Fictional animals come in all shapes and sizes, as it should be when the only limiting factor on biodiversity is the imagination of the artist. So why is it that, out of all possible forms, sapient creatures are overwhelmingly depicted as bipedal with an upright stature? What a coincidence that they should all so strongly resemble humans!
Actually, there are some good reasons for this resemblance and valid arguments to support the belief that, intelligent or not, real-life aliens would look, well, alien.
Why Are So Many Fictional Aliens Bipedal?
First, because of constraints. In movies, television, and, to some degree, video games, there are often technical and budgetary limitations. Before CGI, aliens had to be either puppets, animatronics, or actors in makeup, prosthetics, and costumes. Often it was the latter because it’s the cheapest and simplest option. Even after the invention of CGI, this is still the norm. Since actors are bipedal, so too are the aliens they portray. In the case of video games, when the models for humans and aliens are similar enough that the same animations can be applied to both, it saves time and effort.
Second, there’s pesky bias to consider: human bipedalism is structurally unique, and humans like to think they’re hot stuff. Thus, bipedalism becomes part of the visual shorthand for sapience, the truly defining human trait. Making aliens bipedal is one way to make them relatable. But while the human brain might be wired to appreciate human-like shapes, this underestimates our capacity for empathy. We care very much about our pets and often attribute intellectual and emotional depth to them, a phenomenon known as anthropomorphism. The same and more can be afforded to aliens.
Now, I don’t think I need to explain how budgetary restraints and lack of creativity don’t reflect scientific reality. However, there are also strong, scientifically backed reasons for bipedal design. So let’s dive right in to the good stuff: theories of evolution.
How Convergent Evolution Could Create Bipedal Aliens
Bipedal aliens could be the result of convergent evolution – when organisms independently evolve similar traits to adapt to similar environments or niches. Examples include sharks and dolphins, which evolved similar streamlined bodies and prominent dorsal fins. Some would say that a species that is intelligent enough to develop language, technology, culture, and civilization on par with our own would have likely taken a similar evolutionary path.
What is that evolutionary path? Well, early human ancestors evolved the inclination to walk on their hind legs before many changes in their skulls, and therefore brains, ever took place. Let’s look at Australopithecus, a genus related to and preceding Homo, that disappeared around two million years ago. The skull of the Australopithecus is apelike, with a relatively small brain case and a relatively robust jaw. But the skull aside, they share traits with us that indicate bipedality.
- The opening where the spine attaches to the skull is anteriorly positioned, allowing the head to be held aloft.
- The lower spine is S-shaped, which aligns the head and torso well above the center of gravity.
- The pelvis is broad and bowl-shaped, improving stability.
Because these features arrived before changes to the skull, it is possible and even likely that the development of erect posture facilitated later changes in the brain and behavior by consequence.
How would that happen? Let’s take the use of tools as a given for any creature capable of forming a civilization. To make complex tools, one needs the ability to manipulate the environment. Hands are certainly good for that, but bipedalism isn’t required for hands. Many tools can be constructed while sitting down. Recall our close relative the chimpanzee, a renowned tool-maker, who has hands quite similar to our own but only walks upright on occasion.
The only advantage bipedalism brings to the table is the ability to carry tools, which doesn’t necessarily influence the ability to craft tools. Or does it? Consider this: if you can’t conveniently carry your tools around, you have to abandon them and start fresh every time you need that tool again, which doesn’t make much sense unless the tools are relatively simple, like the sticks chimpanzees use for termite fishing. If you can conveniently bring your tools with you, refining them isn’t a waste of your time. Think about how long it would take to chisel a spearhead, and imagine having to repeat the process every time you needed one. Not a very effective survival strategy.
A number of animals that make tools can learn to do so by observing another member of the same species. Tool use and even gestures can spread through social transmission. But observing another using a tool and improving upon the design is uniquely human.
What’s more, the oldest stone tools ever found predate the oldest known fossils of genus Homo by half a million years. Remember the ape-brained Australopithecus? They could have crafted these tools, and their ability to carry them might have made that practical when it simply wasn’t before. Reliance on tools might have led to natural selection for larger brains in humans.
How We Became Bipedal to Begin With
If we want to create well-thought-out bipedal aliens, we should consider how they became bipedal, like us. Convergent evolution between two organisms requires a similar environment or niche. The niche of humans can be hard to define, because we have used technology and culture to construct and expand upon our natural niches. The same can be assumed for any intelligent life. So, let’s forget niche and focus on environment.
