Would intelligent life evolve any other body plan than humanoid?

Question

Disclaimer: This question uses some liberal wording to get key ideas across. It's not meant to be academically rigorous, so please try to focus on the ideas and not the exact definitions of the terms used. If you need clarification on something or have suggestions to make the wording clearer, I'll be happy to do so or add them.

Executive Summary: The goal of this question is to explore whether or not space-faring species would have many "humanoid" characteristics. Here are some key terms used in the question with explanations of what they mean and how they fit into the goal:

1. Intelligent life - Life forms which have developed a higher degree of social complexity and demonstrated the potential to learn. For example, humans, dogs, birds and dolphins. Proposed species must meet this criterion, or you must provide a plausible explanation for why it is not required.
2. Apex species - A species which has dominated its biosphere and kept competition to a minimum or has a reasonable means of existing beside another dominant species. This typically includes, but is not limited to, "apex predators". For example, humans, tigers and orcas. Intelligence is a prerequisite unless you can plausibly explain why it is not required.
3. Space-faring race - The ultimate goal of this question. A species which has the time and ability to construct methods for achieving space flight. Keep in mind a harsh environment would significantly impact their chances of surviving as a species long enough to do this. For clarity, things like insects are not a good candidate for this unless you can explain how they would achieve the intelligence prerequisite.
4. Humanoid - Physically resembling a human pattern with two legs, two arms, a head containing sensory organs and possibly communication capability, etc.

There are plenty of theories on what intelligent life would look like on other worlds. Sci-fi creators go to great lengths to come up with aliens who are truly alien to us. But would an intelligent space-faring race be anything other than generally "humanoid"? Would the universe have a reason to develop space-faring species other than rubber forehead aliens?

Consider the following (keeping in mind these are general descriptions and not strict definitions), and remember that evolution succeeds only when it is more efficient than the base:

• Without fine manipulators, we would not be able to use tools, and there's no sensible reason to develop tentacles on land. So, this only leaves hand-like clusters of extremities.

• There's no conceivable need for more than two hands that wouldn't be outweighed by the inefficiency of having to supply them with energy.

• Being bipedal gives us a combination of balance, fast/slow modes of travel, excellent ability to overcome obstacles and chase prey or escape predators.

• Having more than two legs would imply a lack of hands due to efficiency constraints (how many intelligent creatures have you seen with four legs and two arms?)

• A head containing the most critical sensory organs makes sense. Anything we've ever designed that's supposed to have good visibility is tall with all its sensor ability at the top (think air traffic control towers, lookout posts, etc).

• Having the primary method of vocalization in the head also makes sense because the higher up it is, the better it will be at projecting sound (assuming sound is the main method of communication, thank for pointing that out @TimB).

• Concentrating functions like eating/breathing/talking into one system is very efficient (again, virtually every organism we know of does this - certainly every dominant organism anyways).

Sure we can design all kinds of crazy adaptations to deal with environmental threats but do we have any reason at all to believe they would happen in reality (and evolve/survive to the point where they become space-faring species) beyond just "we want more flavor"?

The most variation I can conceive of would be skin composition, to allow for living in various elements. But I think ultimately the dominant species would evolve away from having seriously-protective skin features. After all, they would spend tens or hundreds of thousands of years using their intelligence to craft environmental stabilizers like clothing and shelter, so having fur or even just tough skin would have become unnecessary long ago.

We ultimately have had the chance to hone our intelligence to the point of making technological advances thanks to the fact that life on earth isn't that rough. If we were in a world full of constant threats or changing environments, chances are our evolutionary path would have taken us in the direction of physical survival instead of intelligent expansion (e.g., armor plating instead of a bigger brain).

TL;DR: Is there any truly convincing argument for an intelligent, space-faring species to not develop with very similar characteristics to humans? Or as put more appropriately by @Taemyr: "Do we have any reason to assume that any particular intelligent alien would have a markedly different body plan than homo sapiens?"

Edit: To be clear, I'm asking for fully-explained logic showing why a significantly different "style" of organism would end up not only being the dominant species on its planet but be successful enough to develop space-faring technology. Please focus answers on plausible examples based as much in science as possible.

Answer 1

It's important to understand that our "dominance" as it were, is entirely a fluke. There's no intent or drive to make something intelligent; and there's little about our general design that made it even likely.

We just had a series of flukes.

Our ancestors happened to be members of the groups which:

• on dividing as single-celled organisms, stayed together in a colony, rather than dispersing. Eventually, individual cells became specialized, creating multi-cellular life.

• laid down support structures on the inside (vertebrates) rather than the outside (arthropods). This doomed us to never dominating the planet, as the creatures with exoskeletons have always ruled the world, and likely always will, vastly outnumbering us, out-weighing us in terms of biomass, living in a far wider range of environments, outdoing us in just about every possible interpretation of survival, and infesting and living off us.

• controlled their buoyancy by gulping air into their digestive tract, which eventually branched into fish with swim bladders and lungfish: we came from the lungfish.

• were tetrapods, with four fins, laying the framework for their descendants the quadrupeds (all reptiles, amphibians, birds and mammals).

• lived in seasonally flooded mangrove swamps or tidal basins, so were regularly exposed to air, developed fins that could grasp onto roots and stalks, and eventually began to spend the majority of time on land... though naturally we'd been beaten to it by millions of years by the arthropods: insects, crabs, etc.

• invested in internal maintenance of body heat, which served us well when the skies went dark and the ones who'd chosen external thermoregulation died off.

• climbed (brachiated) in trees to became monkeys, this brachiation providing necessary preadaptations to bipedalism and tool-holding.

• had their tails atrophy away (why? We don't know!) to become the apes, resulting in bipedalism rather than kangaroo-style tripedalism.

• lived on water-based prey, likely in mangrove swamps (which may also have lost us our fur covering), so had ample supplies of essential fatty acids for surplus brain growth.

• were large-brained generalists: omnivorous, adventurous, opportunistic and inquisitive. This led us to start using tools.

• were socially gregarious enough to share the abilities that tools gave.

• had a descended larynx, such that complex speech could be developed, and thence storytelling and passing on of knowledge, eventually leading to writing.

It's language -- and more importantly, the preservation of knowledge that it permitted -- which meant agriculture became a thing, and later, sharing of tool designs, mathematics and science led to the industrial revolution.

We could have accomplished all this in ANY body form that had language and a large enough brain to transfer concepts -- none of the rest mattered.

And it's a good thing that form did NOT matter, as the form we have, with its vast flaws, is a result of all these accidents. With only two legs, we are essentially crippled if we lose one, compared to most quadrupeds who can easily adapt. We get backaches, hernias, obesity, and varicose veins, all because of this darn bipedal stance that the mammalian quadrupedal frame was not formed to take -- we've adapted to it, but it's an obvious bodge job, requiring a reworking of all our insides that makes childbirth almost nightmarish and our children incapable of escaping on foot for years.

None of this was required. It was all just a fluke of chance, unlikely to be repeated anywhere else.

But language, and complex, curious brains? Very likely to eventually be repeated on any planet with life.

Whether that inevitably leads to tool use, and whether tool use inevitably leads to space travel, I cannot say... but to me, intelligence and curiosity beget desire to accomplish things; a desire to accomplish things in an intelligent being, becomes a seeking for methods to accomplish that end; and so tool use feels inevitable, and we see it in most species that we consider even slightly intelligent. In human cultures at least, being able to see the sky also seems to inevitably lead to observing it, and curiosity leads to wanting to explore a thing more closely.

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Edit: the above answer focused on the core assumption that our form was inevitable. But I guess I might as well address the other assumptions, though others have already done this well.

uses some liberal wording ... not meant to be academically rigorous

Understood: I shall avoid definitional nitpicks.

Apex species - [...] For example, humans, tigers and orcas. Intelligence is a prerequisite unless you can plausibly explain why it is not required.

Wouldn't a plausible explanation be "the only one in your list of 'humans, tigers and orcas' which even exhibits tool use is the anthropocentric one"? The tool-using species that we are aware of (see list below) tend to be generalists and scavengers, not apex predators.

