I'm working on a hard-sci-fi story where a scientist discovers microorganisms in our solar system with disastrous results. I was thinking about how any cross-contamination with an alien ecosystem would cause the organism to self-destruct in a similar manner to matter, anti-matter annihilation. I'm more inspired by science fiction works like War of the Worlds, The Andromeda Strain, Annihilation, etc., but I'm also interested in explaining how alien life could have evolved with novel-yet-similar mechanisms to life on Earth.

Leading me to the whole L-sided vs D-sided amino acids and protein synthesis. So how would the organisms interact with each other? I was thinking all the cells would self-destruct leaving behind traces of organic material and a myriad of novel viruses. Any material, references, or input is always appreciated.

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    $\begingroup$ I am a bit confused here, do you want your aliens to trigger Apoptosis in Earth cells, or are you asking if they could trigger it ? $\endgroup$
    – ErikHall
    Jul 13 at 21:17
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    $\begingroup$ Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. $\endgroup$
    – Community Bot
    Jul 13 at 21:50
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    $\begingroup$ ... you're asking the wrong question. How L-sided and D-sided amino acid-based life would react with each other is complete speculation. That's actually our sweet spot... excapt that you're asking as if there's a Real World "right" answer. What you should be asking is, "When L-sided and D-sided amino acid-based life meets, I want X to happen, but I'm having trouble explaining it in a believable way. So far I have Y. What must I do to make Y believable?" (c) Thus, as written, your question is off-topic here, too, because all answers have equal value and that's prohibited. (*Continued*) $\endgroup$
    – JBH
    Jul 14 at 3:03
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    $\begingroup$ I was looking for input, I won't ask anymore questions in the future. $\endgroup$
    – user104821
    Jul 14 at 5:01
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    $\begingroup$ @SupraGrumP.Heir don't mind the comments, this is a pretty good first question $\endgroup$
    – M S
    Jul 14 at 15:03

2 Answers 2


When amino acids/azucars/attibic molecules are supplied in the "wrong" direction 3 things can happen:

  1. Let's take L and D-glucose as an example, in humans (and almost every organism for that matter) both L and D-glucose have a perceptible sweet taste but only D-glucose is efficiently usable, you see if we consume L-glucose simply nothing happens, it is not usable in the creps cycle nor in any anabolic metabolic pathway, even so there are isomerase that convert L-glucose into D-glucose that means that you can use some of the L-glucose but the production of enzymes spends energy and amino acids so it is very inefficient, this is an exception most of the sugars do not behave like glucose.
  2. They do nothing, you see most receptors are stereospecific i.e. they only accept one configuration, for example methamphetamine has 2 configurations one is psychoactive the other is inert despite being chemically identical, simply many things in your chiral aliens would not be recognized by the enzymes and would just go in and out without any change, especially the amino acids which would not be recognized at all by the ribosomes.
  3. They are toxic, remember what I said about most sugars not usually behaving as in example 1, well, what happens here is that your molecular if they can bind to enzymes, but this can not alter it so the enzyme is irreversibly blocked, so in the best case you lost a single enzyme perfectly functional and at worst you broke the entire molecular machinery of a mitochondrion, for example. The interaction of your bacteria will be moderated by these 3 possible reactions in different proportions, it is not impossible that you will kill all life on earth, but most likely the battery will die or nothing will happen at all. Finally, you never ask, but the only reason all life on earth is just an evolutionary whim and that is encoded in hyper conserved regions, nothing makes it impossible or even improbable that what you describe will happen, it is a 50 50 chance, but once an organism appears it will never change meaning. And no, a virus cannot infect a DNA counter-sense and most likely neither can a bacterium.
  • $\begingroup$ Good summary. I think you got your L's and D's reversed in your glucose example. L-aminos are the ones we use. $\endgroup$ Jul 14 at 18:59
  • $\begingroup$ @RobertRapplean Not bad, L-amino acids are the ones we use, but it's not always the case with sugars, especially glucose. D-glucose is broken down into usable pyruvate in a process that involves converting it to L-glucose. However, L-glucose is not directly recognized for pyruvate synthesis $\endgroup$ Jul 15 at 1:03

Darth Momin provided a decent answer, but you don't need to go into that much detail.

Put simply, the organisms would be mildly toxic to each other in numerous ways, but it would prevent almost all infections from crossing over. Entities with inverse aminos wouldn't have an issue with normal interaction, or even touching. Sex might be a bad idea, and you wouldn't want to eat them.

D-amino acids block the pathways that L-amino acids would use, decreasing bioavailability. D-aminos are actually fairly common in our environment, but bacteria in our soil preferentially breaks it down to free up nitrogen, thus reducing our exposure to it.


  • $\begingroup$ "Sex might be a bad idea" I think sex with aliens might range from awkward to dangerous even if their aminoacids were like ours. $\endgroup$ Jul 14 at 18:53
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    $\begingroup$ @TheSquare-CubeLaw, I wish the rest of the world were so rational. Ever watch Galaxy Quest? $\endgroup$ Jul 14 at 18:55
  • $\begingroup$ I wouldn't be so sure about the gender, you definitely don't want to eat them, but depending on their anatomy, sex could be a good idea after all. There couldn't be pregnancies or STDs. $\endgroup$ Jul 15 at 1:13
  • $\begingroup$ @RobertRapplean I'll definetely have a look at that movie. $\endgroup$ Jul 15 at 17:21
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    $\begingroup$ @darthmomin I agree about pregnancies. But as for STD's, only the viral ones would be at lower or zero risk of transmission. Bacterial and fungal ones might be just at the same risk. $\endgroup$ Jul 15 at 17:25

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