Many questions describe hypothetical biochemistries, or alternatives to carbon, water, DNA, etc that aliens could use.

They cite specific examples, given a world we know about, of what creatures would need to metabolize. For example:

(Titan's environment): Low temperature. 45% larger atmospheric pressure than Earth. Methane and ethane are the solvents. Carbon-based life which breathes methane and ethane and expels acetylene. Other possibility is breathing hydrogen, ethane and acetylene, producing methane.

-Victor Stafusa, on link at start of question

What if we don't know a ton about the planet or moon we are designing, ex. the intricacies of atmospheric composition, the exact pressure, - but we want to get a sense of what our organisms should respire or consume, given a biochemistry? Is there even a way? Do you just pick what's abundant?

In other words

Could I say "The species has genetic material containing y" and get a list of things it will most likely eat?

  • $\begingroup$ If it has genetic material (as we know the term), it's the same biochemistry we have. At least on basic level of things. Enzymes that works on DNA wouldn't work in methane / ethane solvents, and would be ridiculously slow at such low temp. $\endgroup$
    – Mołot
    Nov 8 '16 at 23:08
  • $\begingroup$ Our understanding of life is HEAVILY biased on the one example we have of it, the Earth ecosystem. While we have a theoretical understanding of how alternate life systems could develop and operate, as well as a good understanding of how OUR biology could withstand other environments, we are really, really nearsighted in this regard. So you can really be creative, it's not like there are known counter-examples :) $\endgroup$
    – Jason K
    Nov 10 '16 at 17:49
  • $\begingroup$ @JasonK While this is a theoretical idea it is still possible to disprove or correlate theoretical ideas - I know there may not be a technical "right answer" we know of but I want a realistic one nonetheless $\endgroup$
    – Zxyrra
    Nov 10 '16 at 19:14

You can see a host of biochemistries which are possible on Earth by looking at microbial redox towers. The redox tower shows a bunch of chemicals which can be combined (an electron donor and an electron receiver) in a chemical reaction. The 'tower' bit is that they are usually arranged in order of how much energy is involved. The further apart the chemicals are on the tower, the more energy is released when they combine (respiration)... Or is required to make the reaction happen when the equation is run in the opposite direction (photosynthesis, chemosynthesis, etc).

e.g. glucose + oxygen <--> CO2 + H2O

Microbes (especially anaerobic ones living deep in marine muds) have all sorts of biochemistries which are well known: sulphate reducers, iron reducers, methane producers and so on.

Here's a set of notes with lots of equations which is about redox pairs and energy. There's a table about halfway through naming a lot of the chemicals used by microbes.

This is a nice presentation which is a bit simpler. A bit. Slide 5 lists what we call all these types of reactions: methanogenesis, fermentation, etc.

So if your hypothetical planet has lots of acetate and sulphates but no free oxygen, for instance, you might be able to find a tame biochemist who can tell you what the main redox equations will be. If the chemistry of your planet is more exotic (say tin is commoner than iron) or the temperature or pressure is far removed from anything on Earth, you might need a tame geochemist to hand wave for you.

Of course, as well as using chemicals which are just floating around the environment or which can form without biological intervention, life also tends to evolve molecules for specific biochemical jobs to help it along. e.g. NADH and NAD+ Which makes answering your question more complicated!


Plants use sunlight and convert the lower energy CO2 into O2 and organic chemicals. So, given adequate sunlight or geothermal energy sources, many chemical cycles could form the basis for life. Doing it without water would be more of a problem. The polymer forming systems I'm aware of are the silicones and the inorganic polymers PN, SN, BN and SiN. Silicone is, by definition, a family of organic compounds and any ?N based life would almost certainly have organic groups on the ?N backbone. Whether B would be abundant enough to allow life to develop, IDK. Most likely P and S are abundant enough. Certainly Si and N are. Once you have life utilizing an inorganic energy source, you can then have their waste product (eg O2) form the basis for more active (animal) life. N2 is too stable, I think, for this to work. But Sulfur compounds are a definite possibility.


Do you just pick what's abundant?


On any given planet or moon, what the things living there will eat will be the things that are there to eat. From that, you can work backwards and develop a biochemistry for them, but if you step too far outside of what we know of life to begin with, you might end up having to hand wave more than you'd like.

