Why do we assume aliens will be at all like us?

[stutefish]

The various threads about whether this or that environment can support life, etc. got me thinking...

Why do we assume aliens will be at all like us?

I don't mean in the low-budget sci-fi "actor in a prosthetic forhead" kind of way.

I mean realistically.

There often seems to be an assumption that life will be more-or-less earthlike. The morphology may be different, but the overall chemistry will be the same. Or the chemistry will be different, but the overall energy budget will be the same. Or the overall energy budget will be different, but the general mentation processes will be the same. Or the mentation processes will be different--but not too different--but the lifespan will be more or less the same (+/- a couple hundred earth-years).

It seems to me that given the scope of the universe, it's just as likely that alien life will be totally inscrutable to us, and we to it, as that it will be anything we could possibly hope to communicate with.

I dunno. A low-energy environment might have some kind of living ooze, that migrates across the surface of a frozen moon at a glacial pace, and thinks even slower, using interactions between itself and the radiation from the gas giant it orbits to generate its thought patters. Would we even recognize such a thing as life? Would it even recognize us?

Or what about a sentient gas cloud, spanning hundreds of light-years deep in the searing radiation hell of a galactic core, with a lifespan measured in aeons. Would it even notice us? Or we it?

For all we know, the galaxy could have plenty of life, but all orthogonal to us, not just in time and space, but in terms of its very nature.

What do you think?

[NEOWatcher]

There often seems to be an assumption that life will be more-or-less earthlike.
...
What do you think?

I'm not sure if that's really the assumption even if it is the most commonly discussed.
It's just a matter of not having anything to go on. How can you discuss the possibilities without a common framework?
I'm not saying it can't be done, or isn't done, only that the natural tendency is to revert or compare to something that can be related...Us.

[Pickled Tink]

Registered just to post in this thread. Very bored Australian up well past his bedtime.

As for why we assume aliens would be like us or familiar to us, well, probably because we have been educated by the science fiction we have produced regarding the subject, and most of that came before the days of computer animation where we really could create anything we could imagine. Then we had to make do with what was around. We had basic animatronics (You can see some of that in the original Star Wars films, non special editions of course) but that looked pretty fake. It didnt move right. Or, you could use people in suits. Naturally, its pretty hard for a person in a suit to use that third arm, plus theres that pesky budget thing that needed to be dealt with. Thus you got the rubber forehead aliens we know and love (Or loathe, as the case amy be) alongside the dog with the cute little antennae and the "futuristic" ring around its tail.

Anyway, despite this being the case, and the fact that any life out there will have definately evolved to fit the environment in which it finds itself, you will end up with similarities. In similiar environments you will find that life will evolve similiar means of dealing with the challenges it faces. On the Earth alone the eye has evolved several times independantly (Compare human eye to octopus eye for an example of this). Similiarly, both the fish that stayed in the sea, and the mammals that crawled back in (Probably coundn't find a decent pub and said "Sod this" and crawled back in ) have evolved flippers for aquatic propulsion. Both bats and bird evolved wings. So on and so forth. You will get similiar means to overcome similiar problems.

The differences with life on other worlds, at least in terms of complex multicelluar organisms, will be brought about through the differences in their biospheres. For example, life evolving in a chlorine rich environment will have to have means of dealing with the nasty highly reactive stuff. This may result in considerable morphological differences, but then again, it may not.

The thing is, that Earth, which is a known habitable world (Duh) has a rather wide variety of different conditions and such, so much so that we can reasonably expect to see a goodly portion of different survival structures to deal with those conditions. The only things we really cant predict with any degree of accuracy would be the nature of the other animals that want to have the little fellow we are talking about for lunch.

As for intelligent life, im going to have to say this is outright speculation, but you would need at least one means of manipulating the environment. We have these wonderful things on the ends of our arms called hands that do the job, and they are a pretty good solution to the problem of environmental manipulation (Insert global warming/climate change joke of choice here). Other life would have to evolve a means to do it. Really, what a species will do is select for a sufficiently energy efficient means of doing so in its environment. For a limbed creature, the best efficiency would be through two arms and two legs which would provide both stability and decent ability to manipulate the surroundings at the same time, which would work better than anything i can think of as a land dwelling creature, but this might simply point to a lack of personal creativity, though if this is the case ill be mildly offended (How dare i suggest such a thing ). Underwater it would be a whole different ballgame with the medium being far more supportive (Unless you go too deep, then squish) As such you could use a whole different set of manipulators. Fleshy tentacles, little pseudopods, or perhaps even good old fashioned suction (How these would be employed is an exercise bet left to the imagination, you dirty dirty people).

A low energy environment would be, in my opinion, unlikely to get past simply single celled organisms, or perhaps moulds. Larger organisms require more energy to replicate and so would not be evolutionarily favourable. Also, the fact is that brains, which are (At least as far as we know) the only way to get any kind of intelligent thought up and running, are hugely energy intensive, so much so that there are some species on the Earth that have lost them through evolution. A quick trip to wikipedia gives a figure that even if incorrect demonstrates this point. Thinking is energy intesnive, so you arent really likely to get a brain in an energy poor environment. They would be rather strongly selected against in favour of microbes devoting all thier puny energy reserves to the task of making more microbes.

