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*By life, I mean something more complex than microorganisms. Something closer to what we have on Earth.

similar to this question: Is it possible for complex life to evolve on planets without oxygen?

For example:

It is said that Titan is well outside the habitable life zone but we make the assumption that life absolutely need water to survive. Titan has lakes of methane. Could methane or other substances be used as a substitute to water? Could life prosper in a waterless environment?

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    $\begingroup$ Yes, I think this is better posted as two questions: one about whether life (by definition) requires water (dihydrogen monoxide), and the other about whether life (again, by definition) requires a signifcant amount of free gaseous oxygen in the atmosphere. $\endgroup$ – a CVn Oct 4 '14 at 17:51
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  1. There were many SciFi story about a civilization that arouse on a planet where F replaced O (by Yefremov). Francis Carsac had a series with the antagonist lifeforms being non-water-based (crystal/superconducting metal).

  2. Wikipedia has a very extensive list of possible non-water-based-solvent biochemistries (Ammonia, Methane, HF that was used in Yefremov's story).

    This article on "Exotic Biochemistries" from Xenology Research Institute quotes Carl Sagan and Dr. Peter M. Molton at the University of Maryland, with extensive discussion on alternate biochemistries, including non-water based (Ch 8.2.2 "Alternatives to Water"). Ammonia seems to be the leading candidate. The article is very heavy on detail, including analysis of reactivity, energy required to fracture various types of bonds, rarity of elements, possible reactions, etc...

  3. Life can potentially evolve that is NOT based on carbon chemistry at all (energy based, crystal based) - for one example, you can have information-based life that exists in computer memory, and doesn't directly require water.

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    $\begingroup$ The short story Wang's Carpets by Greg Bear has an interesting take on information-based life. $\endgroup$ – Jay Vogler Oct 5 '14 at 21:52
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    $\begingroup$ Egan. Greg Egan. -facepalm- $\endgroup$ – Jay Vogler Oct 5 '14 at 22:11
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It might be possible, but it would be nothing like the carbon-based life we see on Earth. Methane and ethane won't dissolve ionic compounds, so ion transport is right out. Cell membranes self-assemble because they're built from phospholipids with hydrophobic and hydrophilic portions, and that requires water in the environment. Even DNA forms its shape because of water.

Water is considered a "universal solvent," not because it's good at dissolving everything, but because it really can dissolve small amounts of almost anything. Hydrocarbons like methane and ethane are much more limited.

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    $\begingroup$ Is there evidence that there are no other universal solvents that could approach the life supporting properties of water? Is water the universal solvent? $\endgroup$ – trichoplax Oct 5 '14 at 9:27
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    $\begingroup$ Actually ammonia is an excellent solvent as well. $\endgroup$ – celtschk Oct 5 '14 at 10:50
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    $\begingroup$ @githubphagocyte: Water's definitely not the universal solvent. Celtschk brought up ammonia, which can actually dissolve more things than water (as far as I recall), including alkali metals. It would be interesting to see a take on life that used solvated electrons. There are some very interesting differences between water and ammonia, but ammonia's one of the likelier possible alternative solvents for life. I was only commenting that the hydrocarbons mentioned seem like a very poor choice for carbon-based life that's anything like ours. $\endgroup$ – Jay Vogler Oct 5 '14 at 22:09
  • $\begingroup$ Ammonia based life is a curious idea, presumably on colder planets where water would be mostly ice. Life would still be carbon based, cause you need carbon for long chemical structures, but Ammonia replacing water. It's a curious idea. Photosynthesis using NH3 and CO2, releasing . . . NO / NO2 and H2? . . . . seems, maybe, possible. $\endgroup$ – userLTK Apr 22 '16 at 7:02
  • $\begingroup$ Ammonia (like propane) is a liquid under modest pressure at room temperature. Its used as an industrial refrigerant and used to be used domestically before HCFCs. So a planet with a thick atmosphere? $\endgroup$ – nigel222 Apr 22 '16 at 10:02
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Note: The following answer is in addition to the excellent answer by DVK, on it's own it's insufficient to answer the question

To answer this question we would first have to define what life is. Following the biological descriptive definition of life we get:

Since there is no unequivocal definition of life, the current understanding is descriptive. Life is considered a characteristic of something that exhibits all or most of the following traits:[36][39][40]

  1. Homeostasis: Regulation of the internal environment to maintain a constant state; for example, electrolyte concentration or sweating to reduce temperature.
  2. Organization: Being structurally composed of one or more cells — the basic units of life.
  3. Metabolism: Transformation of energy by converting chemicals and energy into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.,[36]
  4. Growth: Maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter.
  5. Adaptation: The ability to change over time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism's heredity, diet, and external factors.
  6. Response to stimuli: A response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of multicellular organisms. A response is often expressed by motion; for example, the leaves of a plant turning toward the sun (phototropism), and chemotaxis.
  7. Reproduction: The ability to produce new individual organisms, either asexually from a single parent organism, or sexually from two parent organisms.[41][42] or "with an error rate below the sustainability threshold."[42]

