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I noticed that when it comes to the idea of humans going extinct, and how human civilization could be detected by future civilizations, a lot of the ideas for how human civilization could be detected seem focused on things like finding the ruins of buildings, or finding other non living objects made by humans. Humans have also moved animals and plants from continent to continent both intentionally and unintentionally.

I was thinking that if some of these animals survived on the continents they were introduced to for millions of years they could leave a trace in the fossil record, and was wondering if a future civilization could work out that their movement was unnatural, and so figure out that there was another intelligent life form before even in the absence of other evidence.

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  • $\begingroup$ The lack of goal and petroleum should be a hint, but the various nuclear waste disposal sites should make it clear. If it is possible to deduce the same through biology/ecology, the science isn't there yet for that. $\endgroup$
    – John O
    Commented Mar 22, 2022 at 15:01
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    $\begingroup$ A good answer requires some more information.. Specifically: what is the technology level reached by the ancient humans and what's the present technology level in your story? The objection put by JonO would count, but was (or is) nuclear tech actually present in the scenario ? These folks could be late 19th century paleontologists, digging up fossils and drawing conclusions about a 21th century level civilization that has once existed. There would be radio-active remains, but the paleontologists cannot measure radiation. So depending on tech level, you get different outcomes. $\endgroup$
    – Goodies
    Commented Mar 22, 2022 at 15:16
  • $\begingroup$ The litter we've left on the Moon provides fairly solid evidence of our existence, and approximately when. $\endgroup$
    – user535733
    Commented Mar 23, 2022 at 20:43
  • $\begingroup$ A future civilisation when? When does ours die? and What technological level has this new civilisation reached? The length of time between our demise and their arrival, the stage of their civilisation etc all changes the answer significantly, a civilisation comparable to ancient Rome in technology a hundred million years after we die arising from a new species that came into being after our demise will have no chance of having the slightest inkling we existed, but if it's only a few thousand years between them and us then they definitely will know, some details needed in the question 🤗 $\endgroup$
    – Pelinore
    Commented Mar 23, 2022 at 22:39

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It is much more likely, unfortunately, that the oddities in animal and plant distribution caused by human introduction of species to odd locations would simply prevent an analogue to the Theory of Evolution from ever arising.

Our own evolutionary theories were discovered because of very specific sets of evidence arranged in very specific ways. And the theory was attacked when it was introduced, and if the evidence for it had not been very strong, it probably would have been discarded and forgotten.

By putting modern species into a blender and whirring it up, as it were, mankind has probably muddied the waters considerably for a repeat of that process, if it ever were to become necessary. So future generations might not be able to use the fossil record to figure out that a world civilization existed, because our actions would have made the fossil record less scientifically significant to those future generations.

We've also probably placed similar obstacles in the way of the re-discovery of plate tectonics.

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  • $\begingroup$ I agree, it could've gotten to the point of such derangement that any patterns would be masked by noise to such an extent that the creatures wouldn't even recognise that there were patterns to look for. $\endgroup$
    – Escaped dental patient.
    Commented Mar 22, 2022 at 17:07
  • $\begingroup$ I disagree that our actions would prevent an analogue to the Theory of Evolution from arising. It does, of course, depend on the timescale assumed between 'end of humanity' and 'the civilization that followed', but given enough time the properties of the Theory of Evolution would erase the distortion that humanity has created. $\endgroup$
    – David Jacobsen
    Commented Mar 22, 2022 at 17:37
  • $\begingroup$ I see your point about Theory of Evolution, but I'm not sure about plate tectonics. The fossil record and similarities between existing species across continents were only two arguments that Wegener pointed out. He pointed out that the continuation of geological structures across continent boundaries, and the existence of coal deposits in Antarctica and of geological evidence for ancient glaciers in the Sahara were best explained by plate tectonics. Further evidence comes from structures like the Mid-Atlantic Ridge. All of these are unaffected by the spread of species due to human influences. $\endgroup$
    – Schmuddi
    Commented Mar 23, 2022 at 13:20
  • $\begingroup$ @Schmuddi You make a very good point. I just am not so sanguine that a discovery rooted in many different lines of evidence would be repeated easily if some of those lines became obscured. Discovery is hard and not inevitable. $\endgroup$
    – tbrookside
    Commented Mar 23, 2022 at 20:45
  • $\begingroup$ We’ve also dug up the easiest to access layers of the fossil record. $\endgroup$
    – SRM
    Commented Mar 23, 2022 at 23:10
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I doubt it. After millions of years, most traces of humanity would have degraded to the point where it would be difficult to identify them as manmade. For example, many stone tools are hard to identify by untrained eyes as manmade. (Arrowheads being a notable counter example.) Metals will have corroded and been transported by water into new forms and locations. There will have been significant erosion moving dirt on top of much of the waste. Without the assumption that intelligent life existed prior, it might be very difficult to come to the conclusion that it existed. Extraordinary ideas need extraordinary proof, and it might be difficult to find proof that humanity was intelligent.

