I think the fact that we have 4 limbs stems from the fact that evolving 4 legs is the most practical to move around on land as a primitive animal that is first evolving the ability to move around on land. Because standing upright on 2 legs probably needs a more advanced infrastructure.
So there would need to be good reasons for 6 limbed creatures to outcompete 4 limbed creatures on a huge scale, enough for evolution to push for such advanced creatures that they start being able to build advanced societies like us. What could those circumstances be?
Something else to consider is that a being with 6 limbs seems less likely to me to become biped because it requires a good amount of more energy to defy gravity when you are heavier, thus they might be outcompeted by 4 limbed creatures, assuming all other factors are similar. Thus perhaps it is inevitable that they evolve to have 4 limbs.
…continued, since it doesn’t allow me to edit posts after 5 mins.
Then again, evolution is definitely not an infallible force, perhaps it just wont happen so there is no opportunity to outcompete. But the probability of 4 limbed sentient creatures being the most common does seem higher.
What about a neck? Lots of creatures evolved (to keep) a neck, I wonder why that is so practical? Necks are weak spots. They can be used to look around, but if our head’s organs were in our (extended) chest, then we could still use our legs to simply move our entire body. But maybe energy is a factor in that again. Perhaps it is a convenient but perhaps unnecessary remnant of when we were still standing on 4 legs. It is harder to move your entire body like that when you are on 4 legs instead of 2.
You’re providing the answer in the way you ask the question. Land mammals have four limbs because land mammals have four limbs; as long as you restrict the question to land mammals, you’ll get answers that only (or mostly) apply to land mammals.
Spiders have eight legs and tons of eyes. Insects have six legs and mostly two eyes. Centipedes and millipedes are named after their number of legs. They’re limited in size (and intelligence, presumably) because of their respiratory and skeletal systems, not because of their number of legs.
And these are just land non-mammals. Go into the oceans and we see that your entire discussion of legs and eyes makes very little sense even on Earth.
The answers to the questions (and the questions) provide more questions and reading my train of thought might give somebody other ideas.
I considered those but those are all insects. Evolutionary incentives and physical limitations differ a lot on that scale. Land animals never had those adaptions (not counting insects of course). Perhaps the evolution of massive insect-like creatures, large enough to carry complex brains, would be practical on lower gravity worlds.
The idea of water based sentient life is interesting too. But perhaps it is less likely because they are less likely to evolve arms with hands to use tools, which could serve as a catalyst for developing thinking ability. Amphibians seem more likely.
Mammals have four legs because their early synapsid ancestors had four legs. Those early synapsid ancestors had four legs because their early amphibian ancestors had four legs. Those amphibian ancestors had four legs because their lobe-finned fish ancestors had four fins.
So mammals (ancestrally) have four legs, not because four legs are the bestest possible number of legs for a mammal to have, but because some fish back in the Devonian period with four fins decided to thrash from one dried up puddle to another slightly less dried up puddle.
Plenty of mammals have stopped using some of their legs as legs and instead for some other purpose, or have lost some of their legs.
So we see that the mammal body plan is highly dependent on the unique evolutionary history of mammals. Even if complex life evolves on some other planet, it is not going to recapitulate the contingent evolutionary history of life on Earth.
But on the other hand, we can make a few guesses, because the body plans of animals on Earth are a response to the physical environment of Earth, and any planet with life is going to share some of those physical characteristics. Such as gravity. Gravity gives creatures a top and a bottom, and we find that the top and bottom sides of animals on Earth are almost always different. This gives rise to radial symmetry.
Animals frequently move around, this means they move in a direction. If they have sense organs or mouths, the creature needs more information about where they are traveling to. They move to things they are going to eat, which means the mouth is on the front, and they move away from things they excrete, which means the anus is at the back. And so gravity and movement combine to create bilateral symmetry and cephalization. Radial symmetry is much more common in sessile creatures than in motile creatures, and there are plenty of motile species that have a radially symmetric body plan that was secondarily modified into bilateral symmetry. Cephalization just means that sense organs and brains cluster on the front of the animal.
Bilateral symmetry with a mouth and sense organs near the mouth isn’t much to go on, but it’s about as much as we can say about alien life.