The transition from quadrupedal to bipedal movement occurred around the same time as certain changes in the African savannah: the forests were thinning, creating vast grasslands. Our arboreal ancestors, adjusted to living in the canopies, had to adapt, driving our quadrupedal ancestors to split into different species.
Ancestors of orangutans would branch off first, becoming extremely specialized for life in the canopies. The ancestors of gorillas and chimps became more versatile. Gorillas and chimps are mostly quadrupedal knuckle-walkers, but they also developed the ability to walk upright on occasion, a locomotive strategy known as facultative bipedalism.
What early humans would eventually do, by contrast, was take advantage of the newly opened grassland niches their relatives had yet to exploit. This is where bipedalism comes in handy, according to the savannah hypothesis. It is more efficient for traversing the flat expanse, and it elevates the head for increased scope of vision.
It’s important to point out that the savannah hypothesis has come under scrutiny in recent years. Bipedalism might have begun in the trees before moving out onto the open plains. The vertical climbing and knuckle-walking of gorillas and chimps uses similar muscles to human bipedalism. The common ancestor of all surviving great apes may have used a sort of hand-assisted bipedalism by grasping overhead branches while climbing.
Regardless, it is fair to say that the advent of human bipedalism was a unique occurrence that happened under extremely specific circumstances. The specific environment had to change when it did. Timing is just as important as place. Without certain pre-adaptations unique to apes existing at the same time as the shift in environment, bipedalism might not have been possible at that time.
While it’s possible for all these factors to align somewhere else, requiring all your alien civilizations to have arboreal origins seems pretty limiting. Let’s explore some alternatives, shall we?
Not All Bipeds Are Humanoid
There are certainly other bipedal animals on Earth we can look to for examples. Aside from leg number, these bipeds share little in common with humans, varying wildly in appearance and lifestyles.
Let’s look at the kangaroo and velociraptor. These are two very different beasts, both bipedal, though the similarities don’t quite end there. Note that both their torsos lean forward and must be counterbalanced by a tail. Contrast this with the tailless human, standing fully upright. These differences have functional consequences, changing the act of walking.
Whereas humans balance smooth strides with alternating rotation of torso and pelvis, the kangaroo hops. Its legs are locked together in parallel motion, unable to move independently, and for this reason it can’t even walk backwards. Despite this, kangaroo hopping is among the most energy-efficient forms of locomotion in the animal kingdom. When there’s no rush, kangaroos will forego hopping in favor of crawling, using their forelimbs and tail in addition to hindlimbs, making for a technically pentapedal gait, though this actually consumes more energy than hopping does.
Velociraptors might have lurched forward or waddled. Unlike the kangaroo, their tails were inflexible and would have been capable of limited side-to-side movement at most. Still, the tail is speculated to have provided stability during speed bursts and sharp turns.
Another alternative body plan that includes bipedalism comes in the form of our close relatives the chimpanzees. They are primarily knuckle-walkers, but they are capable of walking bipedally temporarily. Walking this way appears uncomfortable and just plain awkward for them. They must shift their weight side to side, bowlegged, but they will do so to carry food for short distances. Surprisingly, there appears to be little difference in energetic cost between two- and four-legged movement for them.
While these creatures are different enough from us for their civilization to prove challenging to imagine, there will also be familiar touchstones that will ease the way. Maybe, like us, your creatures put their pants on one leg at a time, but an additional sleeve is required to accommodate their tails. Would a hopping creature use stairs like we would, or would they require a different design? It’s enticing questions like these that make worldbuilding so much fun.
Hands and Carrying Don’t Require Bipedalism
So we’ve established that freeing up the hands gives humans an advantage that any sapient creature would need if they were to get to the point of forming a civilization. Let’s consider ways to achieve that without bipedalism.
Perhaps your alien species has a different strategy for transporting items, maybe in a pouch like how a marsupial carries offspring. Additionally, many rodents and some primates make use of cheek pouches for transporting food, which is another strategy that does not require free hands.
There are also many potential prehensile appendages besides hands. Some animals have prehensile tails, but tails being posteriorly positioned makes them unlikely to be as good for manipulation, as they are out of the field of vision. A muscular hydrostat is a possible option; examples of these used for grasping include octopus tentacles and elephant trunks. Another type of muscular hydrostat is the tongue, but it would require a lot of modification to be useful in carrying or manipulation, and it would need to be dry. The penises of some animals, such as elephants and tapirs, are also prehensile.*
Muscular hydrostats are less firm and more flexible than hands are, which could be a downside or an advantage, depending on the situation. Keep in mind that pre-existing structures can be, and often are, modified to suit new purposes. If an animal starts carrying certain objects with their penis and it improves their success, their penises might become even better at grasping over time.* Think about it—a trunk is a modified nose. You could probably make any structure prehensile, if you tried hard enough.
