I showed a draft of a short story I wrote to some friends and got a very derisive comment from the chemist in the group.
According to him the chemistry just isn't possible.

I'd like a second opinion.

The short of it: It's the year 2970 and humanity has finally cracked the FTL drive and starts to colonize worlds far away.
They find a world with perfectly Earth-like conditions. It even has complex live (non-sentient) similar to Earth. The alien life is even carbon-oxygen based with a bio-chemistry that is very similar to our own. There are thr equivalent of plants, herbivores that eat those plants and carnivores that eat the herbivores.
Unfortunately for the colonists the bio-chemistry of the alien life-forms isn't compatible with Earth lifeforms. Earth live-stock can't eat the local plant-life. Meat from alien animals/alien plant-material is at best indigestible to Earth life-forms (including humans), at worst poisonous.
Luckily Earth plants can deal with the alien soil, so the colonists can setup an Earth food-chain starting with plants.
The opposite isn't true: The alien life-forms can and will eat Earth bio-mass. Alien herbivores find Earth plants tasty. The alien carnivores consider Earth live-stock (and especially humans) a nice snack.
My story revolves about the fight for survival of the colonists against the alien biosphere.

Now the chemist (an-organic chemistry, he is not a bio-chemistry expert) claims that this won't work.
According to him, if the alien bio-mass is indigestible (or worse) to Earth life-forms, the opposite has to be true as well. It works both ways.
Obviously that would kill my story altogether.

I'm not so sure he is right. I would presume that it heavily depends on the exact mechanics of the digestive system of the alien life-forms. If that first breaks down the Earth-chemistry bio-mass into small chemically simple compounds that are easily digestible this could possible work in my opinion.

Who has the right of the matter. Can this work or made to work?


8 Answers 8


Arsenic $\times$ Phosphorous

Make your alien lifeforms able to use arsenic while our earth lifeforms can only use phosphorous.

Voilá. If your alien lifeforms eats earth lifeforms the arsenic in their blood outcompete the phosphorous on the food (preventing phosphorous from being toxic), while if earth lifeforms eat arsenic based lifeforms the arsenic displaces phosphorous and kills earth's lifeforms.

Its possible if someone finds a way for arsenic to work on a cell and do the same things that phosphorous does for our cells.

Arsenic, which is chemically similar to phosphorus, while poisonous for most life forms on Earth, is incorporated into the biochemistry of some organisms.[19] Some marine algae incorporate arsenic into complex organic molecules such as arsenosugars and arsenobetaines. Fungi and bacteria can produce volatile methylated arsenic compounds. Arsenate reduction and arsenite oxidation have been observed in microbes (Chrysiogenes arsenatis).[20] Additionally, some prokaryotes can use arsenate as a terminal electron acceptor during anaerobic growth and some can utilize arsenite as an electron donor to generate energy. ¹

¹ Hypothetical types of biochemistry

  • $\begingroup$ Beat me to it, was going suggest exactly this... $\endgroup$
    – Dider
    Commented Mar 25, 2015 at 1:07
  • 1
    $\begingroup$ I really like this. Simple solution that is pretty plausible. Also satisfied my chemist. $\endgroup$
    – Tonny
    Commented Apr 7, 2015 at 14:52


We may remember this from basic chemistry! There are molecules, especially biological ones, who can have the same formula but be different arrangement of atoms. In a biological context, if you eat something with the correct formula but wrong chirality, it may be poisonous or ineffective for you.

If their biology is similar, but of opposite chirality, the two groups are poisonous to each other but can still use the basic environmental factors to produce an food chain. In earth biology, shape matters, and chirality is important in determining if a protein is the right shape, is ineffective, or is deadly.

Sorry, but it seems your chemist friend has a very valid point.

  • 2
    $\begingroup$ Pre-empted my similar answer by a second or two. Perhaps the alien species has evolved to make use of both left- and right-handed aminoacids. $\endgroup$ Commented Mar 21, 2015 at 17:47
  • $\begingroup$ @SerbanTanasa Which is crazy! However, this appears to be a work of Science Fiction, so I suppose it can be explored. It may need to be addressed early on in the book to keep us science types placated. $\endgroup$
    – PipperChip
    Commented Mar 21, 2015 at 17:49
  • $\begingroup$ It's crazy if you grew up on Earth. Perhaps they have more complex cells, with two nuclei, that can perform wonders our poor one-hand-only kind can't even dream of. $\endgroup$ Commented Mar 21, 2015 at 17:59
  • $\begingroup$ our bodies have various kinds of cells with more than one nuclei $\endgroup$
    – Jorge Aldo
    Commented Mar 23, 2015 at 22:04
  • 1
    $\begingroup$ @SerbanTanasa, you wouldn't need two nuclei, just the appropriate evolutionary history. $\endgroup$
    – Monty Wild
    Commented Mar 23, 2015 at 22:54


Earth-life almost exclusively uses what are known as left-handed amino-acids. What that means is that most amino-acids can be "mirrored" - you can reverse them - but our life is only set up to handle the "left-handed" version. You can create the right-handed versions, but our biochemistry can't process them. You will want to check with a biologist to be sure, but my understanding is that this is just a quirk of our evolution - it's just as likely that we could have ended up using right-handed instead.