I suspected I made a mistake there, but for the purpose of this thread it’s similar because they’re on a similar scale, except for the relatively few relatively large species of insects, and the fact that they are few also shows that how practical being large is for them evolutionarily.
Thanks for all that, it makes sense. Some more of the puzzle pieces. As for the 4 finned fish that we evolved from, maybe that is what life on another planet would also be likely to be evolved from? Because while gravity can be different, I don’t think that gravity affects what is in the water as much (does it do so at all, or is sinking simply determined by the density of the object that is to sink or float?). Therefore, the precursors to sentient life could be similar across the universe, then there is another reason that life there would be similar to ours.
Yes, and it’s easy to remember. “Centipede” rhymes with “Twenty-three” which is a typical number of leg pairs for a centipede. “miLLipede” contains the Roman numeral LL = 100, which is a typical count for millipedal legs.
I’d once again like to ask for some sort of means to know if an entity is sentient (or has consciousness if you prefer) …
The Turing Test need not apply.
The best I’ve yet heard is that once we can adequately describe the sorts of information processing loops (be they Doug Hofstadter’s “strange loops” or otherwise) that are the correlates of consciousness in humans we can then potentially evaluate other entities information processing to see if similar processing structures exist. To the degree that the processing is similarly looped up as ours the entity is likely also experiencing some correlate of what we experience as sentience and consciousness.
I cannot see why the circumstances needed to create that sort of processing are limited by physical form in any way. Any entity capable of processing information with enough complexity and that does so in a manner similar to how we do, especially perhaps if additional levels of “tangled hierarchies” exist, could be sentient.
Four limbed? Sure. A hive? Why not? A sentience/consciousness for different world may be more alien than we can imagine. To believe that other world evolutionary constraints on it would converge on something like us seems arrogant to the extreme.
Now of course we are not at a level that we can reliably know what sort of processing loops are correlates of consciousness even if Hofstadter’s speculations seem reasonable … but that seems to me to be the only rational attack of the problem.
I could see a big resistance to classifying an organism that is only intelligent in concert with millions or billions of others of its kind as intelligent, ditto if it is not cute or anthropomorphic.
I did not consider such technicalities yet. I was simply talking about the kind of creature that evolves to the point where they can build skyscrapers, nuclear power plants and artificial satellites for communication.
This is not a contest of who is the most humble. We’re trying to assess the probability of the shapes of sentient beings. We can do that by looking at which creatures with which properties would be most likely to evolve intelligence. I am also not saying that other types of creatures can’t be sentient, I’m just asking what is most likely to happen.
We really don’t have enough information to go on. Aside from a few very basic concepts touched on above there really isn’t much data to let us conclude anything. We have one example of humans, and lots of examples of other intelligent creatures that don’t resemble us very much.
I responded to this mentality several times now. We can speculate based on what we think their planets would look like and what information we have about life. Can you imagine scientists operating under this kind of mentality? “ok, we don’t know everything so lets not do anything”
We also don’t know how prevalent convergent evolution would be in the universe anyway. Maybe the answer is a lot less different than people think.
Gravity surely has an effect on aquatic creatures. Think of every aquatic animal you know. Does it have a top side and a bottom side? Then gravity affects it. Even if it spends most of its time upside down or standing on its head or with its ass glued to a rock, gravity affects it if only because rocks and sand and mud sink and bubbles rise. Even creatures like earthworms have a top and a bottom.
You have to find very very small creatures before you find ones that don’t have a preferred orientation.
Yeah you are right. A lot of that is probably affected too by the direction of the sunlight, that dictates where the underwater plant life is and thus the fish and their predators. Hence why the strongest part of the shell of a turtle is on top, its predators are more likely to attack from there.
I wonder if life could evolve in a practically gravityless environment, like the core of a comet that was melted somehow, the water of which would have some “construction materials” from meteor impacts.
Are cells inevitable? I guess life has to start small, and anything small, alive and self-contained is going to be essentially analogous to a cell, but can we envisage a scenario in which organisms just get bigger and more complex without becoming multicellular?
What about not being self-contained - is it concievable to have organisms that have no particularly clear boundary or container differentiating them from their environment?