Anyway, you might notice that any lone appendage is not going to be particularly dexterous. Carrying doesn’t require fine motor control, but crafting does. The solution to this is to try combining multiple grasping appendages on the same organism. For example, corvids might be bipeds, but they do not gain the “freeing of the hands” advantage because of their wings. They have been observed making and using tools, however. They might use their beaks in combination with their feet to make tools, like grasping a piece of wire in one foot while bending it with the beak into a hook shape. While the beak and the feet are not particularly dexterous in isolation, they work well together.
Try combining any of the appendages that have been mentioned above. Your species could also be quadrupedal with grasping hands, like a chimpanzee, but with an additional appendage of some kind, like a trunk or a tail, to aid in carrying while the hands are occupied in locomotion.
While there is so much variety just on Earth, you don’t have to limit yourself to using its creatures for inspiration. For example, you could have an animal with six or more limbs who uses at least four of the hindlimbs for locomotion while reserving those in front for grasping. Think of something like a centaur. While not strictly scientific, psychic abilities could also be used to manipulate the environment and carry objects.
A species identical to humans except for a pair of pointy ears is lazy, but so too is a species with wildly alien anatomy that still lives and acts exactly as we do. It all starts with the small differences and builds up from there. Everything is connected; nothing is arbitrary. If we still had feet for climbing, would we shoe them? Would we clasp toes with our lovers? How would we dance, if at all? A different body has different capabilities and limitations, resulting in different psychology, culture, technology, and society.
This article is an abridged version of Questioning the Bipedal Default from Volume 2, Issue 5 of Worldbuilding Magazine. Read the full article.
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Why do you not have any love for Orang Utans? They’re better toolmakers and users than chimps, as it were. They’re truly treepeople.
Apart from that, a very interesting read!
Awesome Article! It masterfully describes why humanoid aliens are unrealistic, while also giving worldbuilders ideas for how to design them anyway but logically. And the section on other types of bipedalism is awesome! Almost no one ever talks about aliens that hop or strut!
I love articles like this, diving into the science of worldbuilding and whatnot is fascinating. Can’t say I’d ever heard, ah, *that* about tapirs and elephants before.
“but tails being posteriorly positioned makes them unlikely to be as good for manipulation, as they are out of the field of vision.”
Unless the tail is long and flexible enough to curl around to the front.
“a trunk is a modified nose.”
Actually it’s partially the nose and partially the upper lip.
The big downside of the prehensile penis (assuming you have a body arrangement where you can see what it’s doing) is that it would be the equivalent of half the human population having no hands. That would make it pretty hard to get a civilization started.
As reluctant as I am to get into this, it could be that females develop a pseudopenis similar to hyenas.
Very good article. It’s a question I was wondering about, because I’m planning on writing about a universe that includes aliens. About ways to get fine manipulation without hands:
How much proof is there that anything besides human hands could be capable of producing technology? Of course, we don’t have any creatures making tools as complex as human tools without hands here, but there could be several different causes for that other than physical impossibility. But is there a way to prove that any given appendage is capable of making tools like that? I wonder whether robots or computer simulations of appendages could help.
Could intelligent tool-users evolve underwater?
>Fires won’t burn underwater, and heat generation is vital for technology as humans understand it.
-Five Underused Settings in Spec Fic
Another common question. It seems that most people I have encountered who thought about that question would say “no,” but perhaps they have a limited view of technology. I believe I have heard of a few suggestions about electric-based technology underwater, using electricity-generating organisms as a power source. Underwater hydrothermal vents can provide a source of heat, although only a few specialized underwater creatures can even approach them. I don’t know how plausible these ideas are, however… There’s also the fact that underwater creatures don’t tend to have hands or anything similar – but then again, there are octopuses.
The thing is that those thermal vents aren’t nearly hot enough to do what we want with them for a huge technological step – namely, smelt ores into metal. Not to mention that they’ll be a lot harder to use safely than a fire, even if that’s surmountable.
