Waitaminnit – how would that work? Remember, we’re not talking about silicon microchips, here, we’re talking about mechanisms that would have had to evolve naturally.
Multicellularity does seem kind of likely, on the following basis:
-Cell-like units as an early form of life - that’s just another way of saying ‘small individual organisms’ (and there have to be multiple individual, replicating proto-life units in order for selection to strain out the viable ones)
-Asexual reproduction pretty much has to happen before sexual reproduction (otherwise, who is the partner?)
-Size limits on the cell analogues are likely - what works in little systems doesn’t necessarily scale up smoothly
-Sticking together instead of dispersing after fission-style reproduction is a pretty easy step towards multicellularity.
I think multicellularity has arisen several times on this planet, so it’s not freakishly unlikely.
Of course, there might be other ways to do it, but we’re not going to be the only planet with cellular and multicellular life on it, unless we’re the only planet with any kind of life.
There’s a lot more to Dark Angel than Jessica Alba in black spandex. That doesn’t mean the other stuff matters! 
Like the movie Men In Black, the alien is wearing a Penelope suit. Watch Sahara and then try to convince me that Cruz isn’t an alien trying to be a human female.
I like the cut of your jib.
Question - would it be impossible for cephalisation to happen on an up-down axis rather than a front-back one, enabling an alien to retain radial symmetry while concentrating sense organs in particular areas? Like the Qheuens in David Brin’s Uplift saga
I don’t doubt it. I’ve thought for a long time that a major drag on the evolution of intelligence and tool using on Earth has been the fact that vertebrates have only 4 limbs.
Well, with Earth life, the fundamental information processing component is apparently the neuron, which as cells can only be only so small. That puts a limit on how compact a brain of a given complexity can be. If the organism in question grew a neural network out of something smaller than cells, then a brain of a human complexity could be much smaller. How much smaller would a network of protein chains be than a network of cells ?
Single cells can grow to great size; as I recall there’s a type of water living plant that is actually an enormous single cell. It has the same number of cell nuclei as you’d expect in a normal plant, but no internal cell walls. It’s flimsy due to it’s lack of internal structure and can easily die from a puncture letting out it’s internals, with no cell walls for strength or to stop the cytoplasm from escaping.
I’ve heard of that.
That’s pretty much what I meant when I said “what works in little systems doesn’t necessarily scale up smoothly”. Certainly it’s a viable organism, but it’s not going to conquer the universe, being so flimsy.
Redundant modularity is pretty likely to arise wherever individual modularity appears, and it’s quite likely to be better and stronger than just increasing the size of the individual modules.
I still say, exoskeletons & multiple legs.
I think the more interesting question would be the likelihood that they are DNA-based. Myself, I tend towards the theories embodied in the notion of panspermia (and, no, that’s not what it means, you filthy little perv!)
Exoskeletons place a practical limit on an organism’s size. (Might not be a problem on low-gravity planets.)
Multicellular has evolved only once on this planet, and pretty late in earth history at that - the oldest multicellular organisms preserved as fossils are no older than about 600 million years old. It’s actually a bit of a mystery as to why it didn’t happen sooner. There are a few different hypotheses out there to explain why, but not enough data to strongly back any of them right now.
Is that the Caulerpa you’re thinking of? There are other giant algae, but I think this is one of the largest.
We could well be talking about silicone microchips or smeothing even mor eradically different form terresitrail life. This is the preblem, everyoine is working within very narrow fields when we have no reaosn to assume that the universe itslef is that narrow.
Let’s start with an organism that has evolved in a sea of liquid hydrogen and ammonia ten thousand kilometres beneath the atmoshpere of a gas giant. We’ll assume it is carbon based. Explain to me what its metabolism is like and why it couldn’t have evolved a sensory system based on carbon nanotubules?
Or let’s imagine a creature that evolved within a sea of molten lead and mercury. What in its metabolism prohibits it from evolving a system based on silicon micro-conductors?
Like I say, the universe is a big place and life could have evolved anywhere in any way. Even if we restrict ourselves to the broad criteria of needing water and carbon for life to originate such things can and probably do exist within gas giants or the clouds over Mercury. Once life has come to exist in such conditions it can then migrate into the molten seas or the hydrogen oceans themselves just as it crept into the saltwater oceans and deepp underground and into the deserts of Earth. We only need water for life to be generated. Once it has generated it can move prety much whatever it likes and as such our intelligent species could live almost anywhere conceivable. And since it could live almost anywhere concievable it could have almost any concievable sensory system.
