Let's not delve into specific genuses, families, orders etc.
Broaden your definition of "classifications" to words like "mammal" or "fish", "crustacean" or "avian".

Science fiction depictions of species often resemble Earthlike categories - there will be some variation of a mammal, or some slightly altered fish, or maybe crabs with an extra pair of arms - but generally these categories (classes, etc) are adhered to.

Is there a good reason for this, or are there scientifically plausible, drastically different "categories" of organisms that evolution may produce?

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    $\begingroup$ most classification compare anatomical traits that passed down from a common ancestor however I suspect there are alien lifeforms which bears unique anatomical characteristic unbeknownst to science. $\endgroup$ – user6760 Nov 25 '16 at 5:56
  • $\begingroup$ Iain M. Banks has some amazingly detailed descriptions of all sorts of "none of the above" life forms in some of his books. Even enormous gelatinous beings that live in a gas giant planet. His main characters are usually humanoid, though, as I guess it makes them easier to relate to. $\endgroup$ – Carmi Nov 27 '16 at 13:27

12 Answers 12


Some classification systems are logically exhaustive. Like if I say that I will divide all numbers into "those divisible by two" and "those not divisible by two", it's hard to see how you could have a number that doesn't fit into either category. Either it divides by two or it doesn't.

Some attributes used to classify living things are like that. For example, we have vertebrates -- creatures with a skeleton -- and invertebrates -- creatures without a skeleton. It has a skeleton or it doesn't. That would seem to be all the possibilities.

But many, probably most, classifications of living things are not of that sort. We talk about some animals reproducing by bearing young alive, others laying eggs, and others budding. It's not at all clear that those are the only three possibilities. (As I write this, I'm thinking to myself, did I miss any?)

We talk about creatures that get oxygen from the air with lungs and creatures that get oxygen from the water with gills. Might there not be other possible ways to get oxygen? Or might there not be creatures that do not need oxygen to live?

All living things on Earth are built around the chemistry of carbon and oxygen. Science fiction writers have long speculated about the possibility of creatures with chemistry built around silicon and fluorine instead. Is it possible? I don't know. I don't think anyone has proven it's impossible.

  • $\begingroup$ This provides some helpful information but could you summarize it / address the question with "therefore we've covered it all in scifi" or "there is yet more to evolve" etc $\endgroup$ – Zxyrra Nov 25 '16 at 5:38
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    $\begingroup$ Some invertebrates have skeletons. It's backbones that they don't have. $\endgroup$ – Mike Scott Nov 25 '16 at 6:15
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    $\begingroup$ @Innovine Divisible/not divisible by two is only a meaningful idea when talking about integers. i.e. it's meaningless to ask whether a rational number is even or odd. I suppose I should have said "integers" instead of "numbers". Maybe there's an important point there: yes/no sounds like it covers all possibilities, but in some cases there's a third possibility of "not applicable". $\endgroup$ – Jay Nov 25 '16 at 6:40
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    $\begingroup$ @Innovine It's not really accurate to say that the imaginary numbers "don't exist". The imaginary numbers are just as real, and no more imaginary, than the real numbers; they're only called "imaginary" and "real" for historical reasons. $\endgroup$ – Tanner Swett Nov 26 '16 at 5:05
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    $\begingroup$ As for numbers neither divisible nor non-divisible by 2? Consider the number system consisting of those numbers which are divisible by 3. In this number system, there is no such thing as the number 2, so the standard definition of "divisible by 2" doesn't work here. $\endgroup$ – Tanner Swett Nov 26 '16 at 5:07

Because we haven't found anything that can't be classified this way. If we do, or if more information about currently classified organisms is found, that puts them outside our structure, we make a new classification and slot them in there. E.g., somewhat recently, Archaea were given their own kingdom, whereas they were previously classified as bacteria.
Sci-fi writers tend to use existing structures, since otherwise they would have to develop entire new evolutionary paths of multiple species and work out the relationships within an entire ecosystem, for something that is merely incidental to the story, which is either adventure or drama, usually involving human(-oid) protagonists. Much easier to simply take an existing species, add in the changes you want and stick in in your story, with the advantage that your reader can follow along without having to get a PhD in biology.


Certainly! What makes you think that all possible (rough) categories are currently present on Earth? Suppose modern birds didn’t exist, or it was a time before they evolved: would you ever think of such a thing, or consider it “different” from the other choices available?

