Let us consider that humans leave earth after it becomes deadly for us. And then many years pass. Would there be a chance of a different but intelligent life evolving in our place.

Edit_1 Forgot to specify the time. Let's say 100 million years... So after the climate changes, maybe even after a new ice age or a meteor strike. Enough time should pass so that all that we have now, would be deep beneath the soil...

Edit_2 It does not have to be similar to humans. Let's say that the earth becomes uncomfortable to live on for humans and similar species. Not deadly but a place where as a human you would not want to be. Be it the temperature or lower levels of oxygen.

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    $\begingroup$ Welcome to worldbuilding stack exchange! $\endgroup$ Nov 27, 2016 at 10:56
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    $\begingroup$ What is "many years"? A hundred years? No change. A billion years? Definitely a chance. $\endgroup$
    – celtschk
    Nov 27, 2016 at 12:13
  • $\begingroup$ Once the doomsday clock starts ringing we probably can't stop/reset it... then any complex lifeforms must complete for basic essential such as clean air and non-toxic water in our place! Chances are if there are any post historic intelligent life they will most likely to follow suit... leave Earth! $\endgroup$
    – user6760
    Nov 27, 2016 at 12:24
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    $\begingroup$ @user6760: What is toxic for us need not be toxic for them. Especially if they evolved in that toxic (for us) environment. $\endgroup$
    – celtschk
    Nov 27, 2016 at 12:40
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    $\begingroup$ Why says that we are not the 2nd generation of intelligent species on earth? $\endgroup$ Nov 27, 2016 at 14:21

7 Answers 7


The main point is exactly as @nzaman said, the state of life we left behind, specifically multicellular life. Life has taken billions of years to get to (what seems to be) the first technological sentience on the planet, but what would happen after we vanish is a different story, mainly because a lot of that time was taken up with evolving some fairly major jumps in basic complexity of life on earth - cells, multicellular life, cell differentiation, the ATP energy cycle, photosynthesis, land based life, the basic neuronal//brain/spinal cord system, the additional various specialised regions of the brain and the extra functioning they enabled... and a load of stuff I don't know as I'm no expert. Some of those developments also got prompted by climatic conditions.

If those (or similar) have to be re-evolved, then it'll take billions of years and there won't be time.

Also, if too much has to evolve again, we lose a large part of planetary life conditions in about 1-2 billion years (our protective magnetic field, ozone layer, axial orientation, atmosphere, and surface water, are all unstable on that timescale). Life may evolve to cope and 2 billion is a lot, but it may make it harder.

If not, we can assume that developing comparable life will happen relatively fast (see below) but we have no data how long it will take to develop technologically capable life, as we just don't know how readily that happens.

If we lose humans and (say) mammalian life, then we may see an "evolutionary explosion" as often happens when a large niche for life becomes vacant - in a very few million years (about 10-20m), life in other areas explodes in diversity to occupy the niches. (Also relevant term: "evolutionary radiation"/adaptation)

In that case, and given the record, it looks (casually and fairly speculatively and unscientifically) as if rewriting the dominant life form happens on the order of 50-150 million year part on average. For example, dinosaurs took over and died out over a span of about 165my (of course they existed before and some survived after); mammals took over and reached dominance in 65my... So maybe a pure guess could be that this sort of overturn happens every 50-150my and maybe a couple or so of these happen before we get to one that leads to intelligent /sentient life? Its not much to go on, but we only have one data point from one life form on this planet, its hard to do much.

(Note that circumstances such as physical state, volcanic/solar activity etc won't make much difference unless very extreme. Life will simply evolve to handle the planet as it is, if it can, as it always has)

Update As an afterthought, I also remember that TV series "The Future Is Wild" also looked at this question, and specifically how life might evolve if humans vanished. It suggested possible new intelligence in the order of 200my, and pointed to molluscs (=squid, octopi) as candidates to migrate to land and take over the intelligent life niche, after mammalian life subsequently happened to vanish. I'm not sure how realistic that is, so its more a resource or info for the OP.

