Is there a storybuilding device that can explain why alien worlds could possibly have similar creatures to that of Earth?

I can understand why stories set in alternate Earths and fantasy versions of Earth have the same animals with a few/lots of unique creatures.

But on a alien planet would it be a different story?

This question asked about if alien lifeforms would be Earth like in general. But it was closed as being 'unclear what you are asking'. It also concluded that convergent evolution would only result in life forms following the basic evolutionary paths, so we would recognise arm, head, mouth. They still wouldn't look like anything we would recognise as cat or dog. This further related question asked why 'sentient' aliens would be so similar to humans. But I am definitely not looking for alien humanoids (maybe their alien monkey predecessors). So this won't be relevant either: Justifying alien humanoids without convergent evolution. This is actually the most relevant, I think.

Say, the alien planet, planetX, is 'Earthlike'. Similar distance from the sun, similar day/night and seasonal cycles, similar atmospheric composition and climate dynamics. Let's ignore the niggling question on how life initially started for now. Once 'life' was started, could it evolve along similar trajectories?

Yes, there will obviously be differences, as the planetary history of PlanetX would be different to that of Earth's. eg a single mass extinction event wouldn't have wiped out 90% of all life in one swoop. Mammals as we know them might not have developed as we know it as maybe the dinosaurs where never wiped out. Then again, maybe the dinosaurs never achieved millions of years of dominance and maybe mammals evolved a lot sooner. Maybe we would have giant trilobites scurrying along (they are my favourite extinct critter).

How do I explain that an alien world, that has had no known contact with Earth before, has such similar and recognisable creatures and plantlife, such as horses, dogs, cats, whales, oaks, pines? with the obvious exception of those obviously alien creatures...

NOTE: I don't need any sentient human like creatures to evolve/have evolved. This is to explain the general alien wildlife and plantlife evolving along recognisable Earthlike paths.

If I can't use convergent evolution as a 'valid' reason...How do I explain that my world has Horses, and cattle, etc without reinventing the wheel? Evolve a super galaxy spanning deity that started it all? give lifeform's some sort of masterplan, that it will try follow as the environments and situations allow them too?

(reading between the lines: if you are worldbuilding on an alien planet, do you have to reinvent every single creature and plant to be 'different' and 'alien'?...)

EDIT I realise that alot of creatures and plantlife are the result of thousands of years of domestication. I can work around that.

I'm not looking for an identical 'horse' in everyway. Just that they appear similar and recognisable. I really don't mind of their internal structure is completely alien to our earth vets.

I think what I'm really asking is it possible (STORYWISE) to have creatures that have evolved on Earth in the past, that would appear alien to a modern day Earthlings, such as those awesome pictures in one of the answers below. But at the same time have more recognisably mammal type creatures.

Sort of like several evolutionary branches at once?! Follow some branches that were wiped out on Earth to get the 'aliens' EDIT

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    $\begingroup$ Possible duplicate of Would convergent evolution favor aliens being similar to Earth life? $\endgroup$
    – TrEs-2b
    Aug 8, 2016 at 19:25
  • $\begingroup$ So pretty much, just say 'convergent evolution' and add alien creatures at will? keeping the useful, horse, cow, chicken, dog, cat etc (with maybe an extra tail)... $\endgroup$ Aug 8, 2016 at 19:37
  • $\begingroup$ No, but you are asking basically the same question and has the same answer; it wouldn't have earth-like animals. You have to hand wave that $\endgroup$
    – TrEs-2b
    Aug 8, 2016 at 19:39
  • $\begingroup$ @TrEs-2b, Do you think it has been edited enough to be a bit more unique? or am I just knocking on the same door? $\endgroup$ Aug 8, 2016 at 19:53
  • $\begingroup$ If alien planets evolve creatures or functions are similar to or resemble creatures and functions found anywhere else in the universe, that's convergent evolution. It only has to evolve independently, because that's what convergent evolution is. The classic example is the eye. " Complex, image-forming eyes have evolved independently some 50 to 100 times." That's just on Earth. Alien planets can have both species similar to Earth's where evolution has produced similar creatures, and creatures which are distinctly different because selective pressures and the environments were different. $\endgroup$
    – a4android
    Aug 9, 2016 at 7:16

7 Answers 7


In-universe, there are tons of explanations you can use.

Aliens seeded every planet you explore, intentionally or not. Maybe they were explorers like you and only found barren worlds, but each time they left bacteria that sparkled life millions of years later. Maybe they terraformed every planet they visited to be like theirs. Maybe Earth is one such planet and humans are modeled after those aliens.