Without fine manipulators we would not be able to use tools,

Tool use has been observed in the following animals:

• Primates (humans, chimps, bonobos, orangutans, capuchins, baboons, mandrills, macaques; earliest known evidence of tool use in protohumans 3.39 million years ago)
• Other mammals (bears, elephants, otters, dolphins, kangaroos)
• Cephalopods
• Reptiles (alligators, crocodiles)
• Insects (ants, wasps)
• Fish (wrasses, stingrays, damselfish, cichlids, archerfish)
• Birds (finches, corvids, warblers, parrots, vultures, nuthatches, gulls, owls, and herons).

Not all of these use hands or "fine manipulators". In fact, they can be said to fall into these categories:

• Hands/feet: primates, bears, otters, kangaroos, some insects.
• Beaks/mouths/mandibles: dolphins, reptiles, insects, fish, birds, ants.
• Limbs without fine manipulators: insects, fish.
• Tentacles: primates, elephants, cephalopods.

and there's no sensible reason to develop tentacles on land so that only leaves hand-like clusters of extremities.

I admit I bent the term "tentacle" above, to include all prehensile (="grasping") things other than hands.

The list of prehensile things includes (non-exhaustive list):

• Hands/feet: just about anything that climbs trees, primates, bears, otters, kangaroos.
• Tails: reptiles (lizards, geckos, chameleons, skink), seahorses, various fossil animals.
• Tongues: giraffes.
• Noses: elephants, tapirs.
• Penises: tapirs, dolphins, maybe elephants?
• Lips: manatee, sturgeon, orangutan, horses, rhinos

Given this range of fleshy grasping items, most of which are on land, it seems strange to assert that tentacle-style grasping on land is not something that would be selected for.

There's no conceivable need for more than two hands that wouldn't be outweighed by the inefficiency of having to supply them with energy.

The Orangutan would beg to differ.

In fact, the following creatures have more than two separately-controllable prehensile appendages, tailed ones often having five, and some having over ten:

• mammals (monkeys, opossum, anteater, binturong, kinkajou, harvest mouse, porcupines, tree pangolin, rat, potoroidae, monito del monde)
• reptiles (kink, chameleon, snakes, gecko, alligator-lizards)
• amphibians (salamanders)
• cephalopods (octopi, squid, cuttlefish, nautiluses)
• Also just about anything other than birds which regularly climbs trees.

Being bipedal gives us a combination of balance, fast/slow modes of travel, excellent ability to overcome obstacles and chase prey or escape predators.

Being bipedal also gives us hernias, instability, inability to run from predators for years, hip, back, and circulatory problems that quadrupeds don't have, agonizingly painful childbirth, and so forth: from a medical point of view, the changes to our body that were required for bipedalism are an absolute disaster.

Having more than two legs would imply a lack of hands due to efficiency constraints (how many intelligent creatures have you seen with 4 legs and 2 arms?)

Our 4-limbed skeleton is entirely because we developed from a tetrapodal fish that happened to have four fins and a tail. Skeletal changes are, evolutionarily speaking, slow and hard. The formation of a new bone almost never happens, let alone the formation of an entirely new limb. This can happen as a developmental mutation (merging of two embryos, chimeraism), but not, I believe, as a genetic one, so the mutation cannot be inherited.

A head containing the most critical sensory organs makes sense.

I agree - it's also important to have the sensors near the central nervous system, because nerve length relates to reaction time. However, there are other patterns such as that of the octopus' distributed nervous system, also seen in some of the larger dinosaurs.

Having the primary method of vocalization in the head also makes sense

Agreed. Though really low bassy sounds might be better coming from as low as possible. And for aquatics and fliers, of course, it's irrelevant.

Concentrating functions like eating/breathing/talking into one system is very efficient (again, virtually every organism we know of does this - certainly every dominant organism anyways)

We're the only organism I know of that does this without separation of the systems, and it's super stupid. Our descended larynx makes diving and speech easier, but makes choking to death common (1st leading cause of infantile death, 4th leading cause of death among preschoolers, and 4th leading cause of unintentional death overall). It also means we can't breathe at the same time as eat or drink. [Edit: also apparently seen in chimps, macaques, and some deer].

The most variation I can conceive of would be skin composition

This may be the saddest indictment of how modern educational systems kill imagination that I have ever read.

they would spend tens or hundreds of thousands of years using their intelligence to craft environmental stabilizers like clothing and shelter, so having fur or even just tough skin would have become unnecessary long ago.

The only reason you can see our skin now is because of a past environmental pressure (I'd argue for a semiaquatic stage, others believe it was to improve cooling, or sexual selection). It was not due to clothes-wearing or the invention of AC (in fact, we now have to wear clothes because of our hairlessness). Bodily hairlessness is now maladaptive for civilized-us. Head hair and beards that have to be constantly trimmed and managed is also maladaptive. Head and facial hair that clearly signals age is maladaptive, unless aging is a sexual characteristic - which it is not, for most.

In short, in a large population with no significant evolutionary pressures, and technological solutions like hair dye and beard trimmers to resolve any issues which might affect their reproductive success rates, "Unnecessary" is not an evolutionarily selective force.

Answer 2

The question is built on a large number of false premises. Once they are removed, the question no longer stands.

The term "apex species" is used without defining it, with an implication that humans are the apex species. This term is normally used to refer to apex predators, and there are many apex predators on Earth, including crocodiles and some snakes. So, by example, yes, the universe actually "has a reason" (even this term is problematic) to develop apex species other than rubber forehead aliens.

Without fine manipulators we would not be able to use tools

Crows use tools, without "fine manipulators", so this assumption is false.

no sensible reason to develop tentacles on land

There are several errors here.

One is that the assumption that intelligence will only develop on land. Dolphins and octopuses are counter-examples.

Another is that there is no "sensible reason" (again, a problematic term, when discussing evolution) for tentacles to develop. This is mainly an argument from incredulity, but also ignores elephants' trunks which could be considered tentacles (for a broad definition of tentacle, as is appropriate here).

Finally, it ignores other options, such as crow's beaks.

There's no conceivable need for more than two hands that wouldn't be outweighed by the inefficiency of having to supply them with energy.

If that was true, there would be no animals on Earth with two hands and a fully prehensile tail. As there clearly are such animals, this argument must be false.

Being bipedal gives us a combination of balance, fast/slow modes of travel, excellent ability to overcome obstacles and chase prey or escape predators.

Despite these claims, four-legged, six-legged, eight-legged and winged animals are all very successful in their niches.

Dolphins, octopuses, dogs and monkeys provide clear counter-examples where intelligence is found in animals that are non-bipedal. (I guess crows count as bipedal, but they have no hands.)

Having more than two legs would imply a lack of hands due to efficiency constraints (how many intelligent creatures have you seen with 4 legs and 2 arms?)

Many oppossums, elephants and new world monkeys, are intelligent, have four legs and one arm-like appendage. It's not two arms, but shows the efficiency argument is wrong.

A head containing the most critical sensory organs makes sense. Anything we've ever designed that's supposed to have good visibility is tall with all its sensor ability at the top (think air traffic control towers, lookout posts, etc).

Neither crows nor octopuses are tall, and yet are intelligent and have good vision, so again this argument is flawed.

Having the primary method of vocalization in the head also makes sense because the higher up it is, the better it will be at projecting sound (assuming sound is the main method of communication, thank for pointing that out @TimB).

Crickets and cicadas create noise by rubbing their legs. Beavers communicate by slapping the water. Pistol shrimp use their claws. None of these animals have trouble projecting sound without using their head.

Concentrating functions like eating/breathing/talking into one system is very efficient (again, virtually every organism we know of does this - certainly every dominant organism anyways)

Once again, this is an anthropocentric view of biology. Collembola are more dominant than humans and they take in some of the water they need through a hole in the abdomen. Another counter-example shows the conclusion is false.

Do we have any reason to assume that any particular inteligent alien would have a markedly different body plan than homo sapiens?

Yes. We only have a limited sampling of species on Earth, but the most successful species are not human shaped, and most of the intelligent animals are not human shaped. Yes, the most intelligent animals are human-shaped, but that is only a sample of one.

