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I completely disagree. I think the 'life can look like anything' (lets call them LCLLAs) train of thought suffers from a lack of understanding of constraint satisfaction.

Think about how some LCLLAS talked about how silicon life is probably a thing. First off, it it were, given Earth's crust is mostly silicon and its the most habitual place ever discovered, it would have evolved here. but it didnt. It turns out silicon is just too ridged, and to get life going you need all sorts of chemical properties that just aren't congruent to life.

We are carbon / oxygen organisms for a chemical reason. despite the availability of other resources, other forms of lifeforms didn't develop for a reason. maybe other forms of life does exist in some methane ocean on Saturn like planets, but its not going to be very complex, and definitely not intelligent life building space ships.

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I'm just always surprised how can we be so certain? Even here on Earth, for millions of years, life was literally nothing more than single cell organisms - also definitely "not intelligent life building space ships". And it would be very hard to see how could it possibly evolve into such, seeing as the atmosphere was full of incredibly toxic oxygen.

The thing is, life has almost infinite time to evolve out of these various elements. The fact that it didn't evolve here on Earth means absolutely nothing, seeing as Earth alone has billions of years left where such life could arise, and there are literally countless planets everywhere in the universe where the random dice of evolution roll every second.

It’s a question of physics not just chemistry. Deuterium for example is similar to hydrogen chemically so while those differences are slightly toxic to us it’s easy to assume a planet similar to earth with an abundance of deuterium and a lack of hydrogen would evolve life forms with the opposite preferences.

Except the ratio of hydrogen to deuterium is a function of astronomy and more specifically fusion. No naturally formed planet is going to end up with that imbalance. Which means no natural deuterium based life forms. So while some alien civilization create deuterium life, it’s not going to be part of natural ecosystems.

And so it goes for most possible interesting edge cases. Dependency on rare elements like Neptunium means any competitors without that dependency have huge advantages.

Of course. But there are planets with abundance of silicon(including ours). There are planets with abundance of methane and even here we have lifeforms which can consume it. I'm just pointing out that as long as silicon based life is possible, then given the near infinite amount of time the universe can spend evolving such life, it will surely exist.

Sort of, it depends on how common various forms of proto life are. If their very rare and the first one to happen wins then you might expect silicon life of some form to be out there.

On the other hand if proto life is extremely common then silicon proto life might always be out competed by more efficient alternatives. For example the fact Silicon dioxide is a solid where carbon dioxide is a gas is a major advantage or using carbon.

The silicides would meanwhile say that the gaseous nature of CO2 makes carbon useless as a basis for life, and even go so far as to offer a plausible reason.

Sorry, no. CO2 being gaseous is a major availability and energy advantage. For example you can’t get plankton equivalents as dissolved silicon is parts per trillion at the oceans surface.

Hybrids are a different story, but I doubt that’s what anyone is talking about by silicon based life forms.

CO2 (or SiO2) is only one of the possible outputs, or inputs, of metabolism. Even on Earth its primacy came very, very late.

You don't get to constrain alien life to your preconceptions. Even some Earth life secretes SiO2. Using carbon for some things would not invalidate its Si-basedness any more than diatoms have given up carbon-basedness.

Silicon's near absence from our favored solvent, like iron's, is a product of toxic levels of biogenic atmospheric oxygen. Our early oceans were saturated with iron products, and are now starved of them. There is no reason to assume oxygen would be their favored oxidant.

> There is no reason to assume oxygen would be their favored oxidant

That goes back to astronomy no other option is going to be nearly as plentiful.

> You don’t get to constrain alien life with your preconceptions.

It’s not a preconception it’s the solubility of silicon and silicon compounds. Yes, CO2 and SiO2 aren’t the only option for respiration, but CO2 was and is a near endless supply of carbon and oxygen. Without that growth simply slows down dramatically. Even with a photosynthetic equivalent you simply don’t get the great oxygen event from silicon lifeforms in even 10 billion years.

PS: To be clear I am not saying silicon life would be impossible just that it’s at a huge disadvantage head to head and would have a vastly slower progression to large scale multicellular life.

Relative plenitude did not dictate the elements we and our relatives use, and would not for other life, either. In any case, many other elements are plentiful.