Perhpas. I’ll have to check out the melting points of various metals and the temperature of hydrothermal vents. As for safety, as I mentioned before, there are a few creatures even on Earth adapted to be able to withstand the conditions, so there might be creatures like that on alien worlds as well. There is also the electricity thing I mentioned. I think I found the idea somewhere in the book “Xenology” by Robert Freitas (available online); there might be other people with similar thoughts, but I don’t remember who it is specifically. Finally, is there a technological path that just doesn’t use metal (at least at first)?
Maybe it doesn’t have to be thermal vents. If the species lives on a planet with lots of volcanoes and earthquakes, the lava from volcanos ashore running into the sea could maybe be hot enough to do things to metal. Whole cultures could be built around these lava runoffs coming from live and active volcanos.
Then again, I don’t know anything about metal melting points, either. (Also, should volcanos be spelled “volcanoes”? I have no idea.)
How about bio-tech based on (cringe) breeding.
https://en.wikipedia.org/wiki/Smelting
https://www.whoi.edu/main/topic/hydrothermal-vents
https://www.universetoday.com/27891/temperature-of-lava/
A quick bit of google-fu reveals that hydrothermal vents probably aren’t nearly hot enough. Not to mention that, unless you live much, much deeper than I think any hydrothermal vents exist at on earth, the water would boil and make itself a lot harder to use if it got to those temperatures.
I do like the lava idea, though, kudos to you Bunny. We now run into the issue of keeping the metal molten long enough to cast it, or hot long enough to shape it. I really don’t think that’s possible unless the civilisation manages to be at least a little amphibious.
My favoured idea for how a civilisation could work underwater is akin to what Leon suggests. “All-natural” technology such as bred livestock and anything that doesn’t need too much heat to make – horn, bone, hide, sinew, et cetera. I read a wonderful reddit post a while back about such a civilisation, probably on r/speculativeevolution about such a civilisation of cephalopod-like creatures that eventually managed to create vehicles that could move along atop their ice-covered home world.
Jonny Wilson
That sounds cool, what was their motivation for breaking the ice (strategic; death from above! Because it’s there; just like us. Or did they just have to get away from those annoying sponge people?)?
Larry Niven’s Puppeteers is a good example of a non-biped race that manipulates tools without hands. The Fifthp (Niven and Jerry Pournelle) would be another.
I’ve heard about them. However, I don’t know how much research was actually done – the books are science fiction, after all. Is there any proof that the kinds of appendages involved could actually be dexterous enough to create complex technology?
I think a good comparrison would be octopus tentacles. You can find some amazing videos on youtube.
But I think the most important question is what circumstances drive the need to develop technology.
I just remembered, the Fifthp didn’t develop any technology. Everything the species knows is inscribed on a huge metal cube that they got from another species, and their world view, being prey animals, is entirely genocidal. Which makes them “circumstances” rather than antagonists (like zombies, or a storm, or the aliens in Aliens).
The Puppeteers achieve fine manipulation with their mouths, which have numerous finger-like knobs for lips. I think it would be more accurate to say that Puppeteers combined the functionality of the hand with most of the functions of the head (they keep their brain inside a hump between their two necks.
here’s a few other ideas;
Terrestrial tentacles, could become possible if the tentacles contained a series of air bladders; torus, sphere, torus, sphere, torus, sphere, etc. this would provide rigidity and reduce weight making the body plan feasible.
Squid hooks could evolve into worm like fingers.
A bird could easily hang from wing claws, or stand on it’s wings while using dexterous talons – though I can not imagine why a bird would need to build complex tools.
A large insect with a very complex extendable mouth would be very interesting – it would probably need lungs though.
A star nosed mole, in a climate absent of predators could become larger living above ground, while its nose becomes a cluster of tentacles.
There is a large centaur like species in a story of mine. It has the lower body of a lion, four arms and a dragons head, it’s horrifying and I love it. But I wouldn’t have it as a natural species in a science fiction story. It’s just not efficient or practical; all of those extra limbs need food and water, it’s an extra 60-80 kilos to carry around, and if it’s running or swinging weapons, those limbs are just going to get tangled up, not to mention the kinetic chain problems.
If your going to invent something that we haven’t seen in nature, it’s good to consider why we don’t see it in nature, and why it would be an advantage. If you can’t find a good reason for the former and you have a good reason for the later, then you probably have something good.
Hi. Do you mean that any creature (at least a vertebrate-like one) will not have more than four limbs? I’ve heard that the reason for this may not be due to physical impossibility, but because the aquatic ancestors of modern vertebrates had only four fins. Of course, that hasn’t been conclusively proven, but it’s something to think about.
