If these Precursors evolved here, you would find many signs of their early civilization in the ground. This is a good route to take because to destroy evidence of this, the Precursors or some other beings either have to know the locations of all Precursor burial sites and fossils and have the correct dirt to fill in the gaps, or else strip off entire layers of the ground and eject them into space. You'd have to fill in the gaps and/or eject the entire layers or human paleontologists would find the tampering very obvious to see and might be onto something. The fact that we haven't found signs of tampering suggests one of two things...

• A. There were no Precursors.
• B. The Precursors or other beings were nearly perfect at doing this cover up.

One should generally use Occam's Razor in this situation, but that is just my opinion and certainly not a rule of science.

This is of course assuming that the Precursors evolved on Earth. Other answers have covered the Ancient Astronaut theory well enough.

• So... Evidence of 'digging' is evidence of "Precursors" – apaul Jun 3 '17 at 8:26
• Seems pretty thin on a long timeline... – apaul Jun 3 '17 at 8:27
• Evidence of burials and other civilized methods would give them clues into the existence of a potential civilization that existed during that time. Burials aren't just "digging." Stuff like that is how we discovered the ancient human species. – Elvesflame Jun 28 '17 at 4:59

I hate to sound like one of those crystal peering new agers, but we should also perhaps take into account that our version of civilization is referenced from our own historical progression. If theoretical quantum physics has shown us anything, it's that there may be realms of "science" that our Pre-ancestors may have discovered that allowed them to develop an advanced civilization from that was not based on industrialization but on the very manipulation of the energies that make up the quantum realm. This could have allowed them to create amazing cities out of natural materials that we could never build even today with mechanical tools, but that would have been erased by natural geological events, such as glaciation and continental drift.

There are also theories of short periods of extreme geological violence, that perhaps accelerated and compounded normal geologic disasters beyond their regular capacity, such as a prolonged absence of shifting, resulting in a massive rupture between plates in previous rings of fire. This could have unmeasurable effect on traces of civilization if say an entire landmass shifted thousands of kilometers in a 60 minute uber quake. Not to mention the possibilities of such an event rupturing surperculderas and venting unprecedented plumes of ash into the sky creating a nuclear winter - and thus a global glacial period. Such an event could result in the complete eradication of all trace elements of any civilization except where possibly where nunataks existed (places where glaciers never covered the landmass but surrounded it.)

The unlikelihood of finding such a place compounded by the coincidence there might’ve been some kind of pre-ancestral presence there, shows the impossible nature of finding traces if such locations even existed. Not to mention without the knowledge of such quantum realms how would we even distinguish say a column of hexagonal basalt rock from a quantum formed pillar of basalt rock such as the hexagonal columns in Northern Ireland. Structurally they could easily be explained as both naturally occurring, or if such quantum manipulation existed so too could it have been formed in such a way that it would look precisely the same. There are a myriad of such places we absolutely know are from previous civilizations of the modern era (circa 10k to present) like the giant stones in Cusco that we have no idea how they were moved or cut so precisely at hundreds of tons and find it difficult to conceive of moving with our current tech. That’s in our own validated history, but to go back 50 or 100 000 years we cannot pass judgement of possibilities we are not knowledgeable enough to even acknowledge the existence of. Such ability to manipulate matter on a quantum scale would also prove far more dangerous than any nuclear weapons. So it is conceivable an advanced civilization might have indeed existed, and given such quantum knowledge just as easily could have been wiped out with the same quantum knowledge.

• Quantum technology would have to be based on industrial infrastructure, because that's how we're getting to create it for ourselves. Unfortunately, it's nigh on impossible to create quantum technology out of effectively nothing. Unless visiting alien space bats gave it to our ancestors. Look I give you kudos for an interesting & imaginative answer. Just that grumpy old me doesn't buy it. Don't let this discourage you. This is a great site. have fun here! – a4android Sep 10 '17 at 9:55
• There was a well-meant edit from a new user that was adding a few points. I will post it here in case you want to incorporate that information into your post and for future readers. Edits should not add things that the author himself has not provided through comments and only be used to clarify things and fix for example typos or formatting issues. – Secespitus Sep 10 '17 at 21:00
• "As far as requiring industrialization to access quantum realms, that is as explained above, an idea based solely on our knowledge base. If there is a way to access quantum realms without industrialization, we could not conceive of it, and therefore would never perceive the possibility of a Pre-ancestral race with such ability. That’s not to say it couldn't happen. Take for instance a man that has created a lightbulb, but has never visited the deep oceans." – Secespitus Sep 10 '17 at 21:01
• " If he saw a bioluminescent fish, would he then assume the fish was part of an aquatic super-race of fish that had gone through industrialization so they could design genetic lighting? No. He would accept that there are things he does not yet understand and that he has only discovered one way to create light. So too must we accept we only know of one way to access quantum knowledge, and given the nature of the quantum realm it is possible there are other ways to do so that we cannot as yet comprehend." – Secespitus Sep 10 '17 at 21:01

If the species lived in one localized area, a single geological event could destroy all evidence virtually instaneously. For example, they lived in the caldera of one of the super volcanos. (Yellowstone with its 35 mile caldera https://en.wikipedia.org/wiki/Yellowstone_Caldera?wprov=sfla1 )

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