This is the first question about colonizing an alien world; the second question on agriculture on this planet is here.

So here's the idea: human beings travel to and land on an Earth-like world with the intention of colonizing it.

This world is remarkably Earth-like: gravity close to 1G, oxygen-nitrogen atmosphere with very close to the same proportions as Earth, similar climate, and similar life forms. So, in principle, a human could walk around unprotected on this world.


The life on this planet, while at a similar level of development to that of Earth, is not based on the same DNA/RNA bases or the same amino acids in proteins. In short, the lifeforms on this planet resemble Earth life, but the chemistry is different.

What would happen if a human stepped out on the surface of this planet?

My thought is that they would experience a serious (probably life-threatening) allergic reaction as they breathe in dust, bacteria, etc. with completely different proteins that they have never encountered before. But is that correct? After all, here on Earth people have allergic reactions to proteins which are made up of our familiar set of amino acids; would they have similar reactions to proteins made up of unfamiliar amino acids?

  • $\begingroup$ Are you asking if the grass would burn through their shoes, or something? $\endgroup$
    – RonJohn
    Oct 8, 2018 at 16:28
  • $\begingroup$ No, I don't expect any extreme chemical reaction here - simply alternate bases in DNA and alternate amino acids in proteins. Otherwise, analogous to the life we see on Earth. I'm more interested in the physiological reaction. $\endgroup$
    – Kryten
    Oct 8, 2018 at 16:32
  • 2
    $\begingroup$ You'll probably be interested in my old question Would humans be able to derive nutrition from foodstuffs found on alien planets? $\endgroup$
    – user
    Oct 8, 2018 at 16:43
  • $\begingroup$ Bear in mind that from an allergy point of view, the further you are away from "needed for that life form" the less likely there's an allergy to it - otherwise allergies to steel and aluminum would be rampant. It's unlikely humans would react to (e.g.) alien pollen at all - and just as unlikely the alien pollen would care anything about humans. $\endgroup$
    – JBH
    Oct 8, 2018 at 22:52
  • $\begingroup$ You may be interested in Cibola Burn, a book from the Expanse series. Featuring (amongst other things) mosquitos that drink human blood then die from it, slugs whose slime is a lethal neurotoxin (but only to humans), and clouds that make you go blind. It’s a fun book. $\endgroup$
    – Joe Bloggs
    Oct 9, 2018 at 18:04

3 Answers 3


We would probably poison alien bacteria etc, and they would poison us.

So we just walk around. The alien life would probably sample us. We have mosquitoes, mites, nematodes, and various other things that get under our skin. Presumably the alien ecology has similar things. They inject us with -- stuff. It happens a whole lot here, and if we have skin exposed, it will happen there. We'd breathe things too, exposing our mucous membranes to them. And lungs some. We have defenses to keep living things we breathe in from hurting us much. Those defenses would not be as necessary -- the alien life probably can't live in our lungs etc -- and also partly ineffective.

So stuff gets into us, and the live part of it will tend to die there. Some of it is just -- stuff. Saliva, excretion products, whatever.

We would be randomly allergic to it. We have developed allergies to things already, things that correspond to not-self stuff we have been challenged by in the past. And they might randomly correspond to alien stuff.

For example we have antibodies. These attach to anything that's the right shape with the right locations of hydrogen bonding etc. Something can fit imperfectly and still fit well enough to get a reaction. The region they fit to (called an epitope) tends to about 8 to 17 amino acids long. Sometimes it's a specific sequence of amino acids that has folded up just right, and other times it's two or more sequences that have folded with each other.

Alien proteins or other alien molecules would fit some of our antibodies. So we would react. We would itch etc. It would be a continual irritation. Probably we could mostly live with that.

Would some people occasionally die of anaphylactic shock? Maybe.

We might easily become allergic to foreign stuff. I think it would happen particularly if we get exposed to it and get problems, about every 2 weeks. Continuous exposure I think would result in less allergy. Maybe. If it's always around and usually doesn't cause trouble, it's less allergenic than if it shows up along with trouble, at 2 week intervals. So that's one thing to be careful about. Arrange the times of exposure to minimize allergies.