Likewise, i also think your sentient gascloud example is a bit far in the realms of fantasy. Such things may well exist in D&D (And if they dont, they should!), but a cloud of dust would, as we see in all those lovely nebulae, be simply too vulnerable to stellar radiation. Also, the interactions between the particles of which it is composed would be purely gravitational. You are more likely to get life out of a lump of sand, at least in the case of thesand its interacting on more than just basic gravitational attraction. Theres also more than just hydrogen and helium, with traces of other heavier elements in there, which does kinda help. Examining life here on Earth (which is the only standard we have for this so im bloody well going to use it) shows that we need a good deal more than just slightly dirty hydrogen/helium. Carbon, Phosphorous, Nitrogen, and Oxygen are all needed as well, at the elast, at least for lifeso far as we can determine it. Somehow i dont think Phosphorous will be found out there in any great abundance. Life is, at its most fundamental level, a series of chemical reactions, and you just dont get that between dust grains drifting at seeming random without touchings (Which is a rather big requirement for a reaction to take place. I may not have paid a great deal of attention in class, but that was something that was drilled into me)

Hmm... I appear to have rambled. It is never a good sign when you finish a paragraph and wonder what on Earth it is you were talking about. *reads back over post and opening post*. Hmm... not as bad as i had thought. very well, onwards!

Anyway, with regards to intelligent life, you would need a few things. Ive already touched on manipulating your environment (In a meaningful way, a little beyond "I bite the walrus!"), you would also require a means of mobility, a means of communication, and a means of thinking. There would also be all the means you need to perceive your environment.

We know that evolution has, at least here, solved many of these problems several times in pretty much the same way. There really is no reason to suppose that wont hold true elsewhere (Unless the environment doesnt favour such things. Two legs may work well here, but different environments where footing is less sure would result in more legs or ever a whole diffewrent means of locomotion, and if you look at the number of varieties of that we have here now then its easy to see how any organism using any of these methods might seem familiar. At least to anyone who has watched your local nature channel for any decent length of time.

Actually, that entire previous point can pretty much sum up the similarity thing entirely. If i go any further i risk repeating myself (Even more than i already have)

Hopefully you will be able to pick something of meaning from this sleep addled ramble. I also apologise for my lack of spell checking and whatnot, but i typed this on the wrong computer and i am too lazy to fix this problem.

To sum up: Nature has shown it solves similiar problems with similiar solutions, simply because they are, at least so far as i have been able to tell, the most efficient solutions for those problems in those conditions. Theres no reason that this wont hold true across other worlds. That said, given the real unknown is the alien ecosystem itself, and the undeniably huge effect such an ecosystem has on its inhabitants creatures will, even if they share many similarities with us and life on our world, will be different to varying degrees. That said, i would not be suprised if we encountered humanoid intelligent aliens (Besides the obvious "How the hells did you actually get here?" suprise) simply because humanoid appears, at least to me, to be a very efficient shape for intelligence, having all those really useful things needed to keep a brain fed with food, water and information.

Life out there may well be weird and wonderful, but im willing to bet money (Though not much because i dont have much. Stupid lousy economy ) that life out there will share a number of features with us. The more similiar the environment, the more things we will probably share.

Anyway, ive got about 10 minutes before the daystar reappears, and must flee its approach.

[Moose]

Why do we assume aliens will be at all like us?

I don't think that especially true within the scientific community.

But in order to posit an alternate biochemistry in the absence of an observation, one must be able to demonstrate that such a biochemistry could function. My understanding is there's some thought being put into the possibility of silicone-based biochemistry, but as far as I know, there hasn't been anything really compelling (not that I've been paying close attention, etc.)

In terms of xenobiological or SETI type research, one frequently "assumes" life will be like us (or will be using similar communications techniques) because that's the higher probability guess. We know of only one species, and that's how we roll. Ultimately, it's all we really know how to search for.

[Pickled Tink]

I doubt silicone based biochemistry would be particularly effective. The need for Si-O-Si-etc bonds in chains (Because Si-Si bonds react spontaneously with Oxygen), particularly in the Amino Acid analogues, would make forming complex chemicals difficult, and i really don't see how it could even remotely compete with carbon, which is so far as i have been able to tell a good deal more abundant in the universe (Though i will admit that this is simply based on personal guesstimates based on what i happen to know of how stars work). I do not see how larger more unwieldy molecules would beat out the smaller, more efficient ones in an environment where both exist.

Not ruling it out mind you, I'm just saying that perhaps we shouldn't hold our breath.

With regard to thinking like us, creatures evolved to suit their environment will, naturally, go about investigating the environments they find themselves in with what sense they have at their disposal, but they would likely go about it in a similar way, at least with the evaluation of the problem. It hurts thus stop touching it will likely be true regardless of the species or planet of origin involved simply because creatures that don't stop touching the bringer of pain tend not to reproduce, as they end up dead, or spend a ridiculous amount of resources healing their wounds.

Likewise, creatures that cant determine the trajectories of other objects, these other objects being other organisms most likely early on, but could also be falling nuts from the nut trees of Zamboojahadeen, or falling rocks, or any other object that has sufficient mass to squash a small lizard and a tendency to become airborne or even merely mobile when provoked. Another point of commonality between us and what they would be. Being able to tell where something will be before it actually is there is simple mathematics, and is useful for predators and prey, and the creatures they tread on.

Then, at least for intelligent creatures, you would likely have some form of group dynamics coming about, which would naturally follow the same base trends as we have. For instance, a population where the individuals in it keep killing each other is unlikely to be stable, unless there is a high birth and maturation rate.

Richard Dawkins covered all this better than I could in his books, and while it relates mainly as to how morality could have been evolutionarily favourable to creature and thus the lack of a need for a god to explain it, this can be just as easily turned to demonstrate means by which intelligent aliens would have evolved those same, or at least similar concepts on their own. He does this in his book, The God Delusion on page 214 onwards. (Which is the only one i have to hand at the moment, and he covers it again in a few other books)

In essence, we can count on intellectual processes, or at least the end results of those processes, to be pretty similar, rather than unfathomably different.