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    $\begingroup$ To add to both this and Pedro Gabriel's answer, I'll just ask why you think intelligent life should evolve at all? "Survival of the fittest" is the general principle of evolutionary theory, and that does not mean that intelligence wins. It's simply one tool among others, and only minimal intelligence is required for success. One price we paid for high intelligence was looooooong foetal and childhood development times, which are a massive survival risk in the wild (and even in today's society), that evolutionary experiment could so easily have gone the other way. $\endgroup$
    – flith
    Nov 28, 2016 at 7:48
  • $\begingroup$ Nobody's assuming. It may or may not. We're attempting to estimate likelihoods not assert certainties. The question itself asked the chances / timescale for intelligence to re-emerge, so the answers will try to evaluate (on very limited data!) what chance and timescale that may be. The assumption is in the question though. $\endgroup$
    – Stilez
    Nov 28, 2016 at 18:24

"Would there be a chance of a different but intelligent life evolving in our place"?

Of course there would be "a chance", even though we don't know what that chance would be. If humans evolved to get intelligent life, then evolving an intelligent life is not impossible, which means that it could happen again.

(even though I wager that the probability is little, because on millions of years of evolutionary life, with so many different species and so many different environments, only humans evolved to this point)

In fact, if humans went away, this evolution would be, IMHO, more probable, since human beings consume too much resources to allow another intelligent species to arise. Human beings are so much ahead on that department, with their technology and knowledge, that there is just no evolutionary benefit in competing with them for it... just in adjusting to the environmental changes that humans produce.

Now, we need to focus on another thing. Your question was not only about "intelligent life", but also "different". On this regard, maybe "different" would be harder to achieve, since other primates, like chimps, are the second more likely contesters to develop intelligent life (in fact, many people here may argue that they already developed intelligent life, even though I don't think they are comparable).

So it would be likely that we would see the emergence of another intelligent primate species.

But then again, you said that in your world, the environment has become too much deadly for us. You don't say how, or hor much, so I may only guess. But this would be probably also deadly for primates, which could go extinct. So we could go on to the next species... are the oceans compromised? If not, why not consider dolphins? They are already pretty intelligent.

Also, if the environment is really screwed up, maybe insects would have the evolutionary upper hand. On that regard, maybe ants or bees could be a good choice, since their gregarious nature would be more likely to develop a kind of language, which is fundamental for intelligence.

Finally, you could ponder an alternative... if Earth has become too much deadly for humans, maybe it was because of the humans own doing. So, maybe those humans could have genetically altered a non-human species before going away... which, left to its own devices, could have evolved intelligence more easily. Or (why not?), go all WALL-E and put the abandoned machines and computers evolving their own intelligence?

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    $\begingroup$ "(even though I wager that the probability is little, because on millions of years of evolutionary life, with so many different species and so many different environments, only humans evolved to this point)" — How do you know? Imagine an intelligent species of about the level of the Romans, ten million years ago. How much would we still find of them that we could identify as clearly of intelligent origin? What about an intelligent species on the cultural level of the Australian Aborigines? $\endgroup$
    – celtschk
    Nov 27, 2016 at 12:18
  • $\begingroup$ @celtschk: There is no evidence of instruments or artificial structures from our otherwise reliable and reproductible fossil record. We find shells, and bones, and even leaves on specific geological strata, but there was never anything that pointed to the existence of intelligent life. You are right that we can't be certain... but it seems unlikely that there was a civilization like that (even on the level of aborigenes) prior to humans. $\endgroup$ Nov 27, 2016 at 14:36
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    $\begingroup$ My point is: There probably would not be any evidence left. You seem to have no idea how long ten million years are. $\endgroup$
    – celtschk
    Nov 27, 2016 at 14:49
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    $\begingroup$ I do. I still find it more likely to find a fossilized knife or didgeridoo than a leaf or a feather, as we normally do. $\endgroup$ Nov 27, 2016 at 14:51
  • $\begingroup$ It is possible for another species to evolve alongside humans, but we would have to wait 100 million years or so. Without sentient beings experiencing time, time passes by in a blink of an eye. For us, we would not be able to witness evolution since our timescale is too small. Nothing can evolve significantly within 10 000 years. $\endgroup$
    – Bloc97
    Nov 27, 2016 at 16:02

It depends on two things:
1) In what condition did we leave earth?
If multicellular life flourished, even if humans did not, the chances of an intelligent species evolving go up; if the only living organisms are bacteria and moulds, then it'll take another few billion years to get back to conditions where intelligent life could evolve. 2) When did we leave? The earth is expected to have a close encounter with the sun in about 6 billion years. Whether an intelligent species will have time to evolve depends on the answer from 1).
If there is sufficient evolutionary data available and enough time, another species can evolve in enough time to get off the planet before it becomes uninhabitable. Otherwise, it can become sentient only to be aware of an approaching fiery death and nothing to do to stop it.

