We all know of convergent evolution, in which animals of similar environments yet with no relations often look similar (dolphin-shark for example). Animals in the real world do this because the best solution is often the one evolution favors, whether or not the animal is related, they still will evolve to the same environment.

In this question the highest voted answer suggested that convergent evolution may be the cause of humanoid aliens. But I am asking it is realistic to assume alien life would be similar to earth life in general through convergent evolution.

Now planets that evolve life will tend to be at least slightly similar; rocky, partly water covered, terrestrial planets. It is reasonable to assume that aliens evolving on an earth-like world will in turn be similar to Earth animals?

  • $\begingroup$ Henry Taylor’s answer just repeats what’s said on the linked-to question’s a swers. So are you asking is it probable/likely as opposed to how would that be possible? $\endgroup$
    – JDługosz
    Jul 24, 2016 at 3:07
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    $\begingroup$ This is a tautology. If humanoid aliens evolve their evolution must be convergent because their planet and ours have produced similar lifeforms. $\endgroup$
    – a4android
    Jul 24, 2016 at 6:20
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    $\begingroup$ The other post you refer to concludes with “summation, when we to go out and actually discover a large number of earth like worlds sentient, we would find the majority of worlds have the physics preferred forms of fish shaped swimming organisms, bird/bat shaped flying creatures, four limbed, big brained land animals and bipedal, largely vertical standing, two armed, big headed, "humanoid" tool using sentients.” so what are you asking, since it appears that this is a yes to how I read it. $\endgroup$
    – JDługosz
    Jul 24, 2016 at 9:00
  • $\begingroup$ @JDługosz I'm looking for more ideas on the subject $\endgroup$
    – TrEs-2b
    Jul 24, 2016 at 16:27

6 Answers 6


I'll answer "Yes", but only because the "would be similar to earth life" clause in your question is extremely general.

So by "Yes", I mean that the outer coating of aliens will probably follow one of the patterns already established on Earth. From the slimey exterior of slugs, to gills, scales, fur, hide and feathers, Earth's life seems to have tried out and kept every possible viable outer coating. Just look over the spectrum of special effects creatures from our movies and it becomes pretty obvious,... all the easy choices are already in use.

Similarly, in the area of locomotion, we've pretty much found all the easy options, already in use, here on Earth. Pseudopods, legs, wings, ballasts and fins leave only the highly unlikely option of natural wheels. So again, "Yes".

And by "Yes", I mean that structurally, aliens will have a lot in common with us as well. They will keep some of their offensive capabilities near their sensory organs as a defensive measure. They may also spread some of their offensive capabilities out to appendeges to maximize their range of effect. They will either be upright or long-necked, because the advantage of elevating the visual, auditory and olfactory sensors is too prescious to pass up.

Will aliens look like us? Probably not. But they will obey the same physical laws that we do, so we should at least be able to recognize them as living and understand at least a little about what they do to survive.

  • $\begingroup$ So you’re basically repeating the linked-to answer on why convergent evolution may be the cause of humanoid aliens. $\endgroup$
    – JDługosz
    Jul 24, 2016 at 3:05
  • $\begingroup$ Sorry, I didn't read follow the link or read those answers. I just answered the question as I saw it. If the unintentional duplication is a problem, I will delete my answer. $\endgroup$ Jul 24, 2016 at 6:49
  • $\begingroup$ Ah, yes, wheels; we discussed those in Naturally occurring wheels - do the 'mech' vs. 'tank' comparison apply to organics? (this site is starting to become a bit like xkcd; no matter the subject matter, there's almost always a relevant question to link to) $\endgroup$
    – user
    Jul 24, 2016 at 8:23

Yes, and no, with a heavy accent on "No". The problem is that "Earth-like" is far too vague a term. Even on our lovely planet there exist creatures which seem utterly alien, and truly frightening.

Different planets will provide different evolutionary requirements, and life forms will develop according to them (higher gravity, exposure to solar radiation, heat/cold extremes, night/day cycles, atmospheric mix, etc.)

When you mention "rocky, partly water covered, terrestrial planets", that's really not taking enough parameters into consideration IMO. A rocky planet with low gravity will evolve different life-forms than a rocky planet with very high gravity, etc.

