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What is the scientific foundation for the hypothesis that human space exploration accidentally could spread Earth life to e.g. Mars and cause contamination which is indistinguishable from any indigenous Martian life?

The argument I know goes that if we later detect life on Mars, we wouldn't know if it is our own contamination or if it is indigenous Martian life which happens to be indistinguishable from Earth life. But that is obviously not a valid argument! Proteins are chains of hundreds to tens of thousands of 20 different amino acids. For any protein length, there are at between 20^100 to 20^30000 possible variants. So we know for sure that two similar proteins never occur independently anywhere in the visible universe.

Note that "convergent evolution" doesn't even come in to play here as a potential trend to uniformity, since variation and selection only works on actually existing proteins, not any of the possible but never existing proteins. Indigenous Martian life will very very easily be distinguishable from Earth life.

Are there any other arguments?

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    $\begingroup$ Commenting because I can't make the tag edit: the term planetary defense refers to protecting Earth from asteroid impacts. This post is about planetary protection. $\endgroup$
    – Chris
    Commented May 13, 2015 at 0:33
  • $\begingroup$ @Chris I have removed it now. I thought it included defence against biological contamination, solar activity and other less known cosmic events too. $\endgroup$
    – LocalFluff
    Commented May 13, 2015 at 1:25

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If life on Earth and on Mars have a common origin (a microbial hitchhiker on impact debris, for example), contamination could obscure that fact.

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  • $\begingroup$ But the biggest question in astrobiology today is independent origins. And independent evolution would yield different proteins, especially in such different environments and with such a freaky impact selection as origin. We would clearly know whether it is contamination from Earth during the 21th century, or 0.1 billion years ago. (And of course still know that it has the same origin as life on Earth) $\endgroup$
    – LocalFluff
    Commented May 11, 2015 at 18:58
  • $\begingroup$ If we're talking about plentiful live organisms, sure, but if we have only DNA fragments in fossils, we may not have complete long proteins to work with. $\endgroup$
    – Russell Borogove
    Commented May 11, 2015 at 19:33
  • $\begingroup$ That would have to be very short DNA fragments in that case. With four base pairs the probability is 4^length and the length is 3 times protein length in terms of amino acids. And using the same base pairs, and all with the same chirality. If we only find DNA shorter than 10 base pairs (4^10 is already one in a million, for one single such find), then there's no reason to interpret that as anything but recent contamination anyway. More likely to have occurred abiotically than ever having been part of an evolutionary process with random mutations. $\endgroup$
    – LocalFluff
    Commented May 11, 2015 at 20:13
  • $\begingroup$ Remember there's a bit of a birthday effect, so you might only be comparing 10 base pairs, but you'll be comparing that to a whole lot more from Earth. The old monkey banging on a typewritter idea comes to mind. $\endgroup$
    – PearsonArtPhoto ♦
    Commented May 12, 2015 at 10:43
  • $\begingroup$ Am I right also that base pairs aren't evenly distributed? IANABiologist, but IIRC, triads of base pairs specify aminos, with 64 BP triads mapping to 20 aminos plus a few "stop codes", and some are a lot more common than others -- so straight-up 4^x math doesn't necessarily hold? $\endgroup$
    – Russell Borogove
    Commented May 12, 2015 at 15:20
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You are correct that our current technology (e.g. polymerase chain reaction) allows us to easily distinguish and discriminate among life forms on Earth, and would allow us to readily identify Earth-derived life forms on Mars, should they survive. Even if Earth and Mars life ultimately have a common origin, we could still distinguish them from each other as we distinguish archaea from bacteria on Earth.

However, another potential impact of introducing Earth life would be to take over an ecosystem and cause the Mars life to become extinct before we had a chance to detect and analyze it. I consider this scenario to be rather unlikely, since the putative Martian organisms have had the history of the planet to evolve to their environment. Still, unlikely is not zero.

Lastly, if Earth and Mars life manage to coexist and they do not have a common origin, we could indeed detect the Earth life, but its presence would confound our ability to examine the Mars life in the same sample by adding a lot of noise on top of the signal. It wouldn't be impossible, just much more difficult. (Maybe we could develop an Earthicide that kills Earth life but not Mars life. If we do though, we should be careful with that stuff.)

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  • $\begingroup$ I'm sorry that I've failed to make my point. That's why all attempts of an answer miss the point. "we could still distinguish them from each other as we distinguish archaea from bacteria on Earth." No. Virus are pre-archea, if anything, and half of your DNA is identical to virus DNA, because virus have injected it into you, since billions of years. All life on Earth is very intimately the same family. Every cell, without any single exception. And not even a single one of our 20,000 different protein molecules can ever have existed anywhere anytime, but here since our common living origin. $\endgroup$
    – LocalFluff
    Commented May 13, 2015 at 1:39
  • $\begingroup$ And you failed again in that comment. I couldn't make heads or tails out of that. $\endgroup$
    – Mark Adler
    Commented May 13, 2015 at 1:47
  • $\begingroup$ Well, out of a collection of 20^10000 or so possible proteins, please let us make a pick of 10^6 of them. I bet against you that you and I pick anyone which are not obviously indistinguishable. I could let you and everyone do it always, and I will always bet against you ever coming up with anything similar to my own random pick. $\endgroup$
    – LocalFluff
    Commented May 13, 2015 at 1:52
  • $\begingroup$ Too many negatives. Against ... not ... indistinguishable? Still couldn't figure it out, nor the relevance to the question. Is your point that different proteins are different?? $\endgroup$
    – Mark Adler
    Commented May 13, 2015 at 3:40
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    $\begingroup$ My answer says that it can't be. Perhaps you didn't read my answer. $\endgroup$
    – Mark Adler
    Commented May 13, 2015 at 5:31
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I can think of 3 reasons. The third is probably most important:

1.

The point made by @Russell Borogove about common origin is one reason. We still don't know that much about the origin of life on Earth, so we certainly cannot rule out that possibility.

2.

Another point is that we don't really know what alien life would look like. The only life we know of is Earth life. It could be possible that Earth-like life could be very likely to occur elsewhere as well and that there aren't that many lifeforms that are truly unlike Earth life. I wouldn't necessarily say that two similar proteins would never occur independently anywhere in the visible universe. Convergent evolution show us that different lifeforms can evolve similarly given the same constraints. Proteins perform certain functions and would probably look similarly to other proteins that serve the same function.

3.

A third point is that we are looking for evidence of life rather than the actual DNA strings. This could be current or extinct (We don't have Dino DNA). Earth-life could invade Mars and change the landscape and chemistry to such an extent that we would not be able to determine if it was altered through Earth or Mars life.

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  • $\begingroup$ 1. You haven't considered the combinatorics involved here. We know for sure that two identical proteins never have or will exist independently in the visible universe. 2. Evolution cannot "converge" proteins that never exist. That phenomenon is exclusively internal to a family of life with the same origin. A fossilized bug on Mars could look much like one on Earth. Functions converge, but sequences on macromolecules don't. 3.Good point! But wouldn't recent contamination by humans be easily distinguishable from billion year old traces? $\endgroup$
    – LocalFluff
    Commented May 12, 2015 at 10:54
  • $\begingroup$ Why do you think convergent elvolution could not happen on two planets with seperate origins? There may be some underlying principles universal to life in our solar system (or even universe) that lead to proteins having similar structures. $\endgroup$
    – Marius
    Commented May 13, 2015 at 12:40

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