So yes, its a safe bet that any Aliens out there listening would be thinking, at least at the fundamental levels decided by evolution, in similar manners. You would have to make allowances for differences in culture and differences in their senses and the importance of each sense. For us, i believe the important one is sight, but another smoggy or foggy world where line of sight is drastically reduced could be ruled by hearing oriented creatures, or scent oriented ones, or those that detect electric or magnetic fields.

This was something i wanted to touch on with my previous post, but i was too tired and the paragraph i had written on it seemed to lose coherence.

[raptorthang]

Life needs specific variables to begin. We don't know which ones and perhaps there are variations but no evidence of that on Earth. In our 4 billion years or so, no 'alternate' life forces are known to exist or have existed.

Also, life evolves to fill niches. Survival of the fittest, etc....the same niches are filled, refilled ,etc. but these are not,as you put it, 'inscrutable' to us. They are all logical and all follow the properties of biochemistry as we know it.

Re body 'plans'...as in different phyla even on Earth. Predators, whether a octopus, monkey or tiger beetle tend to have greater neural development...more forward looking vision, killing mechanisms, etc. There are patterns...the physics to fill niches are similar.

Are there alternate life chemistries? No evidence of it. It's difficult to understand how a life form could out compete one with carbon based molecules. Even life forms that started with another base would have over the aeons evolved into with more carbon based chemistry.

All speculation of life forms breaks down once advanced technology enters the equation. We can speculate with some 'educated' guesses over the next thousand years....but ten thousand years...a milion years...a billion....these are all just words with nothing in our experience to relate to.

[IsaacKuo]

Silicon based chemistry may be perfectly viable for artificial beings, which may be the dominant form of life in the universe since they can more easily colonize space than "natural" species. We've sent silicon based "beings" across the solar system, far more places than we've gone ourselves.

As for alternate natural life chemistries...I'm not sure that "chemistry" is even required. For all we know, there could be forms of life that are fundamentally based on wind/fluid motions, rather than chemistry. Look at the visible weather patterns on Jupiter and Saturn...there are interesting long term self sustaining patterns there. Apparently the colder wind patterns in Uranus/Neptune are even more stable. Excessive heat input apparently makes fluid motion more chaotic.

We don't know what's going on in the vast interiors of gas giants. For all we know, the universe could be teeming with life in Oort Cloud gas giants, using fluid motion patterns rather than chemistry. Or fluid patterns in neutron stars might be conducive to the formation of life. We would be hard pressed to ever say "hi" to any of these life forms, though. They may be fundamentally trapped inside the (to us) exotic conditions of pressure/density that are only available inside deep gravity wells. They could never leave their gravity well prisons, and they'd have great difficulty in creating artificial probes which could.

[WalrusLike]

I suspect the following is correct:

Not only is the universe stranger than we imagine, it is stranger than we can imagine.
Sir Arthur Eddington
English astronomer (1882 - 1944)

Anyone talking about life evolving and repeatedly finding that form follows function, has only been working with the one set of initial parameters. I heard a review of a good book called "Your Inner Fish" or something like that... it makes the point that our body structures are present all the way back to the prehistoric fish.

It may be that our world is typical... and that one form arises then dominates. It may be that symmetry is the norm because that is what generally arises out of chaos... but I wouldn't bet on it.

Different initial parameters in the evolving lifeforms could potentially give rise to truly alien constructs.... it is very difficult for us to abandon our inbred assumptions about how life will be.

Just look at planetary formation... for years we thought that our system was the norm... and that all others would be analogues. Now the difficulty is in explaining how come our system looks so alien to the norm.... why is Jupiter still way out wide?

I bet Arty had it right.

[Pickled Tink]

I suspect the following is correct:



Anyone talking about life evolving and repeatedly finding that form follows function, has only been working with the one set of initial parameters. I heard a review of a good book called "Your Inner Fish" or something like that... it makes the point that our body structures are present all the way back to the prehistoric fish.

It may be that our world is typical... and that one form arises then dominates. It may be that symmetry is the norm because that is what generally arises out of chaos... but I wouldn't bet on it.

Different initial parameters in the evolving lifeforms could potentially give rise to truly alien constructs.... it is very difficult for us to abandon our inbred assumptions about how life will be.

While i suspect I am correct in my guesses, i suspect that you suspect that you are correct (I just felt like saying that. Pay no heed it). In the realm of hypotheticals one can argue forever without reaching a definitive conclusion. At least when we make free form statements of pure imagination and speculation. In such cases i have always found it beneficial to list the assumptions that the argument is based on, so ill list mine here:

1: That because Si-Si bonds do not endure in the presence of oxygen, Silicon based life forms will be rare or nonexistent, and beaten out by Carbon based life.

2: Regardless of the bases used, protein synthesis will still be a fact of life (Which would be hindered by any Si-O-Si-O chains. While the problem could be solved, we know there's a better way involving another chemical called Carbon).

3: That some forms are better than others for overcoming certain challenges (Essentially part of the principle of evolution by natural selection).

4: We have a literally gigantic sample of how life has adapted to challenges here on earth.

4 is only partly useful because, as it has been pointed out, newer adaptations are built on older ones, and in an alien environment life could wander down a different path. That's very true, but to then argue that other subsequent structures that have evolved multiple times independently wouldn't evolve on another distinct alien line is silly. Maybe they wont, but they can certainly do so.

Things like multicellularism and symmetry can be assumed, since, well, we are after complex life forms, and symmetry makes it a lot easier to grow. Not as complex, and like it or not, length of the genetic code is a selective pressure, albeit a milder one than getting eaten (More genetic material means more resources used to duplicate that genetic material with each cell division. This adds up, though the cost is so small that is can usually be ignored).