And so, while some forms are certainly more efficient than others, "Earth-like" is too vague. On a planet that looks remarkably like our own there may evolve life forms which move on two, or four limbs, and thus match many of our own creatures as far as locomotion is concerned, yet "breathe" through their skin, and look like someone turned an animal inside-out.

Add psychology into the mix, and "Earth-like" truly becomes too vague a term.


Resemble how? You look at the life form (shape, niche, way of life) and say “that is an animal” as opposed to “wtf?”?

Well, a jellyfish is just like a cheetah, which is just like a woodlouse, right? A sponge is also an animal, hmm.

So if you mean life will split into autotrophs and those that eat other life forms. That, in general, seems like a likely outcome.

But your Title is similar to Earth life but your conclusion asks about animals. Most Earth life is microscopic. Look at a tree of life and you’ll see that the familiar plants, animals, and fungi is a very small part! Life on Earth went on for billions of years without any of it. An Earth-like planet may be before the cambrien explosion.

So the possibility is a matter of timing and maturity as well.


As I said in my comment above the question of convergent evolution is a tautology. Now certain functions have evolved several times independently. For example, flight has evolved several times and in different ways. Similarly for eyes where the vertebrate eye and octopus eye perform the same function but are structurally different.

In essence what we need to consider is whether on a planet sufficiently similar to our own Earth in terms of its age and physical characteristics, and possessing a well developed biosphere is likely to evolve humanoid lifeforms. So does convergent evolution have a preference** for humanoids?

The real question is whether the tetrapod bodyform is a morphology that has a high probability of arising on earthlike planets. The humanoid morphology is a modified tetrapod, and able to stand upright, at least, enough of the time. What we call humanoid is a bipedal tetrapod, i.e., a four limbed organism that walks on two legs.

Jack Cohen and Ian Stewart's Evolving the Alien: The science of extraterrestrial life (2002)***, reprinted in paperback as What Does A martian Look Like?, proposed evolution will create two broad categories of organisms and ecosystems. They call these two categories parochial and universal. The universal is easiest to understand this are biological adaptations that will appear whether environmental conditions are similar. While the parochial will be unique or extremely rare adaptations.

For example, we can expect adaptations for flight and eyesight to occur on any life-bearing earthlike planet with a mature biosphere. This is a universal adaptation. But woodpeckers may be rare. This assertion is based on the rarity of woodpeckers-like species on this planet. Then this is a parochial adaptation. If this set of conditions applies on other earthlike planets, then woodpeckeroids will few and far between.

If the humanoid bodyplan is a universal adaptation, then we should reasonably expect them to be present on earthlike planets. If it's parochial adaptation, then no convergent evolution won't favour humanoids on earthlike planets.

Is there any way of determining whether the humanoid form is either a universal or parochial adaptation? Scientifically no. Short of exploring earthlike planet similar to our own, and interviewing their sapient inhabitants.

If there is an abundance of humanoids in the cosmos, then we can safely say that convergent evolution does produce humanoid lifeforms. If the most prevalent sapient lifeform on earthlike planets is something that resembles the cross between a giant spider and a jellyfish, the answer is no. Also, on the downside we're the ugly ones in the universe.

This answer has come down on the side of scientific uncertainty. There is no litmus test to determine the question either one way or the other. This is why any question about convergent evolution is a tautology. But you need to have lots of examples of the same form or function evolving multiple times and each effectively independently of the others. It's possible, but who knows?

The prevalence of humanoid lifeforms in science fiction is old-fashioned hominid chauvinism and a lack of imagination on the part of writers. In cinema or TV, it's due to the deplorable lack of non-humanoid actors, despite all their claims of trying to encourage diversity, and the high cost of special effects in realising non-humanoid aliens.

Frankly all we know how humanoids can walk, talk and chew gum. But when it comes to those convergently evolved giant spider-jellyfish hybrid creatures, what do they do?

**: The use of the word 'preference' is only a figure of speech. Evolution only favours probabilities and not specific outcomes. That way teleology and madness lie.

