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Djsnowboy267

Why do we assume aliens will need (roughly) the same conditions as us?

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Dual-strand provides no real protection against mutation. On the contrary, twice as many places for something to go wrong. RNA, on the other hand, has stronger bonds in the backbone, if memory serves.

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Back to the original question: Why do we assume aliens will need (roughly) the same conditions as us? Answer: Chemistry, plain and simple chemistry.

To have any chance of life evolving you'll need a neutral reaction medium, something to dissolve the reactants. On earth water does this perfectly but any liquid would in theory be fine. Even sub-zero liquid methane or scorching hot molten lava. As long as it allows dissolved molecules to flow freely and react. But extreme temperatures do come with a draw-back. At low temperatures chemical reaction speed is severely reduced. This would mean your creature will not be able to rapidly respond to outside stimuli. At high temperatures on the other hand reaction speed is increased but again there is a catch; complex molecules start to disintegrate the hotter it gets.

For life to evolve you'll need a temperature somewhere in between, warm enough to sustain fast chemical reactions, cold enough for complex molecules to be stable. Our human body is a perfect example, 37.0 °C (98.6 °F) ensures rapid reactions but deviate too far either way and we're in trouble. Below 32°C (89.6°F) the body starts to shut down and above 41°C (105.8°F) proteins start to break down.

Assuming alien life is DNA based is short sighted. DNA is an earth 'invention' and there is no way to know what alien life uses. The only thing safe to assume is that it needs some sort of complex molecule to store information. And those can only be formed above and are only stable below a certain temperature.

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Restrictions of our imagination play a role too. We can readily imagine carbon-based, oxygen breating, water drinking beings. Now try to imagine something that is silicon based. Or breathes chlorine. I bet anything we could come with would be a weird variation of some earthly creature - be it octopus, spider, worm, snail or cactus. Simply because we've never seen anything that is not of our carbon/oxygen/water standard. We don't know what to look for, so we are looking for something familiar first.

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Furthermore, 100% of known sentient species are carbon/oxygen based.

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Furthermore, 100% of known sentient species are carbon/oxygen based.

That's like saying all circles are round.

ALL known life is carbon/oxygen based. Sentient life is a sub-set of that therefore those too are carbon/oxygen based.

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Furthermore, 100% of known sentient species are carbon/oxygen based.

Yes, but in science, a sample size of one is considered to be not very conclusive.

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I would like to add some biochemistry.

We can know for sure that all life will be based on carbon. That's cause that is the only atom that has enough binding places to make complex molecules AND can form long chains. Silicon for example can't be placed with more than 3 in a row. It's too unstable. It has been tried in labs, but you can't form chains with anything but carbon.

If life has carbon, than it will also have hydrogen. That's cause hydrogen is necessary to 'fill in the gabs' on the free binding places on the carbon. Without hydrogen, also no carbon chains. This brings us at our first complex molecule: fats.

Fats are mostly composed of one or more long chains of carbon with hydrogen. This way we can assume that alien life could have something fat-like. On earth, we use fats and fatty acids to make structures (membranes) and/or to store energy.

Next, we need some solvent, so that all the molecules can react easily. Two mayor candidates for this solvent are water and ammonia. These are good solvents, posses acid-base properties and can so be used in the reactions themselves. However, according to wikipedia, oxygen is way more abundant than nitrogen, so we expect to see more water than ammonia. When we look at comets, this is also what we see. All together, we can assume that most life we encounter will be water based.

Water has limits on when it can be used as a liquid. These limits are made by temperature, pressure and dissolved minerals. You might want to look for planets where these conditions are met.

With oxygen, our fatty acids are complete. Also the rest of the fat can be made. Why life on earth chose to make these, we do not know, but there might be a reason...

The next complex molecule we can make with oxygen are alcohols and sugars(which are chemically just alcohols). Experiments show that 5- and 6-rings are the most stable ones. However, there are 8 possible 5-rings and 16 possible 6-rings of sugar, and it's possible to make other variations to the design. On Earth, life has mostly chosen for fructose as 5-ring and glucose as 6-ring. Why we do not know, but also here, there might be a reason...

Sugars, as fat, have a structural function, and energy storage.