  • $\begingroup$ Except the principle of a "wing" existed long before birds ... $\endgroup$ – frеdsbend Nov 25 '16 at 22:31
  • $\begingroup$ I don’t know why “except”, but that’s one of the points I was aluding to. Flight developed in many different liniages. Are bats different from mammels just because they fly? If you looked at a therapod (before flight evolved in them) you would find them different from mammels and reptiles. Perhaps “warm blooded” is a significant factor, but the only other warms around (mammels) are different in many other ways too. $\endgroup$ – JDługosz Nov 25 '16 at 23:17
  • $\begingroup$ I did not catch that you were alluding to pre-bird flying creatures. On the contrary, it seems to me like you forgot them altogether. $\endgroup$ – frеdsbend Nov 26 '16 at 0:32

Barring panspermia, ANY alien organism is going to fit in a "none of the above" category, because classification depends on descent from common ancestors - the "Tree of Life", in other words: http://www.tolweb.org/tree/

That said, it is quite possible that categories of alien life forms might closely resemble Earth categories, because similar forms are good solutions for evolutionary problems. There are many examples of such convergent evolution in Earthly life. Tuna, ichthyosaurs, and dolphins all look much alike (and submarines look rather like whales :-)) because that form solves the problem of moving efficiently in water. Birds* resemble bats and pterosaurs, because of the aerodynamic constraints of flying. So we'd expect to find alien 'fishoids' on any world that has oceans, 'avianoids' if alien life has developed flight, and so on.

*And consider the remarkable similarity between the hummingbird and the hawk moth.


Yes, There Is Plausible Scientific Reason To Expect Non-Earthly Lifeforms

I think it is safe to say lifeforms are moulded by the environments in which they evolve. So, if life develops in environments significantly unlike those on Earth, the lifeforms could be expected to be equally unlike those on Earth.

Countless Environments For Life In Our Observable Universe

There are an estimated 2 trillion galaxies in the observable universe and an approximately 100 billion planets in our galaxy alone, each with it's own relatively unique set of environmental parameters. So, it's statistically feasible to imagine the existence of environments significantly different from those on Earth. And, any lifeforms evolving there could be equally unique.

Many Other Dimensions Possibly Containing Unearthly Environments

In addition to the nigh endless permutations of environments in our observable universe, superstring theory states there are anywhere from 10 to 26 dimensions:

enter image description here

If our 4 dimensions of spacetime (3 spatial + 1 time) hold trillions of stars, galaxies and planets, each possibly hosting life, it's possible these other dimensions could also have life-supporting environments.

In a universe then where at least 10 dimensions exist, with an upward ceiling of 26 dimensions, that means there are anywhere from 7 to 19 different dimensions the nature of which we cannot even imagine easily.

It is easy to imagine, though, that any life-supporting environments in those 7 to 19 dimensions could bear no resemblance to those found in our modest earthly realm of 4 dimensional spacetime.

For example, if current string theory states the 7 to 19 dimensions are extremely tiny (i.e. close to the Planck limit or below it), then perhaps species capable of quantum superpositioned evolution could evolve there.

And hence, any life evolving in those higher dimensional environments could easily be unlike anything humans can comprehend.


Take for example Q from Star Trek. Q is a member of a race called Q--a race evolved in dimensions unlike those in which humans evolved. Hence Q is unlike any lifeform humans can even comprehend:

Q is a being who is unconstrained by, and possesses power over, normal human notions of time, space, and even reality itself – his fellow Q and he are said to be omnipotent, and he is continually evasive regarding his motivations. The name "Q" also applies to all other individuals of the Q Continuum – an alternate dimension accessible to only the Q and their guests. The true nature of the realm is said to be beyond the comprehension of "lesser beings" such as humans, therefore it is shown to humans only in ways they can understand.


Between the myrid possible environments in our 4 dimensions of spacetime and any environments in the other 7 to 19 dimensions, I think it's plausible to expect any lifeforms living there to be as unusual as their environments are or as incomprehensible as their dimensions are, and therefore plenty of room for "none-of-the-above" lifeforms.