So, the existence of animals on one continent might be taken as a common ancestor walked over some land bridge but the fossils of that common ancestor haven't been found. Or similar forms may have evolved multiple times in multiple places. Plants might have had convergent evolution. These explanations all fit with observed patterns.

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    $\begingroup$ I'm not sure if the concentration of imported species in Australia would be easily explicable by a land bridge or convergent evolution. Australia has ostriches from Africa, dromedary camels from India, buffaloes from Indonesia, toads from the Americas, and donkeys, pigs, cats, and of course rabbits from Europe. These species will start to leave a fossil record from now on, but with no ancestor fossils for any of them. Wouldn't the future researchers notice the sudden appearance of a number of very different species with close relatives from all other continents? $\endgroup$
    – Schmuddi
    Commented Mar 22, 2022 at 16:37
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What time scale are these events happening on?

You talk of Human society collapsing and another developing in its stead, but this sort of this doesn't happen over night. So:

  • Possibility one: this happens over a short-ish time scale. Say in a few thousand years, some species of animals start to develop higher thinking patterns and establish societies based on transmission of knowledge (most likely primates, but one could imagine other mammals or even octopodes). In this case, there will still be many traces left of human presence through inorganic traces left behind: plastics, concrete, and metal all shaped and worked into tools and buildings. They will most likely be very degraded, but still very much identifiable as being traces of an extinct civilised species. If so, the evidence of Human activity through the displacement and modification of other species is a mute point, seeing as it is not needed to infer their existence. It might be noticed, but would also most often correlate with the other vestiges found, for example by finding exotic skeletons in zoos and aquariums, or large gatherings of cattle and poultry remains next to farming equipment.
  • Possibility two: this happens over a long time scale. Over the next few million year, evolution takes its course, new species evolve and one is (un)lucky enough to develop higher thought patterns and form a society. In this case, it is entirely possible that the vast majority of Human "artifacts" would have degraded or been destroyed by naturally occurring phenomena. But over these millions of years, the species that humans transported and modified throughout the world have also had time to evolve into whole new species. Perhaps some die out (I'm looking at you chihuahuas), perhaps some find a new ecological niche to adapt to, but they will all be very different to what we know today. If the new society develops a penchant for genetics and studies all these species, they might observe some weird divergences in the similarity of some species, and perhaps the theory of a prior civilisation would emerge, but I doubt it would be enough proof alone to certify that this is the case. (Source: I am trained in biology and currently work in analysing genomic data)
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Radiations...

On a very long-time scale, the only remaining trace of mankind will be the nuclear wastes radiations.

Some researches are even currently being run on that subject in order to found a way to prevent future civilizations to dig them up… Including some patterns in wastes placement to make the nuclear landfill itself a message.

If a previous civilization relying on nuclear energy produced huge number of wastes over a very long time and decided to create a unique giant landfill, radiations may become noticable (even on surface) on that place and animals may avoid this area over generations… If a patterned-solution to warn next generations had been chose (imagine a spiral-shaped nuclear landfill of 50 km2 for example) you may then observe strange repartition of fauna on the area...

EDIT

To give some credits to the fact that wildlife can be affected by radioactivity, see below citation about Chernobyl and studies links (first and second)

birds and mammals at Chernobyl have cataracts in their eyes and smaller brains. [...] many of the birds have malformed sperm. In the most radioactive areas, up to 40 percent of male birds are completely sterile, with no sperm or just a few dead sperm in their reproductive tracts during the breeding season. Tumors, presumably cancerous, are obvious on some birds in high-radiation areas. - theconversation.com

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    $\begingroup$ "radiations may become noticable (even on surface) on that place and animals may avoid this area over generations" Yes, just like they have in Chernobyl 👍 Oh Wait! 🙄 they didn't actually do that did they 🤗 $\endgroup$
    – Pelinore
    Commented Mar 22, 2022 at 16:48
  • $\begingroup$ In fact, the Chernobyl example is a bit tricky as it is now a place preserved from human activity... In a world full of humans ! Therefore, the impact of radiation on wildlife resurgence is kind of skewed. On the contrary, studies on animals themselves show direct impact of ionized radiation (See the edit on my post for references). $\endgroup$
    – Freedomjail
    Commented Mar 22, 2022 at 17:36
  • $\begingroup$ High-level radioactive waste will have decayed to practically nothing over millions of years. Some lower-level radioactive materials have half-lives on that scale, but after millions of years sites with those materials will probably just be seen as natural concentrations. $\endgroup$
    – Marsh
    Commented Mar 22, 2022 at 18:01
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On a smaller scale, I believe this has happened; I recall reading about a botanist that noticed certain fruit plants were found in long trails, and showed that these trails corresponded to ancient "circuits" of hunter-gatherers, that probably gathered and carried the food plants with them to eat along the way, and pooped out the seeds along the trail.