Carbon is a good choice for our temperature range, but there are plenty of temperature ranges as well or better represented. Life will not sneer at the others. Most life on Earth cannot make any use of the carbon in CO2, no matter how much of it there is; in fact, the more of it there is, the worse off most living creatures are, only green plants excepted. You would die in a room with the nitrogen replaced by CO2, despite that you use none of the nitrogen at all. (Only bacteria can "fix" nitrogen, and only some of them.)

Iron is very plentiful in the cosmos and in Earth's constitution, and most life depends on it, but it is practically nonexistent in the open ocean; yet life thrives there. Even where iron is plentiful, life uses hardly any. Silicon is extremely plentiful, too, yet used by our life even less -- until just lately!

So, your argument from plenitude utterly collapses. Life as we know it is a product of history and circumstance. Given different circumstances and different history, you should expect a different product. There will certainly be similarities dictated by physics: e.g., cells are very, very likely, and equally so use of liquids. Aperiodic fibers to encode genes, cognate to our DNA, and others to fold up and carry out processes, cognate to our peptide chains a.k.a. proteins, are quite likely. But there is no reason to imagine that any of the details of the encoding would match.

> Even where Iron is plentiful, life hardly uses any

That’s a huge misconception about biology and evolution. Basic cellular biochemistry is extremely well conserved. There is almost no pressure to use more iron, but significant pressure to use less. Photosynthesis for example is Iron dependent and that acts as a major limiting factor on ocean ecosystems. Yet, we don’t have an iron free alternative adapted to utilize that giant ecological niche. Life is seemingly using as little as possible.

That should suggest why primarily silicon life is such a poor option.

> There are plenty of temperature ranges as well or better represented.

For complex life it’s not just about temperature but also pressure and access to sunlight. Anyway, the normal argument is that silicon based life could thrive in extremely high temperatures. That’s all well and good for silicon but not so much the other chemicals it’s interacting with. Life is dependent on being able to reliably create and break relatively weak chemical bonds. However, when you start to look at the actual chemical options at those temperatures you run into serious problems. The kind of large molecules you want for complex life are either unstable or too stable at those temperatures.

Iron is not the key element in chlorophyll; magnesium is.

We don't know of many interesting Si compounds specifically because we have none of the organisms that would produce them. Our ignorance does not constrain nature.

"Weak", for bonds, is exactly a function of temperature. At higher temperature, life will have chosen constituents that bond with the right strength, wholly ignoring your preconceptions of what it ought to be using.

Chlorophyll contains magnesium, photosynthesis requires Iron at about 1000 magnesium atoms per iron atom. However, Iron still ends up the limiting factor in ocean ecosystems. https://zenodo.org/record/1258477

Anyway, Life can only chose chemical bonds that are possible. It’s perfectly reasonable to pick a specific temperature range and say here this is ideal for silicon lifeforms, but that means every chemical interaction must occur in that temperature range which is a major physical constraint.

We don’t know of many interesting and relevant Si compounds in large part because they don’t exist. It’s not even just chemical compounds life needs a water equivalent for all that cellular machinery to float around in and bump into each other.

Again: Relevant Si compounds don't exist here, because there is no Si-based life here. Our favorite carbon compounds would identically not exist in a Si-based ecosystem. That the useful compounds would not be simple substitutions of Si for C is of no importance. As I noted, some liquid-analog is likely involved in any spontaneously evolved ecology, but it does not need to be water, or even, technically, liquid-as-such.

Nature is not constrained to the limits of your imagination. I am frankly surprised to find you continuing to insist it is.

The tangible existence is irrelevant, computational chemistry is more than capable of exploring this territory. The issue isn’t simply finding giant complex molecules that would make up such hypothetical lifeforms it’s looking for the basic building blocks.

In the end all of chemistry comes down to quantum mechanics it really is quite constrained. Put another way you can list our every possible 2-9 atom molecule containing Silicon plus common elements and look at how the behave.

And, you claim to have not just made such a list, but also explored all of their possible interactions with one another and with what could be common non-Si compounds in their vicinity? You and what ten thousand universities?

You are just making things up. Who do you imagine believes you? Why do you want them to believe you?

No, can is not did. My point was it’s a finite space.

People have explored some of this space and found nothing.