  • $\begingroup$ I was going to give this a +1, but I cannot promote such a misunderstanding/misrepresentation of string theory and its various variations/descendants/etc. They do propose many other dimensions, but those dimensions are tiny. They're small enough that the smallest possible thing (the strings, fundamental sub-atomic particles) have room to "wiggle"/"vibrate" inside them. You can, of course, use the concept of other space/time/other dimensions in which complex/living things exist (kinda like Flatland's concept, but applied to our world), but that is not what the string theories mean. $\endgroup$ – mtraceur Nov 27 '16 at 5:27
  • $\begingroup$ Perhaps species that evolved below the Planck scale experience no problems with superpositioned existence. I am not saying this necessarily is what is or will happen, but based on the dramatic evolution of human understanding of dimensions (from a time in the middle ages when no one could even imagine superstrings to modern times where we casually experiment with quantum entanglement), I'm extrapolating that our current understanding of dimensions and life as we know it could evolve to believe things now deemed impossible, like unboundedness effortlessly existing within a point. $\endgroup$ – Thom Blair III Nov 27 '16 at 10:05
  • $\begingroup$ I suppose if one is flexible enough with one's conceptions of what's possible, that's a workable concept. But a reader not already familiar with string theory is likely to interpret it in the way I critiqued, I think. I'd appreciate an edit to reduce odds of such misunderstanding. Anyway, I lack the intimate familiarity with the mathematics of that aspect of physics as we conceive them to derive a logically rigorous position on such a possibility, but it seems sorta plausible, and is definitely an interesting idea to play with in a fictional world. Anyway, +1 given the post + these comments. $\endgroup$ – mtraceur Nov 28 '16 at 5:28
  • $\begingroup$ @mtraceur I'm also almost completely unfamiliar with superstring theory, so I really don't know how to edit to do justice to the current real scientific realities being described by them. One thing I was thinking is that as science evolves, that which is poorly understood now will only become more understood and that which currently is deemed impossible or incomprehensible will likely come to be viewed as scientific reality. That is the basic idea I was working with--that if we now suspect other dimensions exist, they may eventually prove to be vastly different from current understanding. $\endgroup$ – Thom Blair III Nov 28 '16 at 12:29
  • $\begingroup$ @mtraceur I tried to add an explanation to my answer...does it seem valid? Any feedback is welcome. :) $\endgroup$ – Thom Blair III Nov 28 '16 at 12:37

Aliens tend to resemble earth species because human are not very good at imagining something totally alien. Look at extinct organisms or less commonly known species and you start to see the range. organisms inherit a lot of baggage from their ancestors, our classification work for our planet's life becasue our planet's life is all connected and related which leaves identifiable patterns, those patterns do not need to be true of non earth life, certain patterns will likely be true to all life the ones not directly connected to ancestry. thing constrained by physics and chemistry would be common among all life, like how leverage works or energy intake vs output. but our classification will not apply well to life on other worlds.

The phrase bounded but infinite comes to mind, there are constraints about how life works but within those "rules" there are a limitless number of variation most of which have probably not even been thought of.

An example is skeletons, skeletons are helpful for increasing leverage and creating support, especially on land, but what you make them out of, whether they are inside or outside, or how they are laid out are all very flexible and vary wildly. Consider cartilage and bones, vs chitin, vs carbonate shells, vs echinoderm plates, vs wood in plants, those are all different ways of creating skeletons.
You can have real fun by imagining better laid out organisms than earth life, like animals with one way breathing instead of dead end lungs, or brains that are attached to the skull at more than one point.

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    $\begingroup$ Modern birds have one-way lungs that extract more oxygen more efficienly than our tidal (in-out) lungs. $\endgroup$ – JDługosz Nov 25 '16 at 6:44
  • $\begingroup$ two way lungs but still part of one way system air still enters and exists from the same orifice. $\endgroup$ – John Nov 25 '16 at 15:33
  • $\begingroup$ sorry reverse that one way lungs two way breathing system. $\endgroup$ – John Dec 21 '16 at 18:50

Why does all life on Earth fit into seven categories?

Because all of it evolved from single celled organisms which formed when certain amino acids combined to form proteins, making RNA which eventually became DNA. Which was ultimately possible because (of a whole slew of things too numerous to list - some that I already skipped), out of the 94 naturally occurring elements found on this planet (and presumably, throughout the cosmos), carbon has four electrons in its outer valence shell.

No evidence exists that there are non-carbon based lifeforms, and all theories that purport it does are tenuous at best.