***: Essential reading for any budding xenobiologist or junior worldbuilder. Highly recommended.

  • $\begingroup$ Good point mentioning Ian Stewart’s book. BTW I had a chance to chat with him when we both worked at IBM. $\endgroup$
    – JDługosz
    Jul 24, 2016 at 9:05
  • $\begingroup$ @JDlugosz You chatted with Ian Stewart! I am awestruck. Have you read the two SF books by Cohen & Stewart. WHEELERS is OK, but tends to be like humorous novel at times. But HEAVEN is full of the most superb aliens. EVOLVING THE ALIEN was the theory, but the two novels are the practice. $\endgroup$
    – a4android
    Jul 24, 2016 at 11:39
  • $\begingroup$ Yes, I have. I like the intellegent pond made up of a whole ecosystem of collective organisms. @tres-2b should read that! $\endgroup$
    – JDługosz
    Jul 24, 2016 at 12:29
  • $\begingroup$ It's one of those books on my to be re-read list. Just the aliens are worth it. Agreed. Whole ecosystems of collective organisms as an alien was stunning. $\endgroup$
    – a4android
    Jul 26, 2016 at 1:45

To an extent, you could say so. Anything that has evolved on Earth multiple times, you could presume is a good solution that will likely appear on other Earth-like planets.

Examples: Bilateral symmetry (evolved separately in many kinds of animals), heads with sense organs, brains, and mouths (ditto), complex eyes (vertebrates and cephalopods), jointed legs (vertebrates and arthropods), wings (insects/birds/bats/pterosaurs). You'd probably expect to find similar structures on other worlds.

But there are some things we take for granted that only evolved once on Earth, and there is no reason to believe they would be the same on other planets. Vertebrates having four legs is one example. Also, while many animal families have bilateral symmetry with a nerve cord running down the middle, vertebrates have a spine on their back while the insect equivalent is on their belly - an alien might pick either, leading to very different-looking body plans. And as much as we might like to talk about how the bipedal form is nice for intelligent creatures, we only have a sample size of one so it's pretty hard to make any generalizations.


The basic answer to this is yes. The why is harder for people to wrap their heads around, but it's rather simple.

Let's say you have a 100 x 100 grid and each point on that grid has an elevation, a color, and several number for minerals. Above that Grid is a light, heat, and energy source.

Is there an optimal form for each of those grid points to take in energy and reproduce? The answer is yes.

The problem is that a creature doesn't start at that optimal point, it reproduces more of itself, and each reproduction has a random change. These random changes move closer or further away from this optimal form. Assuming all of the less optimal forms die, and the more optimal forms reproduce then eventually all the forms for that grid point.

But there are 3 things that mess with reaching these 100% optimal forms. The grid points change their features slowly over time. Forms from other grid points move. And lastly, even though there is this "optimal form" in theory for a given environment when you introduce competing life forms they have to take each other into consideration and thus changes the optimal form from not several forms within a given environment that balance each other. ie The optimal form is several sub-forms.

So what does this mean with regards to whether aliens will look a given way? The answer, there are only so many environments that can support life and there are only so many optimal sub-forms and strategies that can occur for any given environment, therefor sub-forms and optimal forms will repeat across the known universe and as such you will get variations on a theme, rather than a completely outside the box form.

For example, a creature might have any number of fingers or arms, but will still have arms of some sort that can produce fine motor skills if they are to develop tools of a higher order than holding a stick. We can expect lungs of some sort to have developed because oxygen is what is needed to power brains and such of our level.

Basically, you might see some odd stuff out there, but you won't see anything that won't be variations on something we've already seen here on earth.

Of course some argue that "oh but how do you know life couldn't develop in x condition" and the answer to that is that we know enough chemistry to state that unless there is some fundamental law of physics that is changed where these creatures are originating, even in the most extreme conditions that anything we would call life could ever form in, its going to be, as i said, variation on a theme, not something wholely new... and that goes multiple times over for life we care about, because the type of life we care about can mostly only form in environments that either start like earth or become like earth due to the necesities of biological metabolism, thinking, etc. That's why incredibly complex life wasn't a thing for a long time on Earth. The fuel (oxygen) wasn't there for life to exploit.


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