I just noticed you mentioned four “arms” specifically. While I suppose there might be reasons for four arms being impractical (then again, who knows?) do you think, say, that an alien with four legs and two arms (perhaps somewhat like a centaur) might be plausible?
I really would like to think so, because it would be freaking awesome. But with a centaur like shape the main question mark is “whats the design brief?”
Obviously nature doesn’t design, but a critter has to be either really good at something or pretty good at a lot of things. I can’t imagine arms optimized for grasping and swinging from branches evolving on the same body as legs optimized for running very fast in a straight line. The arms couldn’t possibly hold that body above the ground, and you can’t run fast in a straight line through the bush.
There’s one critter that I could imagine benefiting from some extra limbs. The river otter.
A small six or eight legged otter like creature might retain all of its limbs because the extra calorie cost wouldn’t be that great, it wouldn’t have the same kinetic chain issues that a more upright creature would have, you have redundancy, and I imagine it would also be more mobile in the water.
Many small limbs would also make it an absolute menace in small tunnels where it cold scurry at ridiculous speeds on nimble little legs. Real life otters have pretty good hands, so add that in and you’ve got something pretty cool.
It would probably be pretty handy in thick tree tops too, but I still think primates with longer limbs would have the advantage there.
I think if the otter-like creature did become primate-like, it would be very useful for it to retain small arms for holding offspring, but I think those arms would have to remain small, so they don’t hinder movement (through kinetic chain issues or simply being too heavy).
My next book is having four legged otter people :D
Technically, all dragons except for wyrms (no front legs, only back ones) have six limbs (wings are limbs, too, if they include bones, as dragon wings do). A pegasus is six-limbed as well, for the same reason. Sleipnir from Norse Mythology is even eight-legged.
Any being with two sets of legs would be more stable than humans are by nature (and could balance out an extra pair of hands as well). So a centaur-like being with four legs and an upper torso with arms would be an interesting and physically stable design.
Beings descended from insects or arachnids would also be interesting, because they’d have more limbs, but perhaps no outright hands. Their biology would have to differ from that of earth’s insects or arachnids, though, because in our modern atmosphere, they couldn’t grow big enough.
I love dragons.
I think that they would have to evolve on a very small planet with a very thick oxygen rich atmosphere, but the wings would still have to be a lot bigger.
I think the body plan is feasible. If at some point it was a reptilian version of the centaur-river-otter-people it might take to slinking around rocky mountain terrain and pouncing on small rabbit like critters, or sheep.
The four legged dragons body plan fits the double torso requirement for six legs. The wings could evolve from the middle arms in the same way that a dinosaurs flappy arms became bird wings; the ones with bigger flappy-er arms move faster, and warm up faster in the morning… evolution.
I don’t think a pegasus would ever be possible though. Hooves and wings are very specialised features, that require vastly different body plans to make use of effectively.
Nature is very good at getting rid of features that aren’t pulling their weight (literally), just consider flightless birds, legless snakes, mindless hedgehogs, sea mammals don’t have grasping limbs – if you don’t use it you don’t get to keep it.
If you’re the fastest thing on land you don’t need wings, and if you can fly, you don’t want to be carrying around a horses body.
I love the idea of intelligent insects too.
You could get some really interesting creatures evolving (there’s a short webcomic called Our Intrepid Crew that takes place on a space station run by a giant shrimp – I think of shrimp as sea-roaches).
I think the telescopic jaws of juvenile dragon flies would be :D very handy :D
Big bugs definitely need something like lungs though, unless they could come up with some kind of bellow system to turbo charge their spiracles, like a monitor lizard (I think this is what the snorkels on Giger’s aliens are meant to be).
I think the big question mark with big bugs is “why does it need to be smart?” most insects are perfectly adapted to their niche. But this may be entirely due to short life spans, because of size restrictions, which may not apply on other worlds.
It would be interesting to consider how an ecosystem would have to evolve to sustain extremely high levels of oxygen.
If you like sapient insects, consider the possibilities of a hive mind in connection with telepathy. There’s more than one form of intelligence.
Would they be “people” though?
They would have nothing resembling social skills. In Enders Game, the bugs just casually dismantled the first humans they meet, out of curiosity, and didn’t realize that they were doing any thing wrong. I think its the same deal with the aliens in Ann Leckie’s books too.
Do you know of any theories of how telepathy could naturally evolve? I like the idea, but I just can’t come up with a mechanism that I’m happy with.
(shudders)… how would potential queens deal with each other?