But now researchers have uncovered a bacterium that has five of those essential elements [out of the required six: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur] but has, in effect, replaced phosphorus with its look-alike but toxic cousin arsenic. –washingtonpost.com

'in effect, replaced phosphorus with arsenic', which means it likely acts in the same way and is merely a substitute. This bacterium is also just that, a bacteria, not something we'd call "life as we know it" in the sense that it'd be worth writing a story about it. Note also, that it does not replace carbon anyway.

Even if it did, you'd have to re-write history almost completely, redefining your planet's habitable zone from the start. I almost fell asleep researching this, as the journey took me from biology, to chemistry, to astrophysics, and back again and to everything in between them all several times, when I should've just left a comment that said, because:

E=MC2, evolution, Newton's and the square-cube law, metabolics, thermodynamics, photosynthesis, respiration, reproduction, social dynamics, symbiosis, and the Periodic Table. AKA, science; all of it.

No matter where you go, there you are. And you are a product of your environment. If you wish to remain scientifically plausible, you cannot deviate from what science has historically shown to be the general order of things. That's why it's called science fiction, and The Tree of Life is why you can end a game of 20 Questions in 7 tries or less.

Nature basically already has come up with just about everything at one point or another. It is currently estimated that there's 8.7 million species on Earth, currently, and that 99% of all species that have ever existed are now extinct. Describe something, and any biology professor worth their salt can tell you when and where it belongs.

It is in my opinion, that if you can find a planet much like ours in a contemporary state of evolution, 'they' would not be so different from at least one species that you, could have, or can find on Earth.


I like Jay's answer, so I think I'll quote from it:

Some classification systems are logically exhaustive. Like if I say that I will divide all numbers into "those divisible by two" and "those not divisible by two", it's hard to see how you could have a number that doesn't fit into either category. Either it divides by two or it doesn't.


But many, probably most, classifications of living things are not of that sort.

This little piece from Jay's answer captures the central idea one must explore with species classification. We can often create classifications that are logically exclusive, and complete, but sometimes it just doesn't work out that way.

Something worth noting is the definition of a "species." Or rather, the lack thereof. There really isn't a good definition for species in science. It's something which scientists have found useful, but haven't found a really good way of defining. It's a bit odd, really. We're taught in science class that "species" are a real thing, but in reality they are part of a taxonomy to classify nature into nice neat buckets. What you are looking for are the corner cases that start to challenge the idea that an organism can be put into such a neat taxonomy.

In many cases the defining lines of species are very clear. In the case of sexually reproducing organisms, the ability to mate and produce a viable ofspring is a very effective delineation. Or is it? One interesting corner case is that of the Cicada. There are several species of cicada which are actually capable of interbreeding. However, there's a catch. These cicada emerge on strange prime numbered years. One set of species emerge on 17 year cycles, and the other emerges on 13 year cycles. Due to this, the organisms in these two species only have an opportunity to interbreed once every 221 years! Scientists put these in different "species" because of this, but we have to recognize that there's nothing physical preventing them from interbreeding -- its just temporal.

Once we leave the world of sexually reproducing species, the world gets even fuzzier. It is well known that bacteria trade genetic material between species. Many species have developed immunity to antibiotics by trading genetic material with other bacteria. One may argue this is as close to "breeding" as one can get with asexual reproduction, and yet we draw lines between the species.

However, we can always put the organisms into bigger buckets. While "species" may be the classification that gets the most attention, the other classifications, Life, domain, kingdom, phylum, class order, family, genus species, handle the corner cases. One might question whether M. septendecim and M. tredecim are different species (they only differ in their 13 or 17 year reproductive cycle), but both are clearly part of Magicicada, a genus which easily captures both of them.

As a general rule, the higher one goes up on our taxonomy tree (towards kingdoms and domains), the harder it is not to fit in a bucket. That's intentional... it's why that taxonomy has seen such great success. It's really hard to develop a life form that doesn't fit into a domain (Bacteia -- with their particular cell membranes, Eukaryotes -- with their bacteria like membranes but lack peptidoglycan in them, and Archaeae -- which are neither), but divisions smaller than domain offer more options. When one considers the "interesting corner cases" there's no particular reason another kingdom couldn't form, we just haven't seen one.

Talking to something mentioned by nzaman, we see few things breaking the mold of these lower level classifications because it's hard on the authors. It's hard to invent a brand new way of approaching life which, literally speaking, has not been thought of in the past few billion years. It's easier to stick to what we know. Also, if we stick to what's similar, it's easier to draw on existing emotions regarding those creatures.