"Sorry, no. CO2 being gaseous is a major availability and energy advantage"

At room temperature. Silicon life, if it exists, would be basically using lava instead of water. Glassblowers work at 600 degrees, i imagine thats the sort of temperatures where we will see any silicpn chemistry take place at non glacial pace

Silicon at those temperatures is still not a gas. But more interestingly photosynthesis equivalents become drastically less efficient at those temperatures. Life could still extract energy from chemical processes or even light, but ecosystems would be energy starved which slows down evolution. On top of this the chemistry capable of supporting complex life is again less efficient than carbon based life at lower temperatures.

Net result you might get the equivalent of very simple multicellular life, but intelligence becomes extremely unlikely.

Yet, there are simple Si compounds that are gaseous at such temperatures.

Chlorophyll photosynthesis does not work well at high temperature, but that says less than nothing about whether photosynthesis of different products using a different molecule and a different process could evolve. What we can say is that if it could, it would, somewhere.

You have no basis beyond your own preconceptions for any statements about likelihood. Thus, your estimate is 100% confabulation.

> Yet, there are simple Si compounds that are gaseous at such temperatures.

Silicon tetrafluoride might be a gas, but it’s not going to be plentiful. If you feel there are real options here feel free to list em.

> that says less than nothing about whether photosynthesis of different products using a different molecule and a different process could evolve.

Carnot efficiency does. You can’t simply scale up temperature and expect equivalent efficiency.

It's an interesting point.

> seeing as Earth alone has billions of years left where such life could arise,

In case you didn't know, Earth has about one billion years of water left.

https://www.sciencedaily.com/releases/2013/12/131216142310.h...

I do know that, but it's few billion before the earth is consumed by the expanding sun. Without surface water the planet is still there.

I would say, get your head out of your edge cases. Before I go on, I hope you understand no one is saying its not physically possible, and that it absolutely does not exist anywhere in the multiverse, just that if we come across other life, it will most likely be similar to our own.

by definition, the most probable form of life in the universe is the 1 we are observing. you only need math and probability to figure that out. We also have observational evidence. As far as we can tell, there is no other life in our solar system. We have 8 other planets and many moons that have failed at creating life in any form - that's literally hundreds of billions of years of failed attempts at life.

you can't just pick an element and think its going to work. you need ALL the elements, ALL the states of matter, and the right ratios and amounts of elements to work. Even if you get that lined up, expecting complex life to form under enough pressure to create diamonds in thousands of degrees centigrade also puts the probability of complex life off the table.

I don't known who this is hard to understand. in the simplest example as the article states, if you want to fly, there are things that necessitate something that will fly, such as wings. no wings, no fly. sure its POSSIBLE you can fly without wings, but UNLIKELY. given that we have multiple lineages of evolution taking different paths that end up with the same solution is evidence of that.

> get your head out of your edge cases

The Earth is the edge case, is the point we're making. All of the conclusions and deductions about what life must be like elsewhere that use Earth as their implicit foundation are going to be overfit to the data, so to speak. Organisms on Earth converge to similar forms: dolphins and fish, bats and birds, for instance, so the form is useful on Earth is a valid observation. Therefore, we can expect to see these forms throughout the Universe is a quite a jump.

> The Earth is the edge case

You do not know that. All available evidence points to the opposite: We have no knowledge, not even a hint, of any other physical process that could sustain life than that of Earth's carbon-based chemistry.

The idea that life could take a myriad of different forms is beautiful, but it is pure fantasy, based on no evidence at all.

>>We have 8 other planets and many moons that have failed at creating life in any form - that's literally hundreds of billions of years of failed attempts at life.

Again, how can you be so certain of it? Life could have existed in some form on Mars or Venus or even Mercury millions if not billions of years ago, only to be extinguished by the changing conditions in the solar system and ground to absolute dust by time. Maybe not advanced space faring civilization, but life could have existed in those places and you simply don't know.

>> you only need math and probability to figure that out.

Yes, and maths and probability are telling me something very opposite to what you are saying. That if you are observing a solar system like ours(forget the entire universe for a second) for a ~100 years out of billions of its existance, then sorry, but you don't know anything about it, certainly not if it harboured life in the past or not, not to a degree that you seem to be certain of.

You won't see any wings on a hot air balloon. If you think it is not common, look at the fish in the ocean.

You don't see a lot of natural hot air balloons floating around do you?