In summary, each of the substantive arguments given in the question for why intelligent beings should be humanoid can be shown to be flawed using only simple examples from Earth. We are left with no compelling reason why aliens should be humanoid, except as a mechanism to save money in sci-fi film production costs.

Answer 3

• Do we have any reason at all to believe they would happen in reality beyond just "we want more flavor"?

Yes we do have a reason; the fact that the universe is a very large place.

In my opinion a better question would be "Do we have any reason to assume that any particular inteligent alien would have a markedly different body plan than homo sapiens?"

Which is a harder question to answer. Evolution is an unguided process that selects for traits that are advantagous in the environment that the organism is in at the time the trait is selected for. This makes it very hard to argue against any particular endpoint, since you would be making an argument that needs to take into account all possible histories that could have lead to that endpoint. Compare your "The most variation I can conceive of would be skin composition..." with the many irreducible complexity arguments that are out there(eg. ).

In particular note that the spesific advantage gained by intelligence is probably not tool use. Rather it's likely the ability to understand and manipulate social networks.

On to your specific points:

• There's no conceivable need for more than two hands that wouldn't be outweighed by the inefficiency of having to supply them with energy.

This is an argument from incredulity. Imagine a centipede that started using it's legs as manipulators, essentially making up the lack of fingers by having a huge number of "arms".

• Being bipedal gives us a combination of balance, fast/slow modes of travel, excellent ability to overcome obstacles and chase prey or escape predators.

This is not why we are bipedal. We are bipedal because we evolved from quatrupededes, and where in a position where it was advantageous to specialize two of our appendages for manipulation. (One could argue for the advantages of a quadrupedal body-plan for large creatures - certainly they dominate on Earth)

• Having more than two legs would imply a lack of hands due to efficiency constraints (how many creatures have you seen with 4 legs and 2 arms?)

See the scorpion in the other answer.

• A head containing the most critical sensory organs makes sense. Anything we've ever designed that's supposed to have good visibility is tall with all its sensor ability at the top (think air traffic control towers, lookout posts, etc).

Probably a point. There is probably an advantage to keeping sensory organs high. - Furthermore you want your sensory organs close to your brain, which implies a clustered design.

• Having the primary method of vocalization in the head also makes sense because the higher up it is, the better it will be at projecting sound.

Why do you assume your creatures communicate by sound?

• Concentrating functions like eating/breathing/talking into one system is very efficient (again, virtually every organism we know of does this.)

I think you have already answered this point. - "Virtually every"?

In conclusion:

If I would told that I was going to meet an alien tomorrow I would expect:

• It to be walking upright
• Having two arms
• Having two legs
• Having an identifiable head with sensory organs and brain
• Having hands on it's arms with a small number of fingers on each hand.
• Sensing smell, light and sound.
• Relying vastly more on one of smell, light or sound than on other senses.
• Having an understanding of math and logic.

I would also expect to be wrong on at least one of the above points, simply for the reason that my sample size in formulating these expectations is 1 - which is far too small a sample to draw any conclusions from.

Answer 4

Actually there are plenty of examples of creatures with more limbs, in particular insects and arachnids. Obviously there are limitations to how large they can grow on our world but in an alien world creatures with a similar body layout could grow large enough to develop intelligence.

The body plan of a scorpion for example:

So lets say on this world a creature similar to the scorpion started working on a more omnivorous diet. One of its claws starts adapting to work as a manipulator and gain fine motor controls while the other remains for cutting. At the same time they grow more social and become pack hunters, growing larger in size.

With the advent of the pack they start gaining social skills and with it steadily increasing intelligence. They hunt in packs and work together to bring down larger prey with repeated stings and then eat the body.

Before you know it you have something that looks nothing like a human but is growing towards sentience. It has one manipulating grasper, one cutting claw, a deadly sting, and the other attributes of a scorpion but is much larger and more intelligent than our earth scorpions.

You could follow exactly the same process for many other alien creatures too.

Answer 5

Here's one clear example where the body plan would be very different:

Fluid-environment intelligent life.

Earth has several examples of relatively intelligent life forms that live in the water, such as octopuses and dolphins. These creatures have comparable levels of intelligence, manipulation ability (in the case of octopuses), etc., to that of apes. Therefore, it is not unreasonable to believe that an intelligent underwater life form could exist.

Obviously, the body plan arguments only apply to land creatures. Bipedal? Vertical main axis? A sea creature would probably look more like a dolphin or octopus or something.

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Another example would be a intelligent creature which developed from a bird - it might have wings, instead of arms.

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There is a more general argument here, in addition to the specific cases outlined above. Every living thing any of us have ever seen evolved on Earth. Therefore, it's really hard to tell which qualities are necessary/effective for life, and which are necessary/effective for living on Earth.

For instance, on a planet with a dramatically different level of gravity, different body plans would be most effective. Higher gravity? Lower to the ground, more spread out. Lower gravity? Maybe flying, maybe something resembling an insect, with thin fast legs.

On a planet with a dramatically different system of heat generation, such as where the heat primarily comes from a molten core, rather than a star, underground living styles would be most effective, and body plans better suited to that situation would thrive.

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The list goes on. Your theories have the potential to be accurate for e.g. land-based intelligent creatures on Earth-like planets, but there is no guarantee that aliens we meet will fit those parameters.

Answer 6

Without fine manipulators we would not be able to use tools, and there's no sensible reason to develop tentacles on land so that only leaves hand-like clusters of extremities.

Or a tail.

There's no conceivable need for more than two hands that wouldn't be outweighed by the inefficiency of having to supply them with energy.

No conceivable scenario? How about 4 species on a planet, trees, supermegadeadly bears that kill anything on the ground, and two species of tree dwelling primates that are smart enough to use spears/clubs - one of which has 4 arms and a tail, and one of which has 2 arms.

Who do you think would be the dominant species 10k years later?

Being bipedal gives us a combination of balance, fast/slow modes of travel, excellent ability to overcome obstacles and chase prey or escape predators.

No, being bipedal gives us terrible balance and top speed. Go take a dog for a run.

Having more than two legs would imply a lack of hands due to efficiency constraints (how many intelligent creatures have you seen with 4 legs and 2 arms?)

You are begging the question here, by assuming anything that hasn't happened can't happen, you come to the conclusion that the status quo is the only possibility.

A head containing the most critical sensory organs makes sense. Anything we've ever designed that's supposed to have good visibility is tall with all its sensor ability at the top (think air traffic control towers, lookout posts, etc).

Think drones, aeroplanes. You are also making some assumptions that the only senses the creatures could have are those that humans have. On earth, there are animals that can sense magnetism, can predict earthquakes, and many other things. Imagine a world in which the most dangerous and significant factor is frequent, deadly earthquakes.the most important sense wouldn't be sight from the head, it would be tremorsense in the feet. It could make sense to locate the brain lower down, the decrease the reaction time for signal to go from the feet to the brain and return. Or perhaps flight would be preferable. I can certainly imagine if there is only a single flying species, it makes sense to have eyes that look down, and a protruding head isn't necessarily the best option.

Having the primary method of vocalization in the head also makes sense because the higher up it is, the better it will be at projecting sound (assuming sound is the main method of communication, thank for pointing that out @TimB).

As TimB says, you are assuming speech Lets say your species has developed tremorsense to avoid earthquakes (see above), why not use that for communication? It would stop other creatures being able to 'hear' you if they have not developed the same sense.

Concentrating functions like eating/breathing/talking into one system is very efficient (again, virtually every organism we know of does this - certainly every dominant organism anyways)

By every dominant organism, you mean human? Again, you are making some big assumptions. You say that in order to eat, the creature needs a mouth, but why does the organism need to eat? Trees get their energy without eating.

You have made the assumptions that the animals are extremely similar to humans, and then have used reasoning to suggest that, assuming they are very similar, they actually need to be extremely similar. But there are so many alternatives.

Another significant thing you have not considered, evolution does not produce the perfect beings, it generally tends to produce beings that are more suitable than their direct predecessors and current competition.

Answer 7

Without even going into each argument individually, there is a large flaw in your logic.