The politics of juvenile queens in an intelligent hive mind could make for a interesting story. It would be very weird though, especially when you consider the way bees reproduce.
A hive mind doesn’t prohibit personality … it merely means that information is exchanged between all members of a hive and that the security and prosperity of the hive rank higher than those of the individual. While I admit that isn’t how humans work, that still is an interesting premise. Telepathy doesn’t have to be absolute, either. It could be limited either by range or by depth – the surface thoughts could be shared, which would allow for the hive to react quickly to emergencies and keep everyone working where they should, but the deeper thoughts would be individual.
Think of sentient insects more like insect versions of the naked mole-rat, the only hive-building mammal. You have a small number of females who are fertile (the queens), a certain number of males, and a large number of female (or even gender-neutral) workers, soldiers, etc. There could either be a ranking among the queens or they could form a committee or a council. You don’t need to work the sentient hive 1:1 from a bee or ant hive.
Those are all quiet different things.
If it’s one mind with many bodies, then there would be no social interaction within the hive. Interaction with other hives would be about nothing but mating, and murder – I don’t think there would be any trade or war because the whole point of a hive is that it meets all of your needs; of course hives would fight each other for resources but this would be every hive for herself because with no trade, there is no reason for a third hive to get involved if two of her competitors are destroying each other.
If it’s people with telepathy then I recon they would act just like modern humans, only instead of looking at phones, they would just stop and stare into space occasionally.
Now a cluster-mind could be very very interesting, especially if you got two different species (say as different as humans and Neanderthals) with different MO’s re-producing together.
Imagine being in a cluster mind with a queen who can reach into your mind, and produces inbred soldiers and workers (like naked mole rats). But most of your genes say “nope” because you want to have your own private thoughts, dream of stating your own colony one day, and are not at all interested in rolling around with your siblings (like the clones in The Forever War (I’m not sure what naked mole rats do for fun)).
Can you still call a story with telepathy science fiction, or has science put telepathy firmly in fantasy category.
If it was possible to genetically engineer a ‘wetware internet (WetWareWeb)’ form of telepathy into humans, along with elf like life spans, space colonies could get very weird in a similar way to naked mole rat colonies.
In the interest of population control you might want to produce sterile short lived workers, queens could use social pressure to control who is educated and who gets tv. in their brain all day. Workers would be sub sub human (in fact, I think that would be about as depressing to read as Low, but with no hope of any of the characters becoming like able again).
The emergence of individual intelligence in a hive mind could produce some drama.
There’s probably some good star trek fan fiction about characters like Odo, emerging from the changeling pond and struggling to establish their own personality :D
While were on non-human telepaths. I had an idea to make a creature like venom plausible.
In it’s earliest origins it could a self like cluster of organisms. Then it’s component organisms could develop telepathy so it could move with purpose. Then it cold become stronger and tougher, becoming a predator while also developing poisons and defensive chemicals. Then it could get lazy and start to use chemicals to lure prey. Then it could get smart, and realize than if it gave some of it’s prey really good chemicals then they would come back and bring their friends. And then it could realize that it could attach itself to these critters and give them extra strength, while they increase its range, and share their prey, a mutually beneficial symbiosis.
I think this is a fun idea, because you could imagine some cave kid, trying to persuade their friend that the shiny black ooze is fun to play with, and the awkward questions once the ooze starts to get inside their brains and learn their secrets.
You could take it in some extremely creepy or x-rated directions too.
Can you think of any other critters that might want extra limbs? I haven’t slept in 26 hours and by brain is refusing to cooperate.
I think one suggestion for, say, six-limbed vertebrate-like aliens developing was given in the “Xenology” book I mentioned. (You can read it! It’s available online for free). The idea was that vertebrates here have only four limbs because their fish-like ancestor had four fins. Now, four fins does appear to give an advantage over more fins for fast swimming – but not all aquatic creatures need to be fast swimmers. The idea was that, say, in a world with lots of shallow seas, creatures could develop that could have more than four fins and move on to land with that many limbs. It does seem unlikely, for various reasons, that creatures will evolve extra limbs from what they already have (the trend seems to often be a decreasing number of limbs), but if they started out with that many… The question is whether more than four limbs is advantageous; we have to consider the demands it will place on the nervous system, energy usage, and so on. It’s hard to tell whether the advantages outweigh the disadvantages for large creatures, because we don’t have any examples of such creatures on Earth and they are unlikely to evolve from four-limbed creatures here for the reasons given earlier.
Thanks for the suggestion, I will read that.