Buoyancy in water is different than in air. Lighter than air natural controlled flight, even in a relatively dense atmosphere like Earth or Titan, is highly unlikely. Fish maintain buoyancy with oil in their livers which is a relatively low energy process.

Hot air balloons and other LTA vehicles work by having a mean density lower than the containing medium e.g. the atmosphere. Even with Earth's relatively dense atmosphere there's not a lot of lifting gases that are effective without significant heating. Definitely not readily made by biological chemical processes. That leaves thermal balloons. The amount of energy to heat the air in a hot air balloon is pretty significant and requires energy dense fuels.

So a hot air balloon animal would need to produce and then burn energy dense hydrocarbons and have a strong yet lightweight natural gas envelope and body structure that could handle the burning of that fuel. Burning I would note at temperatures that would sterilize surfaces.

>>You don't see a lot of natural hot air balloons floating around do you?

Not balloons perhaps, but spiders float on strands of silk using very similar principles.

Pollen and seeds frequently use some kind of floating device to spread as far as possible - again, you probably wouldn't call it flying as such, but they can move through the air for the purposes of reproduction without having wings.

Floating is not controlled flight. Being blown by the wind due to a high surface area to weight ratio is not flight. I move through the air without wings when I walk. I could jump off a diving board and get some forward motion going. Neither are in any way flight.

Anyway, balloons or no, there is a great deal of hot air.

1. Fish do not, in fact, use oil in their livers for buoyancy. They have an "air bladder", which you may read about. You might have heard something, somewhere about shark livers. Fish are much more closely related to you than to sharks, so it is a big error.

2. Buoyancy in air has been achieved with an industrial level of success using hydrogen. Producing hydrogen is easily within the energetic range of biological processes. Air-buoyant animals could have existed for millions, even hundreds of millions of years before being outcompeted by pterosaurs and birds, and then left no trace.

> Fish are much more closely related to you than to sharks

Sharks are fish, so this clearly isn't categorically true. You probably mean either bony fish (for which I think it's kind of questionable that its true) or more specifically lobe-finned fish (for which the case is stronger.)

You miss the point. Humans and bony fish share a most recent common ancestor. We are much closer to the fish than our common ancestor is to the shark.

Um, my point was that that is arguably true of bony fishes, more clearly true of the narrower category of lobe finned fishes, but not at all true as originally claimed of fishes generally, as that includes cartilaginous fishes (including sharks), and sharks are pretty definitively more closely related to sharks than to humans.

In other words, a meaningless statement. In original context, my remark was about fish with air bladders.

It actually, as written, asserted that all fish have air bladders.

“Fish with air bladders do not, in fact, use oil in their livers for buoyancy. They have an ‘air bladder’, which you may read about. [...] Fish with air bladders are much more closely related to you than to sharks, so it is a big error.” is critically different than what you actually wrote upthread, and is obviously kind of silly. It also would have made no sense in response to the post it responded to, which also referenced “fish” in general and not “fish with air bladders”.

now you have de-evolved the conversation by trying to change the definition of fly to swim. if you are going to do that, there is no point in continuing the discussion. Furthermore, you are confusing buoyancy with flight.

Both air & water are fluids (same equations can be applied to many problems). Equating them in the discussion about aliens is not a step too far which it doesn't require much imagination (compare it for example with an idea of a living nebula that might require some knowledge what is entropy, what is life in terms information, etc)

So now you're presupposing matter. Look, your myopia is not universal.

I disagree with this assessment of silicon based life. While it is true that Earth's crust is mostly silicon, it's also at a temperature that would make metabolic processes using silicon very difficult until you get down to the mantle.

It could be that silicon based life is in fact constrained by temperature and can only arise in planets where the mean temperature is in the thousands. This could in fact mean that we do have a parallel silicon based biosphere underneath our feet. There is obviously no evidence for this whatsoever but then again, there is no reason to assume that the occasional leakage in the form of volcanic eruptions would leave any trace that we could use to deduce that these samples were once living entities.

Doesn't it intuitively seem like the life by definition would evolve out of any particular niche - say 1,000 Kilometers under the surface - to other areas?

I'm at best doubtful about life on Mars because I find it hard to imagine some form of life that occupies a small part of Mars i.e. it's only in the parts we haven't seen. I tend to think if there's any life on Mars it'll find a way to deal with the conditions on all of Mars. By the same token if there's any silicon based life then it'll pop up in places that we can find as well as those we can't. Maybe it does best in places we can't but surely there would be some oddball silicon based creature that would beat the odds.