You're assuming only the best possible outcome can and will end up fostering intelligent life.
While this would make for easier argumentation, as it is often much easier to find the best solution than to count the amount of solutions. It is completely unrealistic.

The first species to be sufficiently dominant in its eco-system and has the capability to mature its intelligence over subsequent generations is enough.
You don't need anything perfect, you don't even need good, you just need good enough.

Answer 8

Most of the other answers have already covered the counterexamples to the arguments put forth in the question, so I will attempt to cover the biological theories behind it.

The question assumes that all intelligent species will evolve into a humanoid body structure by convergent evolution due to the humanoid body structure being the "best" structure for this purpose.

To evaluate whether this assumption is a good one, we can look to examples of convergent evolution on Earth, and determine whether they produce similar body plans.

Aquatic animals

Water resistance is a major factor in the speed of animals that swim through the water, and therefore a commonly cited example of convergent evolution can be found in the similar body plans of dolphins and icthyosaurs, animals which evolved from different ancestors but nevertheless have the same streamlined ogival body shape.

Despite these similarities, however, the two animals are nevertheless very different. The icthyosaur swims by a left-right motion of its tail fluke, and the dolphin swims by a up-down motion.

Flying animals

Similarly, the pentadactyl limb of flying animals is also cited as an example of convergent evolution. Different parts of the animal's limb became adapted into the wing structure in different animals, the pterosaur uses one finger, the bat uses four fingers, and the bird uses all its fingers together. Despite the fact that the structures are outwardly similar, their internal structures are extremely different.

Therefore, we can see that convergent evolution, even amongst animals that are already related to a large extent (ancient amniotes in the aquatic animals, ancient tetrapods in the flying animals), produces animals which are superficially similar in form but vastly different when analysed in detail.

Furthermore, the evolution pressures on intelligence are also much less specific to a certain body plan than water resistance or aerodynamic structure. Many theories for the evolution of intelligence exist, but none of them involve body plan. The most widely accepted evolution pressure theories involve a self-competitive Fisherian runaway process, which can occur in any kind of animal that has the capacity to compete intellectually with others for mates.

There is no good a priori reason for the number of legs or limbs of an intelligent animal to be fixed to a specific number. As the previous answers have already shown, the argument from incredulity is a very poor argument when it comes to imagining aliens.

Answer 9

Stanisław Lem is a very good example of an author who dedicated most of his sci-fi books to examine this very question, and created plenty of worlds inhabited by intelligent life which doesn't differ just in a few facial features from humans, but is so different from anything we've ever encountered that we might not even recognize each other as "life", much less as "intelligent". He often even gives detailed explanations about how these lifeforms could have been evolved.

Answer 10

You seem to be assuming that an intelligent organism would have to be a vertebrate. It wouldn't.

The requirements for intelligence are a reasonable size to accomodate a big brain, social behaviour, and an ability to use tools.

From an anatomical point of view it's unsurprising that (despite the relative lack of social behaviour) the most intelligent invertebrate is the octopus. It needs intelligence to make the best use of all those limbs. Octopi have been shown to learn by demonstration as well as experience (An octopus who watches another octopus solve a puzzle, such as getting food out of a screw top container, is able to solve the puzzle itself.)

The main thing holding octopus civilization back is that their reproductive cycle means they never knowingly have contact with their offspring, and therefore have no incentive to care for them. An octopus that evolved live births would probably soon evolve good care of its offspring and a rich culture quite quickly afterwards.

Holding back octopus technology is their rather stable environment. They have no need for shelter, as warm blooded humans in a cold climate do. On the other hand, if they start going to war, they will need weapons and castles.

I imagine them forming a society like the Greek philosophers, with major advances in maths, and maybe less so in technology. Still it would be interesting to see what technologies they came up with, and that could be the subject of another question.

Concentrating functions like eating/breathing/talking into one system is very efficient (again, virtually every organism we know of does this - certainly every dominant organism anyways)

Actually eating and breathing through the same hole (the throat) is a terrible piece of design. It means we can choke on our food. It comes from our vertebrate heritage, where fish's gills were joined to their mouths, so they could gulp water across them to breathe. As this was the only bodily opening available, the lungs were also accessed through the mouth. Arthropod and mollusc lungs have their own separate openings, which is a much better idea.

Answer 11

Humans evolved from primates, which evolved from small mammals that evolved from even smaller mammals that survived a cataclysmic event some 65M years ago.

Rewind the evolution timeline, and pretty much everything alive originated from underwater life.

One extremely interesting way to actually witness evolution, is to look at how a human embryo evolves in the first few weeks.

We have two eyes because we're genetically programmed as such; we breathe through the nose (/we have a nose) for the same reason. If our ancestors weren't fish, our skulls wouldn't be structured the way they are; there's a reason our eyes start off on either side of our embryo heads, and for the "intake" end of our respiratory system to start off with a support for... gills.

All vertebrae share a common ancestor, and that common ancestor's ancestor was a bacteria. Everything between that bacteria and the Homo Sapiens is a fine combination of trial-and-error adaptative evolution, over millions and millions of years, and with a great deal of luck - without that asteroid impact some 65M years ago, mammals would probably have never been given a chance to evolve and take over the Earth, and if nothing stood in the way of these giant creatures we called dinosaurs, who knows what could have happened.

Being a biped is the result of millions of years of evolution, in a path that includes being a quadruped and growing such limbs as a result of adapting from an aquatic environment - with the advent of the need to move out of the water, most probably being nothing more than a fortunate accident.

My own personal conclusion, is that alien life that would have evolved from a similar original bacteria, over millions and millions of years, with its own "fortunate accidents" and massive extinction cycles, has no reason whatsoever to be anywhere similar to anything we know.

Answer 12

Yes. You think too much within our human world.

• You don't need tools for intelligence. "Uses tools" -> "Must have a certain level of intelligence" "Can't use tools" -> "No further implications regarding intelligence".
• Yes, there is, if you, say, also use them for walking.
• I'm not sure scientists are sure why we are bipedal, but one big reason I know is that's it's better for endurance. You can keep up a moderate speed for days at a time (not you, specifically. Me neither. But people in general if running was as important as being able to spell "specifically" correctly). Deer can't. They'll eventually have a heart attack and die if an animal like a human tracks it. No weapons necessary! But there is no reason intelligence could not evolve in an animal which is better at sprints (and then, say, hiding, or trapping it's pursuer).
• First, examples to the contrary were given. Second you assume hands and legs must be separate.
• Thinking/input complex: sigh What about hive minds? Just as a single example. For example cancer would be much less dangerous for such a creature (what do you care if a couple percent of your "cells" die per year?). Why not bees with some way to communicate, with hives reaching an intelligence as a whole?
• "primary method of vocalization" "eating/breathing/talking" From where do you know they breathe/talk/eat like we do? What about a creature which communicates purely using pheromones? One which doesn't "eat" but one which drinks body fluids of other creatures?

Answer 13

There is a big trap when extrapolating from what you see on Earth: all life on Earth shares a common ancestor. Long before a group of ape-like mammals started evolving towards the intelligence humans currently have, there was a long history that limited what was available.

All vertebrates share the same general body plan with a spine, hollow with nerves inside it, and a mouth somewhere near the top and an anus somewhere near the bottom. Not because that was best, but because that's what we started out with. A mutation that changes the body plan drastically will almost certainly die very early on after conception, because this body plan is the first thing that the embryo develops.

The reason we ended up with this body plan was not because it was best for intelligence, but because some very early "fish" had it, and ended up becoming the ancestor of all later fish.

Elsewhere, evolution has a completely different history. We can't make any assumptions.

Answer 14

I read some flavor text for StarCraft some time ago that explained the background story of the Zerg. I can't find the source, so this is by memory:

The Zerg originated as parasites similar to Captain Higgins. They are worms that invade a host and manipulate its behavior. Through a Xel'naga evolution boost (which may be optional) their repertoire was enhanced to DNA manipulation. Need to crush a nut? Grow a fist made of bone. Need shelter? Grow a thicker skin. Need more shelter? Make a plant grow into a house. Need to move fast? Catch the fastest animal you can find, absorb the DNA, enhance it a bit. Need to be smarter? Grow a bigger brain. Need to fly to space? Grow ... something (Zerg can go to space). Whatever is required is grown or adopted from creatures around the universe.