Source: http://en.wikipedia.org/wiki/Life

So, looking at the above we next can address the actual question. Now we can go the speculative road asking "can be imagine an entirely alien system that fullfills these requirements?" into which category I would categorize all of Hypothetical types of biochemistry (from the currently top voted answer) or we can ask ourselves "which other systems do we know that could potentially fulfill those criteria?". Now, let me present a crazy example: computers. Or rather, a computer connected to a fully automated manufacturing plant, automated mining plant and solar cell plant. Those things could easily fulfill at least 5 out of the 7 criteria and with additional work there is nothing that would stop them from fulfilling the other criteria and thus be considered alive. As the definition is a descriptive definition in certain cases it requires a slightly open interpretation of terms like "cells", "organic" etc., but to prevent it from becoming a cellular definition this seems a reasonable course of interpretation.

In conclusion I do believe it safe to say that life is possible without water.

Now, the actual question is about whether such a system can evolve and then you get in a lot of other murky territory (the likelihood of evolution occurring by chance are... well... let's just say pretty slim) and even disregarding that it's safe to say that such a mechanical system would not evolve by it's own, however it does give a more intuitive argument that life is something far more complex than just "could we get a workable solution when we replace water with methane".

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Life as we define it requires certain conditions, in specific the ability for an organism to form and evolve. The one organism that we know resulted in intelligent life requires water and various other factors, so it is logical to assume that a planet with similar conditions might contain life as well. Aside from that there are alternate possibilities:

  • Life was brought to a planet from an external source. For example by an Asteroid crashing on a planet, or actual other intelligent beings.

  • Life could be formed with other/unknown materials that we do not know about (yet). There are still many things that we do not know, so it would be foolish to assume we know everything about life.

In essence intelligent life requires a construct of materials that are capable of processing information, basically computers made out of random luck. Since there are many ways to construct such a "computer", for example with silicon and copper, there are many forms of intelligence possible.

To survive it would require additional factors, such as the ability to interact with the environment to gain nourishment in some form. The ability to adapt would be necessary if the environment changes over time, as otherwise such a species would be extinct by now. The ability to move would be a great evolutionary advantage, however given the right (paradise) environment not required.

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There are self reproducing masses of chemicals at one confined location in the universe. These are fuelled by the photons emitted from a nearby nuclear furnace. The major elements in the self reproducing masses are hydrogen, oxygen and carbon. Because carbon is such a promiscuous element, it likes to join with other elements into large orgy of elements. Oxygen and hydrogen are happy willing partners which make for a better bigger orgy. With such willing partners it only took a couple of billion years to go from a mass of chemicals to a mass of self reproducing chemicals. From that moment it was inevitable that some self reproducing masses would vary with time and still reproduce. None of the reproducing masses can reproduce if conditions are not at a minimal and they will no longer reproduce. There is no requirement that the fuel source is a nuclear furnace, or that the promiscuous molecule is carbon, or that the happy willing partners are hydrogen or oxygen. Until I know how to make life than I won't rule it out.

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  • $\begingroup$ Please see meta for an explanation of the downvote. $\endgroup$ – trichoplax Oct 5 '14 at 10:23
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    $\begingroup$ @Lohoris Please don't call concerns about language that harms real people "silly." Such dismissals reinforce the harm such language causes to people, and both the initial word and defending it is contrary to SE-wide policy. $\endgroup$ – SevenSidedDie Oct 12 '14 at 19:00
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    $\begingroup$ @Lohoris Tough then. That attitude is being eradicated here. See, we like and value the people who are the target of ugly words, far more than the ability to thoughtlessly throw them around. $\endgroup$ – SevenSidedDie Oct 13 '14 at 8:39
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    $\begingroup$ @Lohoris The fact that you used the word "she" to refer to the molecule indicates that at least some part of you understands which real people reading the word are its target. There's hope yet! But defending careless slurs used where they have no value except a negative one is moot: the word is gone. $\endgroup$ – SevenSidedDie Oct 13 '14 at 17:13
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    $\begingroup$ Sexism, not racism. Women ("she") are not a separate race. $\endgroup$ – SevenSidedDie Oct 13 '14 at 17:56
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If by possible you mean that we're not aware of any physical law being broken by such an occurrence, then yes, life evolving on planets without water is possible.

If by possible you mean that we have identified a specific series of reactions that can naturally occur that lead to the development of life, then the answer is no.

(Yes, I'm in a bit of a pedantic mood.)

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