One might see a similar thing in the fossil record. A concentration of many fossils in a particular area of origin, that are exclusive to a time period before say 5000 BC, followed by a growing diaspora of scattered fossils after that period.

We can tell by various means when fossils were laid down, when animals migrated, etc. Even delicate plants fossilize, we have many fern fossils from tens of millions of years ago.

That would indicate unnatural transportation. I mean, squash a zoo (somehow), fossilize the animals, and it is hard to devise a non-intentional explanation for why so many different animals that originated all over the world happen to be in the same place in well ordered and distinct locations, in such small numbers (a handful each of tigers, giraffes, hippos, elephants, seals, etc.)

Eventually the hypothesis would have to be artificial transportation of the species. The key here (as you seem to intuit) is time and fossil dating: A concentration of something for many millennia, followed by a near instantaneous spread, but in very thin numbers, to other continents and places, such that it is nearly impossible for that to be a natural migration; the only plausible explanation is an assisted migration, which means an intelligent entity capable of transporting the animals (or plants) long distances is responsible.

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They will know "something" happened, but not necessarily intelligent life

The fossil record will show both a massive redistribution of species and a massive extinction level event. The geological records will also show a massive spike in atmospheric carbon, lead, and all sorts of other anomalies. It will be clear that something "really bad" happened at this time in the Earth's history, but it may not be clear that this mass extinction level event was intelligent life. A much more likely explanation would be that there was some kind of massive impact similar to the Chicxulub impact. Such an impact could burn away the planet's forests, pump massive amounts of carbon and other unusual stuff into the air blocking out the sun and cause an ice age that would allow life to migrate all over the place over new temporary land bridges.

Explain enough stuff with what you believe to be true about asteroid impacts, then you can use all the other unexplainable stuff to change your model on what a giant asteroid impact can do.

It's also possible that your intelligent species will not follow the same line of technology as us; so, things that we've done to the environment wont necessarily be the same as what they will do to it. So while we've dumped tons of lead and radioactive waste into the air as part of our technological chain, thier civilization may have never used tetraethyllead fuel additives or nuclear reactors; so, when they see our layer with a bunch of lead and radioactive waist, they might rule out intelligent life believing that no intelligent species would pollute the world in that way. Inversely, thier civilization may mass produce a different pollutant that would be missing from our time period.

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Humans as intelligent life form

First of all, the future civilization has to distinguish between humans (as intelligent life form) and animals. Only then they can think that humans transported animals and plants.

But if there are fossils of humans, apes, chimpanzees, gorillas and other animals, how will they find out which was the intelligent life form or who spreads who.

They will find ships wrecks or airplane wrecks and know that these were used to travel to other continents and may be they assume that all the life forms spread from one place to other but 'from where to where?' or 'who did this?' may not be known.

I remember a story by Arthur C. Clarke, in which, all life form is finished on earth. Some aliens come and find a plastic strip with pictures. They consider it as the life form living on earth but they could not read words written at the end "Walt Disney Productions", "Mickey Mouse".

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    $\begingroup$ "There are fossils of humans, apes, chimpanzees, gorillas and other animals": Yes, there are. And one of those species left a million times more fossiles than the others, and the fossils of that species are frequently associated with metal artifacts such as rings and artificial teeth. Which of them was the dominant species? $\endgroup$
    – AlexP
    Commented Mar 22, 2022 at 9:27
  • $\begingroup$ @AlexP OK! They find that humans are most abundant and may be intelligent also, but when human are everywhere along with other animals and plants, the future civilization can assume that everything was everywhere and not deliberately spread from here to there. $\endgroup$
    – imtaar
    Commented Mar 22, 2022 at 9:39
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For a species to realise that they were not the first civilisation, but came after humans, you would need a long timescale, because a lot of our ruins and remains will stay for an enormous period of time, and will be a definite giveaway.

What will most likely give that our society existed and was civilised will mostly be the climate record, in which future scientists will see that the climate changed in an extremely rapid way within about 300 years give or take. And add on top of that evidence that there was a huge extinction event.

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The most striking evidence for our current existence even into the very distant future (many millions though possibly not billions of years) would be our mining activities. We have simply fundamentally changed the distribution of mineral resources in the Earth's near-surface volume. And it might not be often but I suspect some worked products (glass in particular) will survive in enough locations in great enough quantity that it will be unmistakable that no natural process created them.

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