The Zerg don't even have a body the way you described it, they change their form to whatever is useful at the time. Combined with a hive mind you keep a collection of creatures instead of tools for reoccurring jobs or grow from DNA as needed.

Things like languages, politics, culture and vehicles were unknown to the Zerg and even after learning about those things they didn't adopt them much, but when they did find a useful design they enhanced and kept it.

Answer 15

Many people when dealing with the concept of extraterrestrial life make a number of false or incomplete assumptions. Firstly, the probability as far as we currently know is that life is just as likely to exist only on Earth and nowhere else, as it is to exist elsewhere. We have no frame of reference for 'What is the probability of life existing?' as no comparison exists.

Given an identical Earth, with identical properties, we can still only give the probability of life being between 0 and 1. So to assume life would ever evolve in the same way is many levels further of not knowing the probabilities. It is just as likely that all planets in the universe that have life, evolve in the same way Earth has, as it is likely they have a completely alien idea of life, possibly too hard for us to even comprehend.

How could we possibly know that the human-form is the most likely to exist? Think how strange the entire process of the body working is;

Vision: Discrete quantized packets of energy flow in a sinusoidal waveform, hitting a lens to focus this 'light' onto a transducer, transforming the light's energy into a flow of electrons that will travel across millions of neural connections, acting like complex transistors with 100s to 1000s of possible states, thus forming a 2D representation from a 3D world (And this is a large simplification)

So, why would it be likely that any of that would be the most logical process for a being to "see"? That just happened to be the way evolution began with, and it was better than anything that didn't have that. Why is our way of walking as a bipedal the most likely or efficient? Going to an extreme; Why wouldn't it be more efficient that a life-form would instead of "walking", bend space-time in order to move from point to point? Why does it need legs? How could you or I possibly know the efficiency or likelihood of that occurring? Perhaps this creature can harness zero-point energy...

Answer 16

I think that the answers already posted are great. However, they focus on examples of earth, which one could argue are not intellegent enough to count (which i feel boils down to moving the goal posts and 'not a real scottsman" arguments). Or they speak in very general terms. I still agree with both of them, but lets try another route, proof by counterexample.

Already a list of presumptions made was pretty detailed, so I won't go into them, but one implied one was the presumption of land based intellgence.

Lets imagine a world with an extremly heavy atmosphere, and limited solid mass. You would be crushed by pressure or roasted by heat before solid mass was found. In this world all creatures fly, or more accurate 'float' in the atmosphere, must like creatures in water, but without quite the level of resistance water provides.

In this world a tall structure would likely be quite bad, it would not be aerodynamic. The need to float in a lower density air (compared to water) would likely require a more balloon like structure as well, larger species would have to have large but light bodies to displace as much gas as possible.

appendages to manipulate objects would almost certainly not look like hands, which evolved from feet and thus are tied closely to land animals. Instead their 'wings', whatever structure they use to control their moment in three directions, would likely evolve into their means of manipulating objects, or perhaps grabbers used to catch prey (which in turn likely evolved from 'wings')

This species would likely have a harder time leaving their home planet, due to difficulty of getting solid supplies to construct ships with, but could eventually find ways of doing so. They would look entirely different and work entirely differently, but they would make sense as a sapient species for the world they evolved in.

Answer 17

Your comment on Mat's Mug's answer prompted a new thought and a slightly different answer to the others; turn the question the other way round.

Dinosaurs, for example, ruled the earth for far longer than mammals have, and yet they developed no technology at all.

Let's look at dinosaurs. Velociraptors are bipedal, with two hands, two legs, and a tall raised head and stance containing major sensory organds. Similarly theropods such as Allosaurus and T. Rex.

And yet, as you say, they did not, in millions upon millions of years develop technology, or even, as far as we know, the beginnings of language.

So, given the various species over the life of the planet with your proposed body plan that have not come up with technology, is it still likely that highly specific humanoid physical characteristics are so important as to be universal?

Answer 18

Most of the features you list are kinda convenient but not really critical. A few are critical. Despite all the answers, nobody seems to have pointed them out yet.

It is really, really important, evolutionarily, to have something like a "head", where the main processing is done in close proximity to clusters of sensory organs that provide the most (and most timely) information. This lets you react quickly to stimuli. If you ever wonder whether reacting quickly, is a big deal, watch mongeese attacking a snake. Tiny, super-fast mammals are often able to kill a big fast snake because they have enough of an edge on speed. This is true everywhere in the universe. So, yes, they're going to have a head. And it's going to have to be not extremely tiny, because the universe is a complicated place and you need to have room to store your brain. And it's going to have to be positioned so that it can observe it's environment pretty well (i.e. view not occluded by the rest of the body).

Second, it's really important to have some redundancy. If you're going to live long enough to amass a lot of intelligence, you're going to have to deal with a lot of potential problems along the way. Any organ that is so critical that you'll die without it in time T had either better nearly never have failures for as long as T, or you'll need two. So, two eyes and ears: almost surely, as the physics of capturing light and vibration require sensitive and compact organs, and those organs are key for rapid responses. It's easy for them to be somewhat out of sorts. Also, determining distance is really important and you need at least two to do it by parallax. But there's no really great reason you couldn't have more than two. (Does your mouth go near your head? Probably, because you want to make fast decisions about what you're eating and how you're biting it. But if your mouth is tough enough, or redundant enough, it might not matter.)

There is also a very good reason for bilateral symmetry, which is that motion is difficult if you're asymmetric, and movement is really important. Witness the incredible success of bilateria vs. everything else when it comes to moving around. So if you have more than two of an organ or limb, it's probably going to be an even number. There's a possible exception for certain kinds of fluid-dwellers that look like octopi or jellyfish, but those would be in the minority. (And note that our octopuses are bilaterally symmetric.)

Some way to communicate reasonably complex concepts is also essential, since it allows organisms to take advantage of each others' experience. It could be auditory or visual or even chemical or (for water-dwellers) electromagnetic. For instance cuttlefish are masters of visual display of information. So we can't really predict what form it would take, just that it's there. (Possibly in a form we would initially overlook.)

All the details about two arms and limbs and bipedal and all the rest works okay on Earth, but could (and probably would) turn out differently. Even ability to finely manipulate the environment is fquestionable; social interactions drive intelligence so as long as there's something to do socially, you're probably okay. (Of course, being extremely clumsy makes you vulnerable to parasites and such.)

(These constraints are only true, of course, until they start bioengineering themselves. Then all the former constraints may come off, and they could be anything that works physically with arbitrary materials.)

So: head, yes, and up high or at one end or otherwise out of the way of the body. More than one eye and ear: yes. Bilateral: likely. Communications channel: yes. Everything else: probably different.

Answer 19

For all these answers, every one omits fire. Fire is absolutely vital, yet none but a few who post in comments consider it.

Here is the crux of the matter. Fire is vital because of the energy boost involved in consuming cooked food. The human digestive system is 25% shorter and consumes proportionately less energy than the immediate primates, and is attuned away from the ability to digest cellulose at all. Cooking also kills numerous parasites. The full benefit of this would not be immediately available, but this puts rotting meat back on the table with far less immune system energy consumption. The changes involved free up energy for the intelligence required to wield fire.

Fire is at least four steps in toolmaking. That is it is the equivalent of making a tool to make a tool to make a tool to accomplish a task. This in turn requires the intelligence to understand the steps, and the grasping hands to manipulate fire, and I'm pretty sure the loss of any significant amount of fur on the arms.

(Wait what you say. Fire is vital for intelligence but intelligence is vital for fire. Yes I know. No wonder people don't want to talk about it. The claims are simply not made, but archeologists know where they find cooking fire it is human, and the ability to use fire is the best test of intelligence.)

The fine development of the grasping hand and tactile sensation and the hand-eye coordination would tend to make the hand not-so-suitable for walking on (you want arthritic hands in short order, start developing the knuckle-wakers for fine work). This means unless you started with six limbs, you end up with bipedal. None of this, however, required an internal skeleton at all; however in this world there are reasons there aren't any large exoskeleton creatures.