However, what if your alien life, for some reason, evolved to use both left-handed and right-handed amino-acids?

Then they can consume earth life, although probably not exclusively, because they can handle both types. But if we try to consume them, our chemistry can't do anything with the right-handed amino acids, rendering them indigestible (or at the very best, extremely un-nutritious).

Edit: looks like I got scooped by the other answer + comments.

  • $\begingroup$ Hey! May answer said the same thing! Only it was here first! $\endgroup$
    – PipperChip
    Commented Mar 21, 2015 at 17:51
  • $\begingroup$ @PipperChip: Not quite, you mentioned only the opposite chirality which would render them incompatible. Serban was the other one who mentioned life that uses both, so it can consume us but we can't consume it. $\endgroup$ Commented Mar 21, 2015 at 17:52

Actually I can offer a completely different answer to the others - what you describe is entirely possible.

Have the earth and the native species both basically compatible in terms of both being able to eat each other. However the native species would use some chemical which is poisonous to earth species within their bodies.

This can either be a deliberate defensive evolution that local predators have adapted to overcome or it could be a coincidence that a chemical they all use is toxic for earth lifeforms.


Let's talk about some of the common chemical types used in life:

  1. Sugars
  2. Amino acids
  3. Nucleotides
  4. Other factors
  5. Putting it together

Sugars As mentioned above, both Sugars (the chemicals used for energy transport and storage) and amino acids (the chemicals used for cellular structures) are chiral (meaning they have left & right handed types). Use the above answers to see what handedness matters.

At the worst, the odds of using the same sugar chirality is 50% (some experiments have shown a preference for one handedness over the other in some reactions).

If non-terrestrial biology uses sugar in its energy storage and transport mechanisms and we share chirality, then we should be able to absorb and utilize the sugar from non-terrestrial organisms.

Amino Acids Another aspect of amino acids is that I read somewhere (but can't recall where - so no reference) that terrestrial biology uses 47 of the 50 amino acids most commonly found in nature. Meaning it isn't a stretch to suppose that other biospheres using amino acids in its biology will likely use most of the same amino acids.

As above at the worst, the odds of using the same sugar chirality is 50%.

If they also share chirality, then we should be able to break down and absorb at least some of those amino acids.

Nucleotides Terrestrial biology uses DNA and RNA to store and transmit genetic information. In terrestrial life, DNA uses 4 nucleotides and RNA also uses 4 nucleotides. However, RNA uses Uracil in place of Thymine in DNA. That means terrestrial biology uses a total of 5 different nucleotides.

I'm not a biologist but I assume that other nucleic acids would serve the same purpose, however, those used in terrestrial biology out-competed all other methods of storing and transmitting information over the last 4.3 billion years.

I assume this is both due to it's ability to preserve that information as wells as providing an extremely small probability of mutation (which gives the species a chance to adapt to other environments).

Other Stuff Let's assume chirality isn't a factor and the non-terrestrial biologic uses at least some of the same sugars and amino acids. There are some other interesting things to explore.

One that I found fascinated was the fact that much terrestrial animal life uses a protein analogous to hemoglobin to transport oxygen.

In octopi and horseshoe crabs, this protein is called hemocyanin and uses copper instead of iron as the critical metal ion.

Other metals used in hemoglobin analogs are vanadium and manganese. However, all three of these analogs are much less efficient than hemoglobin (<1/4 for the best).

Another thing to consider is that terrestrial life develops to survive in a specific environment. Freshwater fish can't live in sea water (it's too salty). Salt water fish can't live in fresh water (not enough salt).

Organisms which consume plenty of a vitamin through its diet, lose the ability to produce that vitamin (e.g. human bodies can't make vitamin C).

Elements become toxic to organisms which are unused to exposure to that element (e.g. the arsenic comment above).

Putting it together No one knows what alien life will be, how it will function, what chemical processes it will use, and whether that chemistry could digest ours. However, if the non-terrestrial life uses process similar to that of terrestrial life (which isn't entirely improbable) then we can make some guesses.

If we both use sugars and share chirality, then terrestrial and non-terrestrial organisms can digest each other's sugars. Otherwise we can't.

If we both use amino acids and share chirality, then terrestrial and non-terrestrial organism can digest each other's amino acids. Otherwise we can't.

Organisms from one biology are unlikely to be able to co-opt, infect, or otherwise bother the cellular operation of each other directly (no viral cross infections).

However, organisms from one biology may happily set up shop in the body of organisms of the other biology (e.g. alien bacteria living in our mouths). It may take a long time for our own immune system to adapt a response to shutdown those opportunistic colonies.

For planets with different elemental abundances, we may find that common composition of the organisms contains enough of certain elements to be toxic to the other. I would expect that in most but not all cases, this would mean the toxicity goes both ways. However, in certain special cases this might not be true.

So if you're writing a story of the type you outlined, then you could sprinkle in some of these factoids and then make comments that it is highly unusual for two biologies to have an asymmetry in toxicity like the one you describe.