So far as life in fluid patterns and even in stars, i find such things to be more in the realm of fantasy than reality. I don't see how life as we have defined it for the moment can exist in that manner (Though this may just be a failing of my own imagination). Saying space is big, and therefore almost anything is possible is more than a bit silly, since we have perfectly good physical laws and observed phenomena to go on. Yes, these will naturally introduce bias into our attempts to imagine whats out there in the universe, but the base is still fairly solid (At least in my opinion).

[timb]

The various threads about whether this or that environment can support life, etc. got me thinking...

Why do we assume aliens will be at all like us?

I don't mean in the low-budget sci-fi "actor in a prosthetic forhead" kind of way.

I mean realistically.

There often seems to be an assumption that life will be more-or-less earthlike. The morphology may be different, but the overall chemistry will be the same. Or the chemistry will be different, but the overall energy budget will be the same. Or the overall energy budget will be different, but the general mentation processes will be the same. Or the mentation processes will be different--but not too different--but the lifespan will be more or less the same (+/- a couple hundred earth-years).

It seems to me that given the scope of the universe, it's just as likely that alien life will be totally inscrutable to us, and we to it, as that it will be anything we could possibly hope to communicate with.

I dunno. A low-energy environment might have some kind of living ooze, that migrates across the surface of a frozen moon at a glacial pace, and thinks even slower, using interactions between itself and the radiation from the gas giant it orbits to generate its thought patters. Would we even recognize such a thing as life? Would it even recognize us?

Or what about a sentient gas cloud, spanning hundreds of light-years deep in the searing radiation hell of a galactic core, with a lifespan measured in aeons. Would it even notice us? Or we it?

For all we know, the galaxy could have plenty of life, but all orthogonal to us, not just in time and space, but in terms of its very nature.

What do you think?

I think that's false.

[raptorthang]

"We don't know what's going on in the vast interiors of gas giants."

Actually we have a good idea because the physics is within our grasp. Matter and energy is not a mystery we know nothing of. The forces that hold an atom together and define its existence is based on physical properties that are universal.

[IsaacKuo]

"We don't know what's going on in the vast interiors of gas giants."
Actually we have a good idea because the physics is within our grasp. Matter and energy is not a mystery we know nothing of. The forces that hold an atom together and define its existence is based on physical properties that are universal.

If we knew what was going on, then the surface features of Jupiter and Saturn would not be so mysterious.

Yes, we know the forces of the atom, and we also know a lot about chemistry. We also know a lot about biochemistry...but that doesn't mean we understand everything about Earth's biosphere and its inhabitants. That doesn't mean we anticipated the existence of deep sea non-photosynthetic ecosystems. That doesn't mean we anticipated the existence of deep underground bacteria living literally inside rocks.

Sure, we think we have good theories about the bulk composition of gas giants, but we're still just guessing in the dark about the internal flow patterns.

[raptorthang]

If we knew what was going on, then the surface features of Jupiter and Saturn would not be so mysterious.

Yes, we know the forces of the atom, and we also know a lot about chemistry. We also know a lot about biochemistry...but that doesn't mean we understand everything about Earth's biosphere and its inhabitants. That doesn't mean we anticipated the existence of deep sea non-photosynthetic ecosystems. That doesn't mean we anticipated the existence of deep underground bacteria living literally inside rocks.

Sure, we think we have good theories about the bulk composition of gas giants, but we're still just guessing in the dark about the internal flow patterns.

The comparison, again, is not based on our knowledge of matter and energy. I've been a paleontologist for 32 years and have never heard any colleague state life wouldn't be possible in the conditions you state underground, etc.. You aren't quoting science just as the premise of this whole thread doesn't quote science.

Your statement is based on the general ''they' who once said 'Power flight was impossible but....etc' that statemnet adds zilch to any science logic. Because 'a' was once thought impossible says nothing about 'b' being possible. 'Anticipating' life underground is well within the realm of the properties of matter and energy....anticipating life within a red giant,etc. is not. As another poster rightly points out, statements thrown out without any premise of physics are 'fantasy' In contrast, to give any credibility to such a statement variables should be given as to how a carbon based molecule(or other 'stuff' based on the properties of matter and energy would be replicated, etc. in such conditions...

[jhwegener]

I think I disagree very much, and will give some reasons: Of course we can speculate anything and have any dreams we like. I cannot see those dreams lead anywhere, except perhaps to science fiction and other fiction. Somewhere there may or may not be a reality similar to our fantasies. On the other hand of course it is a fact that "there is life on worlds like earth".

[eburacum45]

The problem with life inside a gas giant is that life, once formed, would be very thinly spead in such a huge environment. An organism attempting to graze on hypothetical sky-plankton and increase its own mass would use a large amount of its energy searching for food.

Perhaps autotrophs (plants of some sort) might be able to exist by absorbing resources from the ambient environment, but they would be thinly spread in the vast atmosphere of a gas giant so would probably provide poor sustenance for any sky-grazers.

Could a semi-permanent storm such as the Red Spot act as a concentrator for these floating plant-like creatures? Perhaps, but for how long? Long enough for an ecology to form?

And speaking of ecologies, what about detritivores? Any dead organism that falls out of a gas giant's sky would sink down towards the dark, hot, high pressure depths, A lot of useful organics could be lost in that way.

[Pickled Tink]

Ive spent a few hours considering the matter of gas giant life forms, and one thing that's pretty much essential to cellular life is the presence of lipids. These can and have been shown to spontaneously form permeable vesicles. A quick google search on their spontaneous formation will keep you busy for a few hours. Suffice it to say these would be the first step in building a cell membrane.