In answer to other claims, intelligence is the game-changer. Humans are the uncontested apex predator in all terrestrial environments that can possibly support a human (we can't live on insects), and now in the shallow oceans.

You ask for a potential space fairing race. This immediately doubles-down on the need of fire. In addition, while I like the idea of an acquatic space-fairing race the essentials of setting one up appear insurmountable. Let us suppose for an instant the water-world with only a few islands, yet they somehow grasp for the stars (for them space is an ocean would seem more true than even to our storytellers). But the first building is the VAB. How do they refine metal enough to build? How can they ever discover rocketry without first the need of gunpowder? How could such a struggling race manage somehow to lift the first of their own with all the tons of water required (although I must say this makes recovery much less to fear). Once put the details to it, it just seems too hard.

If you want a non-bipedal you are likely to end up with something like a large dog with an extra set of arms coupled to the skeleton with another set of shoulders just behind the ones for the forelimbs (before the forelimbs would likely be too front-heavy).

Answer 20

Perhaps at the microscopic level, intelligent aliens are group minded. Their intelligence could be proportional to their population and, if their DNA is anything like our own, they would grow in ways similar to cell division.

They could form spores to protect themselves from the harsh environments of space and reanimate in fairer conditions. Cells can communicate through chemical signaling. Maybe this could be the major pathway used to communicate when cells coalesce to form a superorganism.

They could achieve genetic diversity through cell specialization. With time, and natural selection, the macro organism would take a new form that might be less aware of the intelligence that it possesses for the benefit of cell specialization. But potentially, on the macroscopic level, they could look and behave like human.

I say that only because the diversity of species here on earth is astounding, but humanity today has a much greater potential for intelligent (space-travel) behavior than it did yesterday and I can't say that for another species.

Answer 21

Take a good look at Animorphs. Originally, Author K.A. Applegate conceived the main "good" alien, the Andalites, to be similar to the popular "Grey alien", anticipating possible film or TV adaptation. Her editor wrote back that the Andalite was not interesting, so Applegate decided to make them blue Centaurs with seven fingers, three nostrils four eyes (two on stalks capable of 360 degree rotation) a scorpion like tail tipped with an extremely sharp claw (called a blade) and no mouth (they absorbed nutrients through their hooves). The main antagonists were an alien race of parasitic slugs that could interface with neural tissue. Their primary hosts were seven foot tall lizard men with claws at nearly every joint (we later learn that they were genetically engineered by a more advanced race who pretty much liked to make creatures that filled specific roles but look bizarre and fierce at the same time) and a centipede like alien with multiple claw appendages that were able to manipulate tools and a driving hunger so painful that they overwhelmed the Yeerk parasites, who normally don't change behavior because of host emotions.

In fact, the Andalites on numerous occasions question how Humans are even viable as a species. We have no natural weapons of our own, bipedalism is unique to us (all other creatures portrayed as balancing by some other means, either a tail, having a wide base, or three or more limbs). One yeerk expresses confusion that Earth, in general, having such a diverse ecosystem is even more insane and we should never have achieved our dominance given the concept of self-doubt was unheard of by the Yeerks until the came into contact with the human mind (the first yeerk to identify this trait immediately sees the benefit of this almost immediately as it allows us to better question a course of action).

Moving to more real world matters, a good number of biologists have noted that if intelligent life exists, it will most likely be arachnoid, not humanoid, in origin. This grouping is among the most numerous of animal life. It is likely that alien life will be a majority insect based. Among our own species, social insects meet a lot of the social and language concepts, but they are not communicating in ways we can ever be fluent in translate. (Bees use dancing, ants and termite languages revolve around complex chemical scents). Even our own human society didn't have the same technological leaps... The Inca are famous for their complex road networks that rival even those of the Romans... but they never invented the wheel (or at least developed it for purposes beyond children's toys, which is pretty indicative of all native American Cultures). From an old world perspective, this seems at odds with how technology progress, but recall the Inca lived in steep mountainous regions... the idea of a wheel never caught on because no one used it long enough to devise the next invention, brakes, that would prevent it from catastrophically rolling off a cliff). And without decent uphill propulsion, it was easier just to use more Alpaca's which already could carry loads up hills. The Mayans and Aztecs never even got to the road stage, because they never had access to beasts of burden until Europeans brought horses. But Mezo-Americans were regarded as having some of the earliest forms of Brain Surgery, developing techniques comparable to modern operations as early as the stone ages... in fact, when the first contact between Europe and The Americas occurred, these cultures perfected surgery to such a degree that patients had a 90% survival rate and many people had gone under the knife more than once... In the West, comparable techniques wouldn't be employed until the late 1800s-1900s (again, consider the regions... a good number of antibiotic agents used today are derived from plants native to South/Central American rainforests. The Old World could cut open a skull but could do little to keep it from being infected, so more advanced techniques like where is the safest place to make the incision, never developed).

Take away tool use, and man's distinct advantage is its endurance. Our bodies are quite efficient when compared to other Apex predators and being omnivores means we can survive on just about anything we can get into our mouth. We may not be the fastest swimmer or the fastest climber or the fastest runner, but we have "best two out of three" against most of our superiors in any one category, and even then, they won't do it for long. Humans are the fastest animal in an Ultra-Marathon (a 100 mile run) and while other animals can outspeed us, we can outlast them at a sustained pace. There is a biological reason for our bodies beyond just supporting a large brain.

Finally, just one quip I like to point out in theoretical aliens, of all the animal kingdom, Humans are one of only three species engages in sex for pleasure. If aliens exist, the stereotype of hyper-sexualized fanservice aliens is likely to fall on humanity as the rest of the galaxy thinks we're way too obsessed with procreation and our planet becomes the Pleasure Planet for the alien deviants.

Evolution does not go to any specific point. In fact, the reason why human intelligence never evolved in other animals is the brain capacity needed is inconducive to survival. Human youth are much more vulnerable for much longer because our brain takes longer to develop (full adult human development has always been decades versus apes and monkeys, which are years). But, the fact that we hunted by a combination of chasing our prey to the point of being unable to run any further, rewards the individuals who can figure out how to reduce that hunt-feast time. Some of these traits would be better mobility and stamina, but cleverness to end the chase sooner... humans are one of the best animals at ranged attacks. A gorilla can throw something at 20 miles per hour. A human can easily throw with three times the speed, and humans who are trained to throw can put as much as five times the speed behind an object. But it's not enough to throw an object, you need to hit your target, which is rewarded by better eyesight, an ability to attain a height advantage AND throw an object reliably while achieving the height advantage and not have the resulting force knock us over. This also develops an ability to notice minute changes in the handling of throw-able objects, the capacity to realize that success relies on hitting not where something is but where it is going to be, which requires a finer attention to detail. All of these lead to overdeveloped fore-arms with less developed hind legs. Alternatively, having buddies throw rocks will also help, which requires a need to communicate complex instructions. These things weren't isolated developments... Humans are not Pokemon... Neanderthal does not learn throw at level 21, evolve into Homo Sapien at level 36, and learn Spear Attack at level 55. These all developed simultaneously over the course of millions of years where those who were better able to live were better able to have children who in turn would better be able to live.

Again, tool use is quite common in the animal world. In addition to apes and birds using sticks to get termites, Dolphins are known to use sea sponges to protect their noses while digging for prey in abrasive sea floor sand. Otters are known to use rocks to break clam shells and identify "My Rock". They also use kelp to anchor themselves to a location while eating or sleeping. Dogs are quite capable of using certain human tools to their ends (my own dog has taught herself how to open door handles).