But to use a more terresitrial analogue, many orgamsism on Earth have evolved to use magnetite as part of their sensory system. Magnetite itself is a conductor. So why exactly couldn’t an organism have evolved to utilise wires and switches of magnetite as the sensory system rather than relying on cellular circuits?
No, you will still only get a benefit from concentrating the sensors on the top of the animal if the top is more likely to encounte food or predators before the bottom or sides, and that can only happen if the animal is always moving top first. At that stage you then have a front end and a back end even if you shoose to call them a top and a bottom.
Firstly, do we know for sure that it happened once for plants, animals and fungi, or could it have been separate for each kingdom?
But when I said multicellularity had arisen several times, I was thinking of (beside our own lineage), bacterial mats, algal colonies and most notably, slime moulds - in the first two of those examples, there might be little more than aggregation happening, with no differentiation of function and little complexity (but maybe the same could have been said of our own early-multicellular ancestors), but in the case of slime moulds, there’s differentiation of function, delegation of reproductive capacity, formation of organs.
Sure, it’s not the same in each and every case, but that’s the norm for multiple instances of a similar thing (for example, the eye or the wing)
And it’s colonies of single cells rather than individual multicellular organisms (although I think that might not be terribly useful as a distinction), but it’s multicellularity and it’s a potential starting point for multicellular organisms to arise.
All that is required to kickstart the process is a mutation causing individual single cells to stick together after reproduction, rather than dispersing - many such mutations may be discarded as too detrimental to further survival, but it’s a fairly easy thing for evolution to keep trying at until eventually something not entirely unworkable happens to arise.
I’m going to go out on a bit of a limb here and say that probably the only thing that Star Trek got right is the whole “bumpy headed alien” motif.
There’s a reason intelligent life on Earth looks the way it does, aside from the fact that we’re “hairless apes” and that all relates to the fundamental nature of the universe. Extra limbs (and hair) take extra energy, which means more time spent gathering food. Humans evolved bigger brains because our metabolisms were low enough that it was easy for us to gather ample amounts of food to have free time to do things. Brains take a lot of energy, and if you add bigger mass to the body, you increase the amount of energy needed to support the body, putting a bigger brain with that and you’re really ratcheting up the energy requirements.
Despite the huge varities of life the Earth has spat out over the past 4+ billion years, only one species has evolved to a sentient state (namely us), which means that while life might be common in the universe, the odds of it being intelligent and wildly different than us (i.e. bipedal, relatively hairless, warm blooded, etc.) are pretty slim.
Remember, in everything the Universe takes the lame way out. Physics says that FTL, lightsabres, transporters, and anti-gravity are either impossible or so unlikely as to make no odds. By extension, we can assume that groovy telepathic aliens with 6 limbs aren’t going to be showing up either. 
That really is a bit of a limb to be stretching out on. The only reason we have four limbs, bilateral symmetry, etc is because our ancestors did, for a long, long, long way back, before sentience was a twinkle in the eye of evolution. In order for humans to arise as an intelligent species, a great deal of ‘making do’ has happened. So all other issues of selection and suitability aside, if it had happened to be a fish with six fins that flopped up onto the mud, and if everything else had continued as it did this time (which it wouldn’t, but never mind), then we might well be something like centaurs, or bipeds with four arms.
I do think nature nestles into the easy spots - so I do agree that there are probably vaguely humanoid alien species out there (but not so many that are just bumpy-headed or pointy-eared humans), but we just don’t know how many different ways there are to do this.
Again, I doubt it because of the resource demands created by such configurations. The dinosaurs had several hundred million years to become sentient, and none of them did.
We’ve got an excellent sample with the billions of years life has been around on Earth. Lots of species have never managed to evolve unique ways of doing things. Even totally unrelated species on opposite sides of the globe which fill similar niches have independently evolved similar mechanisms to accomplish the same things. All life on Earth is carbon based, even though there’s no real reason why at least some of them couldn’t be silicon based.
Erm, is there any carbon in this system? Ammonia is NH3.
Perhaps the lack of silicon?
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That seems almost to be assuming that sentience is some kind of long term goal. Also, there’s nothing inevitable about us becoming sentient - it didn’t have to be this way.
There’s a lot of founder/ancestor effect that I think you’re waving aside - as far as we know, life arose once only here on Earth - but once it had, its presence may have been the reason why another form of life with different chemistry wasn’t able to appear - life tends to mop up interesting chemicals and digest them.
The reason we’re bipeds with arms is the same reason that horses and dogs are quadrupeds - we all came from a common ancestor that had four limbs - it’s like this because of luck, and because it works. Why would there not be other forms that work, but just didn’t happen here?