Now, I may simply be reading the wrong sources (Indeed, i have little doubt I'm missing something there), but i do not believe we have located lipids in the atmospheres of the gas giants, nor do i believe the required conditions are present for the formation of the aforementioned vesicles.

Assuming that the above paragraph is true, and I am perfectly willing to accept that it isn't (It is a rather thin spot in this post, so much so that i can almost see out the other side of my monitor), then cellular life could not arise in a gas giant. That said, however, there are other possible means of self replicating structures capable of operation in such conditions (Possibly through some kind of self catalysing crystal formation), but i doubt that these would meet the current conventional definition of life as I linked to in my first post. Such replicators would indeed be distinctly alien to life on earth because the situations there prevent the most fundamental adaptation of Earth organisms from ever occurring.

There are, however, problems regarding the atmosphere of gas giants, for instance the winds and turbulence would be considerably powerful, and i think this would be rather fatal to any attempts at sizable organisms. There may be patches of calm where such creatures could evolve, but we don't know how long large storms exist in the eyes of which such creatures could take refuge.

Given whats presented here, i am skeptical that life could arise in a gas giant, but i could easily be wrong.

[clint]

All of a sudden Jupiter looks way more interesting
On the other hand, I would still rather place my bets on its moons...

[IsaacKuo]

Again, I don't think that we should restrict our thinking to chemistry based life forms. We could design a clockwork robot which uses only mechanics--no chemistry (such clockwork robots might actually be an important component of one radical method of interstellar propagation).

Fluid flow patterns in gas giants may be persistent and might interact with each other in ways that exhibit self replication and variation. There could be clockwork style mechanisms in persistent flow patterns. The gas giants of Jupiter and Saturn may be far too hot for "flow pattern life", since these hot temperatures seems to cause disruptively chaotic flow patterns. But there could be many "cold" gas giants out there--even some orbiting our own Sun which we haven't detected yet because Oort cloud planets would be so dim.

So I'm not at all talking about "cellular life" in gas giants, or even some sort of "crystal life", but rather something far more radically different. Storm eyes would not be refuges for these life forms, but would actually form components of the life forms themselves.

[Ilya]

There often seems to be an assumption that life will be more-or-less earthlike. The morphology may be different, but the overall chemistry will be the same. Or the chemistry will be different, but the overall energy budget will be the same. Or the overall energy budget will be different, but the general mentation processes will be the same. Or the mentation processes will be different--but not too different--but the lifespan will be more or less the same (+/- a couple hundred earth-years).

As far as I know, people who have professional interest in such matters do not make this assumption.

In mid-eighties I read "Life Beyond Earth" by Robert Shapiro and Gerald Feinberg, which speculates about life not based on carbon and water (Shapiro and Feinberg coined disparaging term "carbaquists" for people who insist life must be so based). The book begins with ammonia-based life (carbon chemistry but little or no water), goes on to life based on liquid hydrocarbons (no polar solution at all), then proceeds to increasingly bizarre "lifes" which are not based on chemistry at all -- but on interacting and self-organizing magnetic fields in solar corona, lava eddies in Earth's mantle, polarities of atoms in solid hydrogen, etc. Not silicon life, incidentally. But Shapiro and Feinberg themselves point out that such "life", if it exists, would be very hard for us to recognize, let alone interact with in any kind of meaningful way. An energy-absorbing, entropy-decreasing, reproducing magnetic structure on the Sun would look to us like any other magnetic structure. A comet in Oort Cloud permeated with interacting, living polarity patterns would look to us like any dead, motionless chunk of hydrogen ice -- since all action takes place at electron level, with no atom or molecule ever moving. (And we'd kill it just by shining a flashlight in it.) Our own Earth could be full of magma creatures, and we'd never know it. For that matter, if microorganisms not based on DNA exist right now on Earth -- right in our soil and pond water, -- we'd have very hard time recognizing them. Under microscope they'd look like any other bacteria, and being hopelessly mixed in with "regular" microbes they'd be impossible to isolate unless you know beforehand what to look for. "Scientific American" had an article just a few months ago on this very subject.

So astrobiologists are looking for liquid water and for "life as we know it" because that's what we know (more or less) how to recognize.

[cjameshuff]

In mid-eighties I read "Life Beyond Earth" by Robert Shapiro and Gerald Feinberg, which speculates about life not based on carbon and water (Shapiro and Feinberg coined disparaging term "carbaquists" for people who insist life must be so based). The book begins with ammonia-based life (carbon chemistry but little or no water), goes on to life based on liquid hydrocarbons (no polar solution at all), then proceeds to increasingly bizarre "lifes" which are not based on chemistry at all -- but on interacting and self-organizing magnetic fields in solar corona, lava eddies in Earth's mantle, polarities of atoms in solid hydrogen, etc. Not silicon life, incidentally.

This is a point of view that seems a bit disconnected from reality, and based on a distorted version of the actual perspective that "carbaquists" hold.

The first...ammonia is composed of stuff that's significantly more scarce than the stuff water is composed of. Oxygen is everywhere. Carbon-and-water biology doesn't rule out the possibility of ammonia being of great importance, but the sheer abundance of water everywhere you're likely to find ammonia and its great usefulness as a solvent will make it a given that water gets used heavily. Same goes for carbon vs. silicon. Life would have a difficult time avoiding the use of carbon.