Language is even more widely used in the animal kingdom. I mentioned bees and ants (depending on the lasting effect of ant chemical markers, Ants could possibly count as having a written language, which is leagues more advanced than several of our closest genetic relatives). Dolphins and whales also have complex songs that communicate over vast distances. Prairie Dogs not only have a complex form of informative barks that has been mostly parsed by humans, but regional dialects have been observed between multiple clans. Once again, what separates domesticated dogs from Wolves isn't the capacity for language but the degree of information domesticated dogs can transmit. A Golden Retriever has the language capacity of a three year old and are able to "converse" with humans by a series of barks and body languages. The purpose of a domestic cat's meow is used primarily to facilitate communications with primary auditory communicating humans and body language communicating cats. A wild member of the cat family or feral domestic cat rarely meows, and when they do, it is never in the range of tones that a domestic cat produces, making this an example of a constructed language, with a degree of accuracy that humans can parse the tonal meows... If I were to describe a sound a cat makes as a "mew" you instantly get an idea of the cat in a calm state of mind versus what a "Yeow" is a cat that is distressed in some way.

Even some repetitively simple mating displays can count as languages. After all, language is the reliable transmission of information from one individual to another. "I'm sexy and I know it" and "Hey there, stud" may be quite simple, but it's still transmitting that information. After all, we humans do recognize a statement is being made by showing off one's buttocks. So we must acknowledge that when a firefly does it, it is conveying a message of equal intelligence to one that is conveyed by a Frat Bro.

Human intelligence developed to provide a specific advantage to survival. At its core, it was the idea that humans could outperform a much more physically superior foe AND do it quickly and with minimal threat to ourselves and our own. Our intelligence facilitated survival, but it also facilitated the survival of ants and grass. Perhaps out there in space, there is an animal that is capable of intelligence equal to our own, but it might not have evolved in the same way as us or to facilitate the same niche as us.

Answer 22

A large colony of ant-like insects could in principle implement a large neural network. Such a colony could then control its local environment, make tools, etc. A civilization comprising of many such insect colonies could arise. They would be able to communicate with each other a lot better than we can, because two such neural networks can interact with each other in a much more direct way.

Answer 23

"Without fine manipulators we would not be able to use tools, and there's no sensible reason to develop tentacles on land so that only leaves hand-like clusters of extremities."

This is assuming that intelligence has to evolve on land when there is no reason that it must. Also there are other types of appendages that animals use to manipulate their environment besides tentacles and hands such as claws, elephant trunks, and the beaks of birds.

"There's no conceivable need for more than two hands that wouldn't be outweighed by the inefficiency of having to supply them with energy."

Octopi have eight tentacles and they use all of them for grabbing things so this assumption is false.

"Being bipedal gives us a combination of balance, fast/slow modes of travel, excellent ability to overcome obstacles and chase prey or escape predators."

Many animals that aren't bipedal are much more agile than us so this argument isn't really valid.

"how many intelligent creatures have you seen with 4 legs and 2 arms?"

When it comes to organisms having the same level of intelligence we only really have a sample size of one so what we see in humans doesn't say anything about organisms that would have the same level of intelligence as us.

"A head containing the most critical sensory organs makes sense. Anything we've ever designed that's supposed to have good visibility is tall with all its sensor ability at the top (think air traffic control towers, lookout posts, etc)"

There's no reason an intelligent life form at the same level of intelligence as us would need good visibility.

"Concentrating functions like eating/breathing/talking into one system is very efficient (again, virtually every organism we know of does this - certainly every dominant organism anyways)"

The only animals that do all three through the same system are humans and the only animals that make noises through the same system that they use to eat are vertebrates so this has only evolved once. Squid have their gills separate from their mouths. Insects make sound by rubbing different body parts together and moving their wings and they breath through air holes along their exoskeleton so animals can make sound without using their mouths. Also aliens wouldn't need to use sound as a means of communication but could use bioluminescence.

Answer 24

Dewi Morgan's answer is my personal favorite, so far, but I'd like to take a crack at answering the OP's question, using the methodology Dewi described, and I decided to apply it to a scorpion based species, being inspired by the comments of by Tim B:

[the] ancestors [of the hypothetical spacefaring species] were:

members of the group of single-celled organisms which, on dividing, stayed together in a colony, rather than dispersing. Eventually, individual cells became specialized.

Members of the group which laid down support structures on the inside (vertebrates) rather than the outside (arthropodae). This made [them] [destined] to be a dominant species on the planet, as the creatures with exoskeletons ruled [their] world, and likely always will, vastly outnumbering [other animal groups], out-weighing [other animal groups] in terms of biomass, living in a far wider range of environments, outdoing [other animal groups] in just about every possible interpretation of survival.

Members of the group which lived in seasonally flooded mangrove swamps or tidal basins, so were regularly exposed to air, became [amphibious] (not to be confused with amphibian) and eventually began to spend the majority of time on land... though naturally [it was] millions of years [before reptiles, birds, mammals, and other animal groups].

Members of the group which [did not] invest in internal maintenance of body heat, which served [them]well when the [vegetation plagues wiped out the high energy food sources required by] the ones who'd chosen external thermoregulation, [and so caused their near extinction as animal groups, with only the most primitive, non-specialized, variants able to adapt and begin a new evolutionary path, far too late to compete with the dominant arthropodae].

Members of the group of tree-climbers who became [primarily ovivorous], so had ample supplies of essential fatty acids for surplus brain growth.

Members of the group of large-brained [ovivorous arthropodae] who were social, adventurous, and inquisitive enough to start using tools and sharing the abilities they gave and were opportunistic and gregarious, always looking for an opportunity to improve [their] lot.

Members of the group which [had enough sets of appendages that they were able to specialize one set of appendages for tool use (which later included writing) and another for both gestural and later audible communication, and still have enough for locomotion and other basic survival needs, instead of having them atrophy away like some other more primitive species], such that complex "speech" could be developed, and thence storytelling and passing on of knowledge, ]including the previously mentioned writing].

It's language -- and more importantly, the preservation of knowledge that it permitted -- which meant [aviculture] became a thing, and later, sharing of tool designs, mathematics and science led to the industrial revolution. [... spacefaring ... ]

As far as I can tell, this meets all of your criteria: intelligence, apex predator/species, could continue to spacefaring status, and addresses the body plan question. In my imagination, this leads to a cold blooded species resembling an oversized mix between a scorpion and centipede, having an exoskeleton, compound eyes, or multiple groups of eyes, mandibles not suited for verbal/audible communication, no significant need for olfactory sense organs at all or externally visible auditory organs, vibrations could be detected in many other ways, skin, hairs on the body, etc. It has 5 to 7 separate pairs or sets of appendages, some customized for specific tasks, like the claws of a scorpion for one task, and the legs for another, but with more complexity and variation in each set. The fact that it's cold blooded allows for efficient energy conservation required to power multiple appendages in a single organism.

In summary, this is a fully-explained logic showing why a significantly different "style" of organism would end up not only being the dominant species on its planet, but be successful enough to develop space-faring technology.

Answer 25

Without fine manipulators we would not be able to use tools, and there's no sensible reason to develop tentacles on land so that only leaves hand-like clusters of extremities.

"on land"; there is no reason why intelligent life could not develop underwater instead (in fact: octopi and dolphins here on earth are intelligent enough to be sapient)

There's no conceivable need for more than two hands that wouldn't be outweighed by the inefficiency of having to supply them with energy.

Have you seen how many manufactoring processes use clamps and grips of some kind (especially in carpentry); having more hands makes it a lot easier to hold a plank in place, place a nail against it and hammer it in place

Being bipedal gives us a combination of balance, fast/slow modes of travel, excellent ability to overcome obstacles and chase prey or escape predators.

Bipedal balance takes rediciulous amounts of energy, quadrapeds have better and cheaper balance. Plus; horses are perfectly capable of balancing, jumping and running away

Having more than two legs would imply a lack of hands due to efficiency constraints (how many intelligent creatures have you seen with 4 legs and 2 arms?)

Why though? this builds on the claim that "more than two hands is not worth the energy", which is debatable. I will return to the octopus; it has 8 hybrid arms/legs, and it is intelligent.

A head containing the most critical sensory organs makes sense. Anything we've ever designed that's supposed to have good visibility is tall with all its sensor ability at the top (think air traffic control towers, lookout posts, etc).

Partially true, but vision is the only one that benefits from being at the highest point of the organism; all other senses can be pretty much anywhere (although keeping smell away from the anus seems practical)

Having the primary method of vocalization in the head also makes sense because the higher up it is, the better it will be at projecting sound (assuming sound is the main method of communication, thank for pointing that out @TimB).