The macro example would be the extinction of the South American marsupials when the North and South American land masses merged. All the placental animals in North America went south and all the marsupials in South America went extinct. Except for Opossums which for whatever weird reasons not only survived but went into North America.

No because if this was true, then we would see carbon based life do the exact same process in reverse. However, there is a limit to adaptability, particularly through the dumb and highly stochastic process of natural selection.

We have no evidence of a void ecology in near earth orbit or of life persisting close to the Moho barrier - where the crust meets the mantle. There are hard limitations to the encroachment of life that only technology or radically different chemistry can circumvent.

The same hard barriers might also mean that life on Mars is largely constrained to those few underground liquid water lakes that we have recently found evidence for. Keep in mind that even the most barren deserts on Earth have several orders of magnitude more water present in the air than on Mars.

> Doesn't it intuitively seem like the life by definition would evolve out of any particular niche - say 1,000 Kilometers under the surface - to other areas?

Other areas that are similar, sure. But if you're asserting that we can be confident there are no subterranean mantle-creatures because we don't see any wandering about out here, I don't think it's a good argument, for the same reason we can't go down there ourselves and look

If mantle-creatures were to exist, they would have a far, far greater volume of living space to expand into than we surface-dwellers do. Maybe they just haven't gotten around to us yet, for the same reason we don't have any large cities in the antarctic.

What do you mean by “by definition” in that first sentence?

Like, if it failed to do so, it would fail the definition of life? I don’t think anyone defines life that way, and I don’t think it would be a good definition (would I even count as alive under such a definition? It seems like no.)

I think of life as dynamic. There are limits of course - it's taken billions of years for life to operate in the vacuum of space but it really does seem like part of the definition life should be that it spreads. I'm confident that you too move around.

What's the path-dependency-ness of life? Like, once you have carbon based life to what degree does it preclude silicon based life from evolving? How often does a feature evolve sui generis when the nich that the feature exploits is already filled? Clearly it happens sometimes so if you've got a good enough angle you can step into something that's seemingly already covered.

For all that after several billion years it's just carbon based life forms in these parts. You'd think that there must be some area where silicon based life would provide an advantage. Unless it's a case that carbon based life is an overwhelmingly superior product but on some hypothetical world it's simply filled with silicon and there's virtually zero carbon.

Carbon is a way better chemical for life than silicon. It can form vastly longer chains and produces IIROCC (if I recall organic chemistry correctly) much more robust bonds.

Additionally we find organic molecules everywhere from comets to gas clouds in galaxies. We don't really find as much interesting silicon-based chemistry everywhere.

"Other forms of life didn't develop for a reason" isn't that convincing of an argument. We are somewhat knowledgeable about what chemical and physical processes can occur between, -100c to 1500C under 1atm or so, +-100 nanoteslas, in a timescale of under a human attention span. Beyond that, we become increasingly clueless.

We have only a single very specific data point. That, and the human tendency to opine confidently about that which we cannot know.

Well, stage one to silicone life could be computer chips. Then for a transition period, the shell could be metallic followed by integration of the human mind to that almost indestructible body shell. Something like the bad guy terminator 2. But step one could be computer chips. Human designed and made rather than by strict definition of natural selection.

> First off, it it were, given Earth's crust is mostly silicon and its the most habitual place ever discovered, it would have evolved here. but it didnt.

Just give it a few more decades ...

If we assume for a second that silicon life is a thing, would it be reasonable to think that it would be utterly alien? Probably it's chemical composition would be entirely different, but would it be unreasonable, that it could evolve eye, muscle, bone etc. analogs just like us, and the end result would be not entirely unfamiliar to our earthly eyes? The article talked about wings being one of the few viable modes of flight, and Richard Dawkins has made a lecture about eyes having evolved independently multiple time here on earth (like in, as the article states, mammals or octopi).

Octopus eyes did not, in fact, evolve independently. This claim is repeated ad nauseam and is wholly false.

The common ancestor of octopods, insects, and humans had eyes, and coded where to put them with the same gene in all lineages that have eyes. This was demonstrated by inserting a vertebrate "eye here" gene into a fruit fly embryo, causing an eye to grow there.

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