Life in non-polar solvents might be possible, but it'll be far less likely. Removing polar solvents from consideration drastically reduces the complexity of the chemical interactions that could occur. Given the above mentioned wide availability of water, it again seems extremely unlikely that it won't be used, even if the life has to collect it in solid form and use potent biological antifreezes to make it a liquid. Perhaps in a hot, sulfur-rich environment, sulfuric acid or something could substitute...if the organisms don't just learn how to crack the water out of it.

Magnetic fields, fluid eddies, etc all seem to lack the complexity and persistence needed to form life. They dissipate too rapidly for structures of any real complexity to arise. Maybe there's room, energy, and a quiet enough environment for some kind of self-replicating plasma structures in the boundary between Sol's wind and interstellar space, but I don't see it being nearly complex enough to object to us classifying it as an interesting solar wind phenomenon rather than as life.

Mineral-based deep life is the other major possibility I see...things behave very differently under the conditions of temperature and pressure experienced in the mantle, perhaps systems of other elements can achieve the needed complexity. The volume of suitable habitat is enormous...it could be many times less likely to arise than carbon/water surface life, and still be the dominant form of life in the universe. It still may make use of carbon and water, though likely not in any biochemistry we'd recognize. For obvious reasons, it needn't be considered when looking for life on distant planets, or for much at all. Maybe we'll someday find some curiously complex and common mineral deposits in a chunk of material from the core of a differentiated, then disrupted asteroid and conclude that they're the long-frozen remains of such life.

[timb]

This is a point of view that seems a bit disconnected from reality, and based on a distorted version of the actual perspective that "carbaquists" hold.

The first...ammonia is composed of stuff that's significantly more scarce than the stuff water is composed of. Oxygen is everywhere. Carbon-and-water biology doesn't rule out the possibility of ammonia being of great importance, but the sheer abundance of water everywhere you're likely to find ammonia and its great usefulness as a solvent will make it a given that water gets used heavily.

O is about 10 times as abundant as N, and ammonia is more volatile than water, so unless the starting cloud had a very anomalous distribution of elements, any ammonia rich planet will have much more water than ammonia.

[zhamid]

I posed a similar question on a different forum.

I think one reason is that you can't look for something you can't recognize. Right now how would you detect a drastically different life form? How would you predict it? Study it?

As for differences, I totally agree. Some said that evolution went down the same path many times on Earth. It didn't. As far as we know, it had same beginning (meaning it didn't restart from scratch, just some larger species went extinct and others evolved again). They also evolved in the same parameter, in the same planet with a very small temperature range. Just look at the diversity of life in our own planet.

I can imagine exoskeleton intelligent life living on Ocean floor that lived in sophisticated groups and communicated using complex language and went extinct long ago. How would we ever recognize such an intelligent life? Or looking at it the other way, if we didn't have hands and fingers then we likely wouldn't have been able to munipilate our environment as much as we do / did. But that doesn't mean we wouldn't posses intelligence, just that we wouldn't have left so much evidence of it for others to find.

Now going a bit too far with imagination, if we were 2D creatures then would we recognize 3D life in its fullest? If there are multiple dimensions then there can well be spieces that interact with more than just the four dimensions us humans interact with. That life would be drastically different than ours.

The interesting thing is that we find life pretty much everywhere on Earth, in some very unlikely places. I find it hard to imagine that in nearly infinite universe (for all practical purposes) there isn't life in environments drastically different than ours.

[I suppose we need to agree on the definition of 'life' first].

[Pickled Tink]

I think one reason is that you can't look for something you can't recognize. Right now how would you detect a drastically different life form? How would you predict it? Study it?

By doing some theoretical work on self replicating molecules (I absolutely reject magnet field interplays and flow patterns in gas giants as life. They fail to meet several of the criteria that define life, such as a metabolism, response to stimuli, and reproduction (If those can be overcome, ill change my mind on the subject, until then I don't think its worth bothering with as at present it's a fantasy).

As for differences, I totally agree. Some said that evolution went down the same path many times on Earth. It didn't.

Then by all means explain why the wing has evolved at least four times independently. Insects, Birds, Pterosaurs, and Bats.

The eye has done it at least twice. If you examine and compare the human and octopus eye you will notice that in the human the blood vessels and nerves sit on top of the retina, resulting in a blind spot where the optic nerve passes through. The Octopus has the nerves and blood vessels behind the retina, and thus no blind spot. This is a fundamental difference in the early stages of formation that still went down the same path.

Flippers have done it at least twice, what with fish evolving them the first time, then some happy land mammals crawling back in again to become whales, porpoises, seals, etc.

The leap from single to multi-cellular life has been performed in a lab. Twice bacteria have evolved a means to metabolise Nylon (The first time in the wild, the second in a lab). Bacteria are constantly evolving, independently of each other, to be resistant to antibiotics.

Adaptive Camouflage has evolved independently in both the Cuttlefish (Along with a few of its cousins) and the Chameleon.

Your statement here is clearly and demonstrably incorrect, as i have shown with just these examples, and I could go on and on like this for hours. But I believe that this error has stemmed from a fundamental error on your part in understanding exactly what evolution is, as demonstrated with the following quote:

As far as we know, it had same beginning (meaning it didn't restart from scratch, just some larger species went extinct and others evolved again).

It appears you are mixing up Evolution, which deals with how life changes over time and adapts to its environment through natural selection and genetic drift, with Abiogenisis, which is how life came about in the first place. Evolution acts completely independently of Abiogenesis.

They also evolved in the same parameter, in the same planet with a very small temperature range. Just look at the diversity of life in our own planet.

Which has a minimum maximum temperature variation across its surface of a 147.7 degrees Celsius, naturally speaking of course (I wont pretend lab temperatures are valid), with a very wide variety of different environments allowing life on earth to display an astronomically large number of different phenotypes attempted in the quest for reproductive success.