If, on the other hand, you'd like to start with a particular biochemistry, then you need to make the things that go into it abundant, and have some cycles in place that produce and consume nutrients and waste (also for the back cycle waste and nutrients).

  • $\begingroup$ This is a helpful start but it I don't know if it's necessarily true on Earth - for instance, there is significantly more nitrogen in the atmosphere than there is oxygen or carbon dioxide - yet plants and animals make much more use of what is less abundant here. $\endgroup$
    – Zxyrra
    Nov 10 '16 at 19:16
  • $\begingroup$ @Zxyrra Well, yes and no. Nitrogen is useless filler mass, and at the time life first developed on earth, atmospheric oxygen wasn't really a "thing". $\endgroup$ Nov 10 '16 at 20:45
  • $\begingroup$ My point being if you pick what's abundant Nitrogen shouldn't be useless filler, and oxygen shouldn't be so important $\endgroup$
    – Zxyrra
    Nov 11 '16 at 3:33
  • 1
    $\begingroup$ @Zxyrra Sure, if you go ahead and ignore how chemistry works. When I say N3 is useless filler mass, that's not to say it's useless filler mass to us; that's to say no energy can practically be extracted from it for nourishment. (Nitrogen being hard to get out of the air is why guano is so valuable; and why the Haber process was [and is] a huge deal.) $\endgroup$ Nov 11 '16 at 7:14

basically take the 4 or 5 most common elements in original atmosphere of the planet throw out any noble gasses and there you go. You can also toss metalloids of you have any. note I am using atmosphere in the geologic sense which includes the ocean.

remember the atmosphere before life is going to be very different than the atmo after life.

Early earth atmo contained the following elements, H, N, O, Ar, C, Ne, He. drop the noble gasses and you have the composition of life. The relative abundances are a bit different, if you are worried about that then use abundance then skew for reactivity.

If it's there and it is reactive life will figure out how to use it. don't worry too much about the compounds because life will change those.

major/ macro nutrients of earth life. Lipids (C,H,O,N), Carbs, (C,H,O) protein, (C,H,O,N), nucleic acids, (C,H,O,N,P*) P is on the list for the atmo, it is just further down the line, more common in life because it is so reactive, and N is less common in life because it is so nonreactive. but they are both used a lot. All the minor constituents of the atmo get used in life too.

so to sum up you rule can be elemental abundance minus noble gases, then skew for reactivity. That is skew what gets used in your organsims by reactivity.

  • $\begingroup$ then skew for reactivity but isn't that the sticky wicket? We're carbon-based because C is reactive enough to be useful for metabolism, but not so reactive that it burns too fast (at STP). Because of that, it forms really long chains that can be broken. One more thing: C, H, O, N and Fe are the five most common non-noble elements in the universe. That's why they're the most common on Earth, and would be the most common on any other planet. $\endgroup$
    – RonJohn
    Sep 24 '17 at 0:23
  • $\begingroup$ @RonJohn I don't know what you are talking about, hydrogen, nitrogen and carbon are not major components of the earth, if you are talking about the earth's surface then oxygen followed by silicon and aluminum is by far the most abundant element, yet silicon and aluminum are almost non-existent in biochemistry. Iron is only needed in a few creatures, so you need to be looking at the atmosphere. universal abundance does not predict planetary abundance by itself. $\endgroup$
    – John
    Sep 24 '17 at 1:15
  • $\begingroup$ @RonJohn Iron is not really that important in biochemistry, it is important for humans because we use hemoglobin, but iron is just a trace element in anything that doesn't use it as a n oxygen carrier, which would be most of life. Phosphorus is more important for life than iron. $\endgroup$
    – John
    Sep 24 '17 at 1:23
  • $\begingroup$ yes, I was mistaken. I should have written, "that's why C, H, O, N are the most common elements in life". $\endgroup$
    – RonJohn
    Sep 24 '17 at 1:28
  • $\begingroup$ except that doesn't explain why they are the most common in life, since they are not that common on earth, just the most common in earth's early atmosphere. $\endgroup$
    – John
    Sep 24 '17 at 1:35

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