Order to chaos. The universe has an underlying structure. No matter what the origin of this structure is, this is what rules evolution. Lifeforms on planets with similar characteristics will always develop in similar ways. There are deviations, but a cow is more-or-less universally a cow and goes moo.

Sheer dumb luck. Everything is random. What are the odds of getting 10 tails in a row? Not even 0.1%. What are the odds of getting any other combination? Exactly the same. Maybe every planet being the same isn't fundamentally less probable that every planet looking different. Or maybe your universe just got the right draw. In a multiverse scenario, the universe across the street might be widely different though.

Selective sampling. There are a near-infinite number of worlds with life, all more exotic and alien than the other. But you yourself only see a fraction of it. And more than that, you only see those that look like yours. Of course you'll always end up looking at the same archetype of planets, because to you life requires a nice Goldilocks zone and liquid water.

Or any combination of the above.

Partly out-of-universe, it's all about the frame of reference. Generally speaking, when explaining a new experience to another human, you'll explain it with analogies you can both relate to.

I don't know what a chicken is, though I hear everything in the galaxy tastes like it.

That's a Mass Effect quote, from everybody's favorite Turian, Garrus Vakarian who is poking fun at a human trope. Everything tastes like chicken because we, the audience, get what this means. Presumably, we all know and like chicken. We understand that if it tastes like chicken, then there's nothing weird or exotic about it. It's just plain food.

Even if it looks like green dead rat and looks like it'll give you cancer, if the character says "tastes like chicken", then you know it's not actually bad (or at least that's what the author wants you to think). So if blorg tastes like chicken, you know what it means. If blorg tastes like gurflink, it's not informative.

Now you extrapolate. A herd of white, fluffy, grass-eating animals from an unexplored planets would be spacesheeps (this is totally not an elaborate ploy to make that pun) to a human explorer. Because the character knows what a sheep is, and the audience knows what a sheep is, and the author knows what a sheep is. You form a mental image of a shmeerp just by saying it is sheep-like.

From the character's point of view, who isn't an omniscient narrator, they don't know what a new species is. They only know what it looks like at first glance. It wouldn't make sense for them to call those animals anything but sheeps.

When the scientists come in and discover there is actually no genetic correlation between alien and Earth animals just like the narrator explained 50 pages ago, the name spacesheep will already be common parlance.

Another out-of-universe explanation for live action stuff is that it's easier to get a prop cow than a prop animal-that-doesn't-exist-on-Earth.

  • $\begingroup$ I did not mark this the accepted answer just because of the spacesheeps 'pun'... $\endgroup$ Aug 9, 2016 at 14:22
  • $\begingroup$ @EveryBitHelps I regret nothing! $\endgroup$ Aug 9, 2016 at 14:24
  • $\begingroup$ haha. sorry, I thought I had accepted the damn answer! My phone needs to die! $\endgroup$ Aug 9, 2016 at 17:03

Planet seeding. It's most likely that life did not start in any form on Earth, but instead traveled here via meteorite, comet, or some other celestial body. This collided with Earth and the life took off from there. The basic building blocks are there now...however that said, it's still alot (alot!) of convergent evolution to get to the same place we have. There is the chance that evolution will follow a similar route had the environments been the same (if the terrain is the same, the same traits advantageous on earth would theoretically be beneficial there).

But I think a better idea for you:

You could move the entire theory into the conceptual range. A 'horse' is a concept to some degree...large sized 4 legged herbivore. Your alien planet can also have a large sized 4 legged herbivore that kinda looks like a horse, as long as it fits the concept it can be called a horse without having the exact same structure as our horses here do. A tall strong wooden plant that kinda looks like an Oak tree but has a completely different cellular make-up can still be called an Oak as long as it fits the concept.