I can imagine exoskeleton intelligent life living on Ocean floor that lived in sophisticated groups and communicated using complex language and went extinct long ago.

I can imagine flying lizard men that shoot lasers from their armpits and scream X-rays. Imagining something does not make it real, nor does it make it suppose that such a thing could actually exist.

How would we ever recognize such an intelligent life?

We would identify it as Intelligent by studying its activities and its response to stimuli. It doesn't matter how it communicates, what matters is that we will be able to see how it responds to actions taken. Also, something you should remember: Brains, the thinking tool we evolved, are very energy intensive. The statistics for the brain on wikipedia indicate:

"Although the brain represents only 2% of the body weight, it receives 15% of the cardiac output, 20% of total body oxygen consumption, and 25% of total body glucose utilization.""
- Wikipedia. Brain energy consumption header. Further referenced from there (Brain page).

Or looking at it the other way, if we didn't have hands and fingers then we likely wouldn't have been able to munipilate our environment as much as we do / did. But that doesn't mean we wouldn't posses intelligence, just that we wouldn't have left so much evidence of it for others to find.

Actually, it would. The Evolution of our intelligence is tied to our ability to manipulate the world and explore. As i have shown above, a brain is extremely energy intensive. So much so that brains have actually been detriments to species and they have subsequently lost them through evolution. You do not build a complex computer without a need for it, and a machine that doesn't get much information isn't going to need such a big computer to process it. Intelligence is an emergent feature of the brain that happened over time and became more powerful.

Now going a bit too far with imagination, if we were 2D creatures then would we recognize 3D life in its fullest? If there are multiple dimensions then there can well be spieces that interact with more than just the four dimensions us humans interact with. That life would be drastically different than ours.

Of course it would, since it would be operating on a completely different set of physics, however i am unfamiliar with the specifics of multidimensional theories and thus will not speculate on the matter.

The interesting thing is that we find life pretty much everywhere on Earth, in some very unlikely places. I find it hard to imagine that in nearly infinite universe (for all practical purposes) there isn't life in environments drastically different than ours.

Of course there will be life in other environments. And it may well have adapted to far more nasty extremes than it has on Earth, but Earth has demonstrated an enormous number of different environments and countless different adaptations to these environments that it is silly to simply disregard what we have seen of convergent evolution here and say that it will not hold true on other worlds when it has clearly and demonstrably happened repeatedly here.

I am not saying that alien cats will look like cats and alien dogs will look like dogs, I am, and have been saying repeatedly that life will likely adopt similar strategies and evolve down similar lines. Some solutions to problems are better than others, and these will beat out their competitors. it is not true that all adaptations are equal, otherwise natural selection would not work in the first place.

My apologies if this post carries something of an off putting tone. It is not intended.

[eburacum45]

Then by all means explain why the wing has evolved at least four times independently. Insects, Birds, Pterosaurs, and Bats.

The wings of birds, bats and pterosaurs are all derived from homologous structures, in relatively closely related creatures which share many of the same genes. No wonder they all look very similar. On the other hand insect wings have a completely different structure and origin. If we find organisms with wings on other worlds, they will likely develop from totally different structures to both insect wings and vertebrate wings; that is because they will share no ancestors with organisms in our world.

The eye has done it at least twice. If you examine and compare the human and octopus eye you will notice that in the human the blood vessels and nerves sit on top of the retina, resulting in a blind spot where the optic nerve passes through. The Octopus has the nerves and blood vessels behind the retina, and thus no blind spot. This is a fundamental difference in the early stages of formation that still went down the same path.

Yet the octopus eye and the human eye share certain genes; they are both derived from a very ancient Last Common Ancestor with primitive eyes, and the tendency to develop eyes has been inherited by both groups. We will share no LCA with alien creatures, so their sense organs need share no features in common with our own.

[IsaacKuo]

The wings of birds, bats and pterosaurs are all derived from homologous structures, in relatively closely related creatures which share many of the same genes. No wonder they all look very similar. On the other hand insect wings have a completely different structure and origin.

The wings of birds, bats, and pterosaurs are also remarkably different from each other.

Birds use a completely different sort of wing structure...it's the sort of design which an engineer would be shocked by. Using many feathers instead of stronger/lighter web structures seems bizarre--especially considering how many birds have webbed structures for their feet.

Bat and pterosaur wings are superficially similar, but even these are significantly different in that pterosaur wings use only one finger (the "pinky" finger) while bat wings use many fingers.

And of course, there are gliding animals with very different sorts of wings. Flying fish use fins which are unlike tetrapod limbs. Draco lizards use wings supported with ribs.

[Pickled Tink]

Sorry i didn't get to these earlier.

The wings of birds, bats and pterosaurs are all derived from homologous structures, in relatively closely related creatures which share many of the same genes. No wonder they all look very similar. On the other hand insect wings have a completely different structure and origin. If we find organisms with wings on other worlds, they will likely develop from totally different structures to both insect wings and vertebrate wings; that is because they will share no ancestors with organisms in our world.

IsaacKuo made a good post about this subject, and I find myself unable to add much to that at this time, save to point out that the divergence between mammals and dinosaurs (from which it is currently understood birds descended) happened a good 2-3 hundred million years ago. To say that they are closely related is incorrect.