In numbers, I'd guess that if we leave in the next million or so years-- and going by present rates we'll certainly have made the planet uninhabitable by then-- it leaves about four billion years and change for new intelligent species to evolve, which is plenty of time, as our oldest species are about that old. A couple million years to get off the plant before radiation makes it uninhabitable, and they're safe.
OTOH, we stick around for another couple billion years, and the earth is just getting back to thinking about vertebrates when the radiation level starts increasing and eventually wipes out all surface life. I suppose, it'll do so gradually enough that most species can evolve defences, but the more resources go into basic survival, the less are available for increasing brain capacity.
Option 3, is between the first two. A potentially intelligent species develops, capable of understanding the world around them, they've just started mastering tools, when the weather gets hotter, the water gets scarcer, harvests get leaner and predators get meaner.

In summary, it's like a house. There's a chance there will be a new occupant, but it depends on what condition you've left it in, and how long you're willing to wait.

  • $\begingroup$ On a science note, forget the red giant in 5-6 billion years. We lose the protection of the magnetic field, water, axial stability and oxygen in about 1 - 1.5 billion..... $\endgroup$
    – Stilez
    Nov 27, 2016 at 14:48
  • $\begingroup$ "going by present rates we'll certainly have made the planet uninhabitable by then" - well, that depends on what you mean by "present rates". If you naively extrapolate e.g. the CO2 increase or the extinction rate in the last 100 years for a million years, you'd end up with an uninhabitable (to oxygen-breathing life anyway) or desolate planet. But there's only so much fossil fuel on Earth, and presumably more vulnerable species go extinct first. So a naive extrapolation likely doesn't mean much. $\endgroup$ Nov 28, 2016 at 0:58
  • $\begingroup$ FWIW my statement above is based on the impact of fairly well-defined future solar luminosity which, long before the red giant stage, will evaporate all surface water, probably ending plate tectonics and free atmospheric oxygen (as free atmospheric O2 is almost entirely renewed by water-dependent lifeforms). The earth's magnetic field vanishes or becomes insufficient due to gradual solidification of the earths core, on a similar or slightly longer timescale. Axial precession and instability does some of the rest. So it's independent from climate change or present climatic conditions. $\endgroup$
    – Stilez
    Nov 28, 2016 at 18:47
  • $\begingroup$ @Stilez: I suspect he was commenting on my post. Strangely enough, I made no reference to climate or CO2 either, merely that it's uninhabitable (to existing species). $\endgroup$
    – nzaman
    Nov 29, 2016 at 7:47

Firstly, if the earth has become 'deadly for us', surely it is deadly for all life in which case I very much doubt intelligent life will arise. Assuming that Earth is only deadly for humans though the answer is:

We don't know

Given that we're still not totally certain how intelligence works and how we evolved it we can't say with any certainty that it will come back if we leave. I would suggest it will based on how many animals we see able to learn and use tools but that is just my personal opinion and it is just as likely that no other animal will make the jump from simple tools to the point where they have intelligence and can make decisions based on logic and reason rather than instinct (I'm not certain humans have reached that point yet).

Even if we set a lower benchmark human-level, or near human-level, intelligence is still hard to evolve. The Earth is around 4.6 billion years old. Life has been around maybe 3.8 billion years. Complex life for around 2.1 billion years. Homo sapiens have been around for about 0.2 million years. Their may have been intelligent hominids such as Homo Neanderthal slightly before this but even then humans have only been around for a tiny fraction of the time complex life has been around.

Given how long it took intelligent life to evolve the first time any intelligent life that re-evolves will not be around for a long time after we leave. Furthermore it will not be anything like a human and its intelligence might come in a different form to our own so we might not recognise it as intelligent at all.