On earth, the rest of our body's are the atoms nitrogen, phosphorus and sulfur, the ions sodium, potassium, calcium, chlorine, magnesium and iodine, and the metals iron, cobalt, copper and zinc. Despite the fact that thy are more rare, it are these that are necessary to make the really interesting molecules in our bodies, like DNA and proteins. This way, we can not know what the molecular composition of alien life will be.

However, some people think it is coincidence that life developed the way it did, here on Earth. Other ones think there must be a reason. The truth is, we will only know when we meet others; like us, or not.

I hope this clears things up (If you can be bothered to read). Chears

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It's the SQUIRRELS, I knew they were up too something!

http://scp-wiki.wikidot.com/scp-1013-j :huh:

But on a more realistic note, the most likely life we'd find in space would either be extremophile bacteria or self-replicating-machine starships, possibly flown by uploaded brains or self-aware AI. I would however love to be proven wrong.

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Nucleic acids and their analogs are the only known information-carrying molecules known. And from there it is all derived. all places have to have conditions in which base pairing works, temperatures and solvents, needed chemicals, pressures, energy sources, radiation shielding etc...

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This is something that has nagged me too. I think our imagination is way too limited when it comes to this subject. Sure, everything is speculative, but I think if we ever encounter life of a different origin than our own it might truly surprise us. It could be very possible we do not even recognize it as life at first and might not be so according to our current definitions and understanding. There might well be a DNA analog that does not chemically resemble DNA at all.

Another thing I sometimes like to speculate about is that life has - at the hands of humans - already spread from the earth. All of our space probes contained life when they left earth and it is not very unprobable that some of it at some point will find a habitat it can thrive in. It for sure is not intelligent, but those life forms could be trying to answer the same questions in a couple of million years :) Information wants to be free and DNA is just a fancy way of spreading it.

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There might well be a DNA analog that does not chemically resemble DNA at all.

.

We would still see its information-carrying properties.

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Yes, but in science, a sample size of one is considered to be not very conclusive.

It's all we've got to work with at the moment :)

Also, it's most likely that humans are part of the most common type of life.

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When trying to make predictions, it is good to leave as few unknowns as possible. Can a silicon-based biochemistry exist? Possibly. Can a carbon-based biochemistry exist? Definitely.

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It's all we've got to work with at the moment :)

Also, it's most likely that humans are part of the most common type of life.

Why? Like it was said before, N=1 is useless for any decent conclusion. It is the only kind of life we are sure of, but any other conclusion beyond that is pushing it. Our type of carbon based water loving might prove to be the odd one out, if we ever find a couple of other forms of life.

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I don't know why, but this discussion reminds me of the first episode of Red vs Blue.

The way I see it though is we can't possibly be the only planet with life. The other issue is actually detecting life is pretty darn hard with out actually being there. It's just one of those things that will take time, but we will eventually know for sure at some point I think.

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Nucleic acids and their analogs are the only known information-carrying molecules known. And from there it is all derived. all places have to have conditions in which base pairing works, temperatures and solvents, needed chemicals, pressures, energy sources, radiation shielding etc...

Why limit information-carrying to the relative complexity of molecules. We are in the early stages of using photon states to carry information in the form of photonic qubits. Maybe photonic life could have evolved elsewhere. The point is anything is possible since at the subquantum level we have no idea what exists(if anything) to build from.

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Why should we, we would know it was an machine or at least something alien.

Some stone age people would not know, anything who moves around would be an animal as only animals move.

How do we know that cows aren't really alien self-replicating organic space-probes?

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Why limit information-carrying to the relative complexity of molecules. We are in the early stages of using photon states to carry information in the form of photonic qubits.

photon states don't work all on their own like nucleic acids. they need ( compared to the photons themselves ) a HUGE and fragile scaffold of ordinary matter around to hold the qbits and facilitate the computations ). And qbits can't be copied ( no clonning theorem ) and life has to reproduce.

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How do we know that cows aren't really alien self-replicating organic space-probes?

common ancestor and stuff:)

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Slightly off topic, I once read it was possible for life to be plasma-based on grand scales, like within a star nursery or nebula. I don't know where I read that, but can anyone elaborate on the possibility of large DNA like structures being formed in a plasma or Nebula?

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