I'm not sure what else to say. Some of the foreign stuff might be very poisonous, and I see no way to tell which would be. Some of the foreign DNA might be highly mutagenic and carcinogenic. If your body's DNA-building machinery mistakes foreign nucleic acids for regular nucleic acids, it might incorporate them into your DNA and then mis-copy them the next time the cell replicates. That can be very bad.

Or they might interfere with regulation of various kinds. If your cells mistake a foreign amino acid or nucleic acid for one of your own -- just by the proteins that shut down production when there's too much -- then it could stop you from making stuff you need. Which you could have made plenty of if the signalling wasn't screwed.

There are lots of ways for things to go wrong and some of them could be pretty subtle. You might go years before cancer shows up, or months before you get deficiency diseases of various sorts even though whatever you are deficient in is plentifully available in your food or even in your bloodstream.

Lots of unpredictable effects possible. It might be possible to get lucky and none of them happen. Luck of the draw.


nice question :)

the short answer is things would probably go badly or well, depending on how disjoint the native molecular machinery is from our own, the ability of their microbes & plants & animals, if any of any size, to poison or infect or bore into or attack us.

our enzymes are tuned to digest proteins composed of l-amino acids, d-sugars & right-handed dna, and only a limited range within these classes. non-conforming bio-molecules will be for the most part inert, apart from possible deleterious effects. native bio-matter will not be digestible, almost certainly. the native bio-material will be of no use to us, but may be harmful to us. hence the need for sterilization.

this leads to the broader, far more interesting question of what to do with such a stupendous opportunity, should it ever present itself.

a rough sequence of the essential steps in a process of terraforming a planet might be:

  • create a well-sealed native bio-environment on an off-planet moon to preserve the native flora/fauna exhaustively - a pragmatic & moral imperative, within practical limits of space & time.

  • administer a total extermination event, down to below the lowest extremophiles on the continental and oceanic crust, to prevent any competitions with or complications from alien microbes/viruses & etc.

  • condition soil, rivers & oceans with a starter-kit of microbes, then basal fungi,animal & plant life to form a stable atmosphere and condition the environment, step-wise, for sustainable living, on the land and in the oceans. this is the hard part. humans would have to design a stable bottom-up tree of life, something far beyond our capabilities at present, i should imagine.

this process, sterilized planet to self-sustaining terran derivative, would almost certainly take millennia to effect, in a realistic time-frame. unlike a lot of science-fiction, this is a challenge humans might actually aspire to confronting some distant day. we likely have a good amount of time in which to learn the lessons needed to initiate the process of planning such an undertaking in something like adequate detail, by elaborating the process upon a lunar terran zoo.


It depends on whether these novel proteins have structural elements that allow them to interact with human enzymes and cell structures. That's basically a coin toss but I would think that there will be at least some so there will probably be some interactions.

What interactions?

  • Nulls, reactions where there is an interaction but it is shortlived and, for the human system, inconsequential.

  • Normals, the two structures interact the same way that they do on Earth. This could be fine and it could kill people, it depends on what the structures are.

  • Locks, unexpected binding that doesn't result in any reaction but locks up a receptor site for an unexpectedly long time. The real world example of this that comes to mind is Carbon Monoxide binding to Hemoglobin, it's not Oxygen or Carbon Dioxide; it fits the receptor site but not properly so it takes a lot longer than it should to unbind.

  • Breaks, catastrophic damage is caused to amino acid structures. This could kill cells and eventually cause enzyme deficiencies with prolonged exposure. Prions may be formed this way also.

  • Other, unexpected interactions that don't quite fit into any of the above. This includes direct chemical reactions that form new compounds, etc...

One would expect to see some of all of the above interactions, the exact proportions would depend on the exact structures involved and their precise concentrations. The human body is extremely adaptable but it is possible that the rate of disruption could overwhelm people's immune systems.

This answer assumes that the new environment shares the same amino acid Chirality as Earth, if it doesn't then possibly the same but actually one would expect very little interaction.


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