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    $\begingroup$ Some primates can learn sign language and perform simple negotiations. And lie. I think this meets "decision based on logic" requirements. $\endgroup$
    – Mołot
    Nov 27, 2016 at 11:10
  • $\begingroup$ @Bellerophon: It's hard, we agree, given the absence of precedents. But it is not impossible. Also, I don't think this would be a reason to shut down an otherwise interesting idea for a book. $\endgroup$ Nov 27, 2016 at 11:15
  • $\begingroup$ @PedroGabriel I didn't say it was impossible. $\endgroup$ Nov 27, 2016 at 11:54
  • $\begingroup$ @Bellerophon: Fair enough. $\endgroup$ Nov 27, 2016 at 11:57
  • $\begingroup$ I don't know that the "it took intelligence a long time to evolve the first time (us)" argument is valid. High-energy complex life probably required high oxygen levels in the atmosphere, and that took a long time to develop - that rules out anything except the last 500 million years or so. And it took large and complex brains a long time to evolve, once the oxygen levels were high. Even 65 million years ago I don't think there was anything with the encephalization of modern parrots, corvids, monkeys, or dolphins. I don't think we can say anything useful about how likely (character limit) $\endgroup$ Nov 28, 2016 at 0:55

It's certainly possible; we can't tell how likely it is with the current state of knowledge.

100 million years is a long time - our direct ancestors 100 million years ago would have been very unspecialized things that would look shrew/mouse/rat-like to a non-mammalogist. That was before the major diversification of placental mammals.

It would also depend on how widespread the effects of the event that drove humans off Earth was.

If it was something specific to humans (maybe a super-pandemic?), intelligence might evolve from other primates, or maybe from raccoons (they have decent grasping hands).

If it was something like a K-T extinction that destroyed all mammals (or all large mammals) but some other vertebrates survived, birds are also possibilities for high intelligence, though their technological ability would be very limited due to lack of hands.

If all vertebrates died out, human-level intelligence within 100 million years looks much less likely, though octopi evolving intelligence might be believable.


The other answers already give very good information on evolution, so I'd like to point out something else:

This question has some similarities to the Fermi Paradox.

The Fermi Paradox, in a nutshell, is this: with all the countless planets out there in the universe, there should be at least thousands of intelligent life forms like us out there. But why haven't we heard from any of them?

The main idea is that there exists a kind of "great filter" - somewhere between microscopic lifeforms and a space-faring civilisation, there exists one or more barriers that very few lifeforms get past.

If the great filter is in front of us (e.g., if progressing from where we are to space-faring is the problem), there's definitely a chance new intelligent life will evolve in your world. How quickly this happens is going to depend very much on what kind of life forms remain on your planet after the humans are gone (it'll be much more plausible for animals to become intelligent than bacteria).

However, if the great filter is behind us (the better option for humans), there's going to be problems. If the filter is between microbes and multi-cellular life, and your world only has bacteria left, it'll be practically impossible for new life to form. If the filter is between animals and intelligent creatures, there's also no chance.

So, the answer is going to depend highly on what kind of lifeforms remain and where the great filter is- no one knows the latter, so you'd have to decide in your story.


One of the problems about a sample size of one is that it can tell us very little about how likely it is to occur again. As far as we are aware, there has never been another animal like us living on Earth, despite a fairly decent number having brains that are comparable in size to those of our close ancestors.

The last common ancestor of humans and chimpanzees lived about 13 million years ago, so if there are still apes around it is certainly possible for the same events to happen again in the same amount of time. There are a good number of other candidates besides the great apes that could plausibly be the ancestors of another sapient species: dolphins, crows, parrots, and elephants are all quite intelligent already and may be able to go through the same evolutionary steps that we did. (The intelligence of octopuses is overrated; while they are certainly impressive compared to other invertebrates they are only mediocre when compared to mammals and birds...but since they look so strange and alien the thought of octopuses growing sapient is just fun to think about.)

The problem is that intelligence is only one solution to the problem of survival, and it might not be a very likely one. There have been many species that evolved fairly high intelligence and then simply stopped developing in that direction for millions of years, because they didn't need to. We don't know the exact circumstances that led to our own evolution. So while we can say that the development of a new intelligence within the next 10 million years or so is possible, it is hard to determine how probable it is.

One thing that we can say is that, depending on the conditions we leave behind, it may be difficult for a new species to make that leap. One of the drawbacks to large brains is that they have a high energy cost and long growth periods. The aforementioned intelligent animals typically breed slowly, take a long time to grow up, have few children, and require a lot of food for their size. Because of this, intelligence is most useful in a relatively safe environment with a lot of available resources, where the main evolutionary pressure is intraspecific - the same kind of environment that favors other costly adaptations such as bright colors and sexual displays. Hostile environments, by contrast, tend to favor fast breeders with short lifespans that can survive on few resources and adapt quickly, where the death of any one individual will not be a great loss to the species as a whole. If we leave Earth as a wasteland, it is unlikely that we would see new intelligences arise before the environment recovered.


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