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    $\begingroup$ +1 Yes! A lot of old Science Fiction would use what your describe as the conceptual range. Have a POV character say something like, "That looks sort of like an oak tree if you ignore the weird color and the tentacles." Then from that point on, you (the author) can refer to that alien thing as an "oak tree". $\endgroup$ Aug 8, 2016 at 20:25
  • $\begingroup$ @Henry Taylor, how often does one need to mention the tentacles? $\endgroup$ Aug 8, 2016 at 21:06
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    $\begingroup$ Only when they are relevant to the plot. If they never grab anyone, then after the first character's commentary, just call it an oak tree. $\endgroup$ Aug 8, 2016 at 21:12
  • $\begingroup$ "There is the chance that evolution will follow a similar route had the environments been the same" - Broadly similar maybe, but the chances that they would re-evolve animals that looked nearly identical to Earth creatures like horses would still be astronomically small...your second suggestion would be a way around this, but if the OP literally wants creatures that look like the Earth counterparts, seeding with single-celled life won't really work even if the cells are identical to our own distant ancestors. $\endgroup$
    – Hypnosifl
    Aug 9, 2016 at 0:02
  • $\begingroup$ Yes. Peter F Hamilton and Neal Asher frequently call this "analogues", I.e. horse analogue, dog analogue, even if physiologically it's completely different $\endgroup$ Aug 9, 2016 at 10:43

If you modify the phrase "no contact with Earth" to "no known contact with Earth", you leave yourself open to a useful alternative, Star Trek's The Progenitors.

Just postulate that an ancient star-travelling race seeded both Earth and your alien planet with the same DNA and that within that DNA were time-delayed gene's which expressed themselves as different useful yet similar species on both worlds.

  • $\begingroup$ I approve. edit done! $\endgroup$ Aug 8, 2016 at 20:13

The lazy worldbuilder's answer to this problem is simple. Make up names for these creatures, let them fulfil the same social and economic roles as familiar Earth animals, and don't describe them in detail. This gets round the problem of convergent evolution working implausibly thoroughly, without requiring you to make up everything in the ecosystem.

  • $\begingroup$ That is pretty much what I'm trying to avoid. I want to say, eg horse. why? because there will undoubtedly be a scene where the horse owner is admiring it's form, or grooming it, or explaining how his is better than someone elses. I don't want to then have to go back and check the entire story for such slips... $\endgroup$ Aug 8, 2016 at 20:05
  • $\begingroup$ Surely that's what word processors' search facilities are for? $\endgroup$ Aug 8, 2016 at 20:07
  • $\begingroup$ you would have to search lots of different words to get every single slip. there will be a time, where you thought you were being clever by using some not so repeated word... $\endgroup$ Aug 8, 2016 at 20:09

Life requires energy. The Earth's energy comes from the sun. Via photosynthesis, this energy is not ephemeral. It can be stored as complex carbon chains in plants. When those chains are oxidized (like in your tummy or in a car engine), energy (and carbon dioxide) is released. Energy is released because shorter chains are more stable than long chains. And this is why living things must "breath" oxygen, they need to extra energy from carbon chains.

Maybe, on another world, the chemistry of how to store energy via photosynthesis would need to happen.


Considering the vast differences between earthly organisms which arose from the same common ancestor, it is difficult to believe that any planet with an entirely different evolutionary history is going to come up with anything resembling Earthly life forms.

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Hallucigenia. Cambrian worm

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Giraffe. Modern Mammal

To make it even more strange, consider that there were several evolutionary paths "not taken" deep in the past. The Ediacaran period has fossils which are sometimes interpreted as multicellular life forms which have left no modern descendants, presumably either dying out during snowball Earth or perhaps driven to extinction by early Cambrian creatures. Stephen Jay Gould's book "A Wonderful Life" suggests that many of the creatures found in the Burgess Shale were doomed not only to extinction, but to never have descendants (many body plans and features have no modern counterparts, for example).

enter image description here

Opabinia. It has five eyes, and the pincers on the end of the "snout" is not the mouth, but a grasping organ

If Gould's hypothesis was right, random extinctions could have changed Earth's biological history. All modern vertebrates are descended ultimately from one species of Cambrian worm, if that creature had become extinct 500 million years ago, there might not be any vertebrate creatures today.

enter image description here

Our esteemed ancestor

So since so many factors are in play, the odds of anything even remotely resembling Earth life will be very small. There will be some convergent evolution, if a creature swims then for hydrodynamic reasons it will resemble a fish, but could have a radically different lifestyle. How you deal with this is up to you as the writer, but taking the lay way out and having cows with three horns or six legged horses is a cop out.



The same aliens that placed life here on Earth placed the same life on other worlds. Because these aliens were not that advanced in the life sciences, they couldn't do much more than place their planet's life forms, with minor changes (like what we can do now with genetic engineering).

However, their large advances in the physical sciences led to them experimenting a lot with the impact of life on planets (with their experiments running for billions of years), until some catastrophe wiped their civilization out entirely, leaving no trace behind unless someone stumbles upon the ruins of the civilization, which would be entirely by chance as their dead ruins would not be something easy to find in the vastness of space.


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