Even if it turns out to be unlikely it's certainly possible and know one knows whether it is truly plausible or farfetched.


Well, I'm not a biologist or biochemist, but I think your scenario could be made to work (although still very unlikely) as follows:

Our genetic code is based on four nucleotides which make up two pairs of complementary bases. Now there's no reason to assume that the genetic code of the aliens is the same; indeed, there's no reason it should be restricted tot he same base set. However, in principle, it could be that it uses those four nucleotides we also use and then two more, so it is based on three different base pairs. Since the genetic code is that fundamental, every single of those alien life forms would have those extra nucleotides. Now if those extra nucleotides are poisonous for us, we cannot eat anything from their life. However we don't contain anything that's poisonous to them (all our DNA bases are used by them, too), so they can eat earth food quite fine.

Alternatively, their DNA could be made up of the same bases, but they could use a superset of the amino acids used by earth life forms to build proteins. Again, if one of those extra amino acids is poisonous for earth life, then you get what you want.

Now there's one open question: How can it be that it is poisonous for all life forms on earth? I think that would be easiest explained for the extra nucleotides: Since our genetic machinery (like the ribosomes to interpret the genetic code, or the DNA duplication/repair mechanism) is not prepared to those extra nucleotides, their presence might cause those processes, which are at the very base of everything going on in our cells, to fail. On the other hand, those alien life forms would have no problems with earth material since the nucleotides are the same.

A similar mechanism could be at work with the amino acid suggestion; our ribosomes may erroneously build those alien amino acids into our proteins instead of the correct ones, because they are not prepared to distinguish them; this leads to dysfunctional enzymes and proteins, and thus to death.

Note however that while this would be a possible solution to the problem, it would still be a highly unlikely one: It is far more likely that the alien life forms would have evolved to have a completely different biochemistry, which means we would be as useless as food for them as they would be as food for us.

Well, unless one assumes that evolution on at least one of the planets (earth or that alien planet) didn't start naturally, but as experiment by some intelligent species; then that intelligent species could have intentionally made that similarity plus specific difference, probably in order to study what effect that difference has on evolution.


Changing the alien digestive system to break everything down to a level where chirality is not an issue may be possible, though is going to be quite complex compared to our approach of just dissolving the food and using whatever we find in it fairly directly.

The main thing you will need is a way to synthesize all those complex proteins more or less from raw materials. Using bacteria is probably the easiest approach, rather than adding a bunch of new organs. The conditions for forming new proteins would be quite different to what you need to break down the old, possibly incompatible ones, so the aliens will need multiple stomachs.

Ideally the usable proteins would get filtered out at some point so they don't have to be recreated, but I don't see a way that could have evolved - breaking down everything could be useful since absolutely anything they can find will be a balanced diet, but they have no reason to expect minor variations that only exist on another planet.

I don't have the math to confirm it, but I suspect this approach is going to have some energy efficiency problems - there is a reason we let plants produce most of what we need. At least some of the food will need to be usable as is to power the complex digestive system.

It seems a little unlikely that something this inefficient would end up dominating the planet, but it's not completely implausible, and it gets you a creature that can eat absolutely anything as long as it eats massive amounts of it.

For a simpler approach, make the basics completely compatible, and add some poison. There are plenty of examples of plants evolving poison and the animals that eat them evolving a resistance to that poison. No reason it couldn't end up happening with pretty much every life form on the planet.

Even if the animals are not affected by the poison, it may make the alien plants taste awful - given a choice, they will always eat something from earth.


Use biology

You can actually displace this phenomenon to biology instead of biochemistry. Consider the mitochondria, which is believed to be a the result of an ancestral symbiotic relationship between eukaryotes and prokaryotes.

Now, what if a similar symbiotic relationship exists with these alien organisms which makes them indigestible to earth organisms? That is, earth creatures have no problem eating these alien organisms, only with these symbionts (which are poisonous, let's say). The alien organisms would naturally have no problem eating each other (since they evolved together) and may or may not have problems eating earth organisms (if the chemistry is as similar as you claim, I would say the latter).

This is really the only way to have "similar chemistry" and still have the phenomenon you want. The above posters give ideas which create huge differences between alien-earth biochemistry (chirality, biochemically speaking, can be as different as an apple and a suspension bridge; its like the idea of tones in Chinese languages, a different tone makes the word have a completely different meaning).

EDIT: Found a much better solution:

Use L-Glucose

D-Glucose is used by almost all higher organisms (multi-cellular) because one of the first enzyme in the glycolysis pathway, hexokinase cannot phosphorylate L-Glucose. Hence, L-Glucose cannot be used as energy by most organisms (though it tastes just the same as D-Glucose).

However! L-Glucose can be oxidized by the enzyme: D-threo-aldose 1-dehydrogenase which found in certain organisms such as Burkholderia caryophlli (a plant pathogenic bacterium, according to wikipedia. Thus the solution is simple, just make all the organisms on the alien planet utilize L-Glucose as their primary energy source.

To allow them to also metabolize earth organisms, give them non-stereo-specific enzymes for metabolizing D-Glucose


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