However, if you're really interested in pushing the bounds, the only limit is the scientists' willingness to invent a new taxonomical category for you. If you create something that is unique enough, we'll change our tree of life to fit. It's not cast in stone. Then again, perhaps that's what you're looking for?

  • $\begingroup$ Another difficulty with "species" is that the ability of two categories of animal to interbreed is not a transitive relation. If animalss in group 1 have genes for a trait on both sets of chromosomes, and those in group 2 have a different set, and any animal with both traits will be non-viable or sterile, groups 1 and 2 would seem to be different species. There could, however, be a group 3 which lacks those traits, and could breed with animals from either group. If one doesn't know of a group 3, but one might possibly exist, should groups 1 and 2 be the same species or not? $\endgroup$ – supercat Nov 26 '16 at 19:12

Yes it's possible, in the past there exists some species that does not fit into any modern classification.
For example the Vendobionta were multicellular species that cannot be classified in plants, animals or fungi.
Like Vendobionta, in a past era, or in alien world, can exist lots of unknowing species that actually we are unable classify, but new categories can be created for those species.

  • $\begingroup$ It's not that they are in a different category: rather, we don’t know whether tney were animals, lichen, or what. $\endgroup$ – JDługosz Nov 25 '16 at 23:20
  • $\begingroup$ Actually we have no enough information to classify them, but there are lots evidences that suggest they are not animals, plants or lichen. There are lots of hypotesis about classification (cnidarians, protozoans, new phylum, new Kindom, lichens, etc...), read the section "Classification and interpretation" on wikipedia. My post doesn't prove that Vendobionta Creates a new category, but shows that new classifications are possible, and Vendobionta can be one of them. $\endgroup$ – Stefano Balzarotti Nov 25 '16 at 23:51

The science of classifying organisms by a hierarchical taxonomy is called Systematic Biology. It uses all the available information of features related to the organisms in order to place them in a structure derived by evolution (taxonomic tree). That is why turtles, crocodiles and serpents are children of the class Reptilia and not to the class of grasses (not so related). Putting all these organisms together in a single tree of life it's quite difficult because we can only see living species and there are a lot of holes by lack of evidence.

All alien life must satisfy the properties of our life, i.e self replication, mutations and adaptations to a changing environment (entropy). Therefore, it must follow evolutionary diversification driven by the same evolutionary forces of preserving information in an highly entropic environment.

Nevertheless, as in current systematic biology, the number of orders, classes, etc can change with time as we understand more and more about our nature. Therefore, the taxonomic tree structure is NOT DEFINITIVE but it relies on evolution.


The disciplinary reason to classify exobiology into similar categories as Earth biology is because of the convergent evolution principle. Creatures that evolve independently and in different environments are likely to develop similar structures if those environments are similar. For example, water creatures are likely to develop gills and fins, eventually, because those structures are so powerfully useful in water environments, and will likely retain them once evolved.

There is a slightly tenuous approach to this concept in regards to exobiology: The idea that alien life would even be elementally similar to Earth life is entirely speculative. There may very well be life that is not carbon-based, or has no need for water, and we should expect this life to live in environments unlike anything on earth. Yet, in response to this criticism, if such life were possible, and say lived in a kind of methane or amonia sea, it may very well look like a fish anyway, being that it practically does live in a 'water environment'.

So, continuing the example of gills and fins, we should expect to find 'gills' and 'fins' in alien creatures that live in environments that are similar to Earth's water environments, even if not elementally (save for the real possibility that our plethora of life variations is not a stupendous rarity, where something analyogous to the Cambrian Explosion marks a principally unlikely evolutionary development).


No, there is no good scientific reason for thinking that life forms on other planets would sensibly fit into categories like "fish", "insect", or whatever. People might find it easiest to describe a creature by thinking of the earthly animal it most resembles, but the only sure thing about alien life is that it will differ from earthly life in unexpected ways.

The things that make mammals mammals are mostly the result of a billion years of historical accidents. There is no law of nature that associates hair with female production of a high-calorie food for infants. Many features from earthly life are likely to be found elsewhere, like skin and shells, claws and teeth. Limbs. But, if biologists ever find a world teeming with life, Linnean classification will have to start from scratch.

(I think a lot of people who began their answers with "Yes" were answering a different question than the one you were asking.)


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