You already mentioned that sound need not be the main communication method; but height is not as important for projection as you might think. In fact; if the mouth is facing "up", it can project in every direction at once, instead of in the direction the "face" is facing; this is very useful in larger social groups

Concentrating functions like eating/breathing/talking into one system is very efficient (again, virtually every organism we know of does this - certainly every dominant organism anyways)

concentrating eating and breathing is a terrible design; it allows the organism to choke. Also, depending on the diet, the mouth might be occupied for long periods of time (see: snakes, who have a breathing tube under their tongue to breath while eating).

We expect and display other lifeforms as vaguely humanoid becouse that is what we know, and it allows the (human) protagonists in fiction to share air and tools with the aliens. There is, however, no scientific reason why it should be so, and there are so many viable permutations even on earth that we can be pretty sure that aliens won't be humanoid

Answer 26

One of your assumptions is that such a creature would both sense and communicate in the same way we do. However, imagine a creature that lives in the dark, perhaps in a cave system.

One possible evolutionary process might have it communicate by radio waves. Imagine sensory organs all over the skin which produced radio pulses and detected reflections as a means to navigate. I don't think it is hard to imagine an evolutionary process that would allow for this. Perhaps they ingest a type of mineral that naturally produces short metal fibers which were excreted through the skin. OVer time these might pick up a signal causing an electrical potential affecting, for example, sodium density in cells, and from this extremely basic beginning an evolutionary ramp could build a phased array radar of sorts.

Such a creature would have a very different body plan, their sensory objects distributed over their skin rather than concentrated into single organs. Moreover, it is not hard to imagine that they manipulate their signals as a means of communication instead of sound. In a sense this is similar to the telepathy that seems all to common in SciFi.

Because of the speed of radio waves this would not give a very fine sensory detection structure, but finer manipulation could take place with dermally extruded fibers that can detect small movements, much like whiskers, and perhaps a related series of prehensile manipulators. So imagine this creature has no hands at all, but instead is has cylindrical body covered in small worm like protrusions that can be moved and detect touch. Essentially the body has no hands but is covered with thousands of fingers.

Moreover, instead of bipedal locomotion it doesn't walk, it rolls, using the "fingers" to manipulate and control the rolling. With a non skeletal body (see below) this may well better adapt them to movement in the rocky unreliable surface of a cave.

Much as an octopus's tentacles are controlled independently from the brain, each finger might have its own neural system, and the brain power would be distributed throughout the body rather than centrally.

Moreover, since it rolls and is small, squat and cylindrical, it is less clear that it would need a skeletal system. Without one it could squeeze through small spaces very effectively, which might be useful given our premise that it lives underground.

In fact the system might have various muscular systems that allow it to reshape its body. For example, it might be able to extrude out a longer appendage out of its body by filling a cavity with "blood" (for example, like a penis), and because the surface is coated with fingers, give a very useful type of "arm", and there is no reason that it could have six or more arms pointed in different directions, not so much as preformed appendages, but as a natural ability to extrude parts of its body.

Anyway, just a few thoughts. I think there are a lot of unjustified assumptions in the original question.

Answer 27

My ideas that make these aliens most unlike humans based on them needing to space travel. This requires at least propulsion and rocketry tech (if we assume they can do calculations in their head like those black women in that movie Hidden Figures).

So for chemical propulsion: depending on their worlds conditions like ambient temperature, hydrogen as rocket fuel requires cooling thru helium to keep it from escaping vehicles. So at a colder world, these parameters can affect what fuels they use. However I don’t believe it will affect other things like the energy produced by burning fuel might still require to have steel and other materials to contain it and to direct the engines. I don’t have in depth knowledge but my answer is still better than others on here

Answer 28

The key to intelligence is a large nervous system. Brains however, are hungry organs. Life would never simply increase caloric needs, it needs to sacrifice elsewhere. Consider humans: our colons are incredibly small compared to apes, and we also have the capability to throw objects with lethal force. The largest growth in the human brain happened when we got tools- we could access brains and bone marrow, increasing our caloric budget.

We can then infer, a creature needs to be efficient, and have a high caloric budget to be intelligent. Tool use is obviously recommended, and as many other answers have pointed out, lots of creatures can grab things. Since space faring is required, omnivorous elephants are likely not an option (Unfortunate) because of weight. On the other end, rats, mice, and most rodent sized creatures are simply to small for a brain.

What about aquatic creatures? The problem here, is NOT fire. (Lots of questions on this site answer this already.) The problem is, if you are the right size for spacefaring, you are a great size for a large predator's snack, and no reliable ranged options, humanity's main survival strategy. You are likely restricted to aquatic burrowing creatures, (Sand Octopus?) or eusocial creatures.

Natural ways to break bone are not recommended. Why would you need tools or a large brain anyway? Don't dismiss it entirely though- Hyenas are highly successful predators.

How about birds? Argentavis magnificens was a large vulture. Since it is extinct, we don't know it's brain size. A vulture with a more parrot like beak would be a great example of a flying, tool using, potentially intelligent species. I am curious as to if they would still develop flight technology, seeing as it is a bit redundant, but so are cars. Rockets are all that matter in this case, and the only flaw I see is that your eyes might be too close to your grabber. (Unless you also have talons capable of tool manipulation, which you should have, just not required.) This brings the idea of giant birds wielding bird guns (shaped for use by bird) and shooting down large predators and prey. (Guns may or may not be required to understand how rockets work. The original 'rockets' were fireworks.) Really, I see no issues besides the fragile bones. They probably can handle the g forces. (Source: falcon diving, not dead.)

So, how do we make a quadrupedal creature work? An elephant's primary defense is it's large size. Deer and similar creatures use speed. We need spacefaring though! My solution? Take the elephant, make it omnivorous. Then you add extreme dimorphism. Females are essentially normal elephants, perhaps with more throwing capabilities. Males are smaller, they don't grow further from normal baby elephant size, but instead are adapted for speed, they travel in packs to hunt. They use strategy to hunt, wielding spears, or perhaps sabers (elephants seem better at swinging. Then again, it needs to be throwable) the males carry meat to the females who stay and protect the young. This is just one potential example, and eventually, these hunter elephants might be able to travel in space.

One last possibility: Kangaroos. With slight improvements to hands, and a larger brain protected from motion, and arms designed for throwing, it is possible to have a tripedal space faring race. They may even be more effective in navigating zero g environments.

To summarize: No, humans might encounter space vultures, space elephants, space kangaroos and possibly eusocial space octopi. (Those are terrifying!) They don't have to look like their earth counterparts, and probably shouldn't.

Answer 29

Recipe for tool usage: https://en.wikipedia.org/wiki/Tool_use_by_non-humans

• flexible appendages: appear mostly in dense, environments providing lots of hiding spaces - coral reefs, forests, etc. open spaces prey-hunter dynamics reward aero/fluid-dynamics and calories not going to brains, unless you can outsize the predators (ala whale/elephants/hippos- but then no pressure to use tools)

• fire: a runaway reaction, that supercharges meals, disinfects/preserves food/denaturates poisons/bacteria, helps to prepare tools and allows for defense against predators

• speech: Allows you to make plans, to communicate structure and school the young, transfer experience and for that to develop, you need to be pressured to the extreme, you need to be predated (aka smaller). Predation super-charges social selection, it allows for the social to sacrifice ("mobbing") aka feed the anti-social to the natural predators and outdo them in any situation.

It sabotages blunt force hierarchical power structures - allowing for a redistribution of resources, it allows for contract-cults (religion) to develop, creating social complexity.

Social complexity is the first "industrial" society, it allows for "trades" to exist (even if its just arrow or spear making) and larger tribes to stick together (conflicts can be resolved verbally) and guardian shifts to be deployed around the clock.

If the trinity comes together, it accelerates one another, sweeping outwards in ever more successful combinations. First hunters & gatherers, then nomads, then farmers. If its not there- it gets eventually eaten or stagnates in a pond, happy to be to big.

Speech is the most important one. Without it, one war with all the elder dead, one plague, can undo all knowledge gathered over generations.