The one stop source for all things generally informational, also known as wikipedia, states in the final paragraph of its opening on the Mammal page:

"The mammalian line of descent diverged from the sauropsid line at the end of the Carboniferous period. The sauropsids would evolve into modern-day reptiles and birds, while the synapsid branch led to mammals. The first true mammals appeared in the Jurassic period. Modern mammalian orders appeared in the Palaeocene and Eocene epochs of the Palaeogene period." -Wikipedia

As IsaacKuo has so kindly pointed out, theer are considerable structural differences between each of the four wing types is pretty obvious, with Mammalian and Pterosaur wings evolving to be membranous, while the Wings of the birds, far more closely related to the Pterosaurs than the mammals, who split off earlier, have an entirely different wing structure as a result of their evolution from the Therapods of the Jurassic.

Yet the octopus eye and the human eye share certain genes; they are both derived from a very ancient Last Common Ancestor with primitive eyes, and the tendency to develop eyes has been inherited by both groups. We will share no LCA with alien creatures, so their sense organs need share no features in common with our own.

Sharing genes doesn't mean that the structures didn't evolve semi or even entirely independently. One of the main ways new structures come about is through the re purposing of old genes. The Evolution of the Flagellum is an example of this. In any event, even if two creatures did. Even if the two structures shared a common foundation, which is highly unlikely given the divergence between Molluscs and Chordates, of which Octopi and humans are members of respectively, occurred about 540 million years ago.

Sharing a common ancestor doesn't have much to do with subsequent adaptations. All it does is provide a framework upon which subsequent adaptations can be built, and while this will in some respect bias those that can come about. For instance, Octopi are unlikely to evolve winged flight due to many factors, not least of which is their lack of a skeleton.

The significant difference between the eye of the Mollusc and the eye of the mammal are an obviously early adaptation difference. The fact that both went down the same line to form a very similar structure, even if they did start from the same base, is indicative of independent mutation and selection in the separate lines, and is still a valid case of convergent evolution.

I consider A-life (some of it, anyway) to be legitimately alive. Yes, they are nothing but patterns of electrons in computer memory, but they use resources, reproduce, respond to stimuli (obviously, stimuli themselves existing in computer memory), and in many cases mutate and evolve. My definition of life is "local decrease of entropy", or anything that uses energy to create order and more of itself.

By that definition you could also include crystallisation to be life. Now, while I am not ruling out a crystal based life form, I'm just saying that the simple process is not in itself life, under your definition, which I am taking to be the part you included in your quotation marks. With regards to that process, even including the following part of your post as part of the definition, the wording is sloppy. "Make more of itself" can go two ways with regards to such a process, the first would simply be the individual crystal "growing" larger. The second would refer to replication of new individual crystals.

I'm not saying you are not on the right track, i'm just saying your definition is flawed.

I don't see any reason why magnetic patterns or flow patterns can't exhibit metabolism, response to stimuli, or reproduction, any more than there are inherent reasons why molecules can't exhibit them.

I sincerely hope I am wrong. However these features would be far less stable due to their complete dependence on local conditions at all times to be about right. Chemical based life is much more resistant to these effects. An example would be to chuck a Cat in a pond (KIDS! DO NOT DO THIS!) it will survive and swim to safety. Flow patterns and magnetic field organisms would be much more vulnerable to impacts, field interactions, etc.

I am even willing to say that chemical life is a better solution to the problem of life (Though as a chemical life form I am probably more than a little biased in this judgment).

[Ilya]

By that definition you could also include crystallisation to be life. Now, while I am not ruling out a crystal based life form, I'm just saying that the simple process is not in itself life, under your definition, which I am taking to be the part you included in your quotation marks. With regards to that process, even including the following part of your post as part of the definition, the wording is sloppy. "Make more of itself" can go two ways with regards to such a process, the first would simply be the individual crystal "growing" larger. The second would refer to replication of new individual crystals.

No. It may not look like that, but crystals are in a higher entropy state than solution they precipitated from. Notice I wrote "uses energy to increase order". Crystal growth is an exothermic process. It releases energy and stops when solution is exhausted, and there is no way to increase the complexity by adding more energy to the structure. Likewise, fire is not alive by my definition, but a computer virus is.

[Ilya]

By doing some theoretical work on self replicating molecules (I absolutely reject magnet field interplays and flow patterns in gas giants as life. They fail to meet several of the criteria that define life, such as a metabolism, response to stimuli, and reproduction (If those can be overcome, ill change my mind on the subject, until then I don't think its worth bothering with as at present it's a fantasy).

I consider A-life (some of it, anyway) to be legitimately alive. Yes, they are nothing but patterns of electrons in computer memory, but they use resources, reproduce, respond to stimuli (obviously, stimuli themselves existing in computer memory), and in many cases mutate and evolve. My definition of life is "local decrease of entropy", or anything that uses energy to create order and more of itself.

[clint]

...Some said that evolution went down the same path many times on Earth. It didn't. As far as we know, it had same beginning (meaning it didn't restart from scratch, just some larger species went extinct and others evolved again)...

Well, that's not necessarily so.

I think it's much more likely the very first life forms on Earth did indeed evolve many many times (from scratch)
- until finally one survived and reproduced long enough to evolve further.

[ravens_cry]

I don''t think we exactly assume that aliens will be exactly like us. But for science fiction, at least the visual kind, it does have certain advantages. For one thing, it is much easier to relate to something that looks like us. Even ET and Wall-E, had arms hands and a face. it is easier for a human to build an emotional connection with such a creature. And in film and television, rubber headed aliens are a lot cheaper. Even in literary science fiction,a certain amount of emotional resonance, even in beings described as physically and even mentally alien, like the Emotional, Rational and Parental triad of Asimov's The Gods Themselves, is helpful for creating an attachment to the characters by the readers.
As dor in reality, I speculate there will be similarities. I think intelligent aliens will have some form of language, and advanced cultures will have ways of storing that language. I think they will have manipulators of some sort, they will use tools in some shape and form. What will they look like? Not a clue.