In my opinion, if we discover some life on Mars, but biochemical analysis says that it is compatible with Earth life (uses DNA, same chirality on the molecules, clear analogs with Earth life in the design, probably could even find an ancestor), we have to stick with the theory that Mars did not originate life. If Mars did independently originate life, it should be clearly obvious.
"Mars is able to natively host small amounts of Earth-derived life" is still interesting and still says some things about the prospect of life in the universe, but I don't think that would really add much to what we already know, the way a truly independent branch of life would.
Good cases have been made that RNA and DNA may be the only suitable bases for life, which would imply that finding those wouldn't be absolute proof, but there's enough other stuff in Earth's base DNA catalog that should be different that sequencing should immediately reveal a different origin. (Not to mention the different starting conditions should result in other choices for basic functioning.)
(It is not necessary that RNA/DNA be the only choices for life; it simply means that if the arguments are even partially true it tweaks the probability that truly foreign life might still be RNA/DNA based up. But above that level I'd expect significant differences; "ontogeny recapitulates phylogeny" may be false at the macro level, but billion-year-old decisions of life are still written into our DNA at deep levels. Truly independent life should have made at least some different decisions.)
Well, thanks for that, but I did read your entire comment and it was an honest question. Your third paragraph, for starters, includes a lot of "should" and handwaving.
In what ways were the starting conditions different?
Earth underwent radical changes in its environment in its early history. Did those changes -- from a methane to an oxygen-rich atmosphere for example -- result in radically different RNA or DNA sequences for the bacteria from each epoch?
Why should different starting conditions result in different forms of basic DNA anyway? Wouldn't the basic problems of evolutionary viability be the same?
There is indeed lots of stuff in Earth's base DNA catalog -- including some terrestrial critters that would do just fine on Mars, which would suggest that they could have independently evolved there. Tardigrades are one such popular example.
Your very last sentence is, "Truly independent life should have made at least some different decisions", but there's that word again: should. So, again: why?
On a completely different side note: this is part of the reason that anti-intellectualism is so rampant in America today. Those that have the combination of both ignorance and intellectual curiosity too often get met with sarcasm and indifference by people who then complain that nobody takes their opinion seriously.
"Did those changes -- from a methane to an oxygen-rich atmosphere for example -- result in radically different RNA or DNA sequences for the bacteria from each epoch?"
It certainly left an imprint. I can't google up a concrete reference, but there are genes in our genome that come from the era when there was a lot more methane in the general environment meant to essentially put the organism in hibernation when they temporarily encounter a certain type of high-methane environment.
How "radically different" it is depends critically on your definition of "radically different". It is certainly detectably different, which is more to my point. Sequencing a Martian lifeform without those markers would stand out and provide very strong evidence that they don't come from a heritage that had that in its past. (I have no idea how widespread those genes are, so I don't know how much Earth life that would eliminate as possible ancestors.)
"Why should different starting conditions result in different forms of basic DNA anyway? Wouldn't the basic problems of evolutionary viability be the same?"
Genes code for chemistry. Different starting chemistry implies different solutions for problems encountered early in life's existence. Not to mention there are multiple solutions to problems, and the odds of life choosing the same initial solutions are just negligible.
You can see this if you play with evolutionary programming, and look at the resulting programs. You can see it in conventional genetic programming, too, if the problem has many unrelated but equally appropriate solutions. In fact, you can see it in that music link we had a couple days ago. Any given evolutionary run tends to create a population that congregates in a relatively small part of the space, regardless of how many other viable paths there are. Evolution's ability to escape local optima is greatly oversold in the popular press.
"So, again: why?"
Because the probability is so long against it. Why would two 100-fair-coin-flip series ever be the same? It's theoretically possible, but you'll never see it.
Basically, what my point boils down to is that a Martian life form won't fit into the phylogenetic tree of life. See http://www.talkorigins.org/faqs/comdesc/phylo.html , which doesn't relate the tree directly to the topic at hand IIRC, but you should see how it relates with some thought. Something that does fit into the phylogenetic tree very probably came from Earth.
"Those that have the combination of both ignorance and intellectual curiosity too often get met with sarcasm and indifference by people who then complain that nobody takes their opinion seriously."
I call foul. You grunted a one word question to a pull quote, a "why" that I answered two paragraphs later. If I had had no answer, I would have understood. This post of yours is not unreasonable, asking for specific further elaboration, but just "why?" didn't cut it. (You prompted me to remember the phylogenetic tree page, which puts my point on much firmer footing, which I appreciate.)
Also, since I can smell the "But you can't prove any of that" coming, I would say, prove for me that life will exactly evolve the same way on Mars that it would on Earth. Actually, I explicitly won't hold you to that, because we're all speculating and handwaving. Even a biochemist of fifty years practice is just speculating and handwaving. Until we start building artificial life forms and start really exploring the space of "valid life forms" neither you nor I nor anybody have the actual data in hand. (We're just barely at the threshold of artificial life and by no means have we even begun exploring the space of possible life.) However, if you go back to my original post, I actually make a judgment call on probabilities, so it's not like that's news to me that it won't be proof.
Thanks for the explanation and link. I wasn't familiar with the term "phylogenetics" -- although it's not too far removed from concepts in high school biology.
> Also, since I can smell the "But you can't prove any of that" coming...
Not from me. I just wanted more information, not an argument.
It's contingent on getting it, of course. Ideally from the planet itself.
There will have to be something to test, though; if there is no biological material to test for anything I'm going to have to lean in favor of unknown-but-non-life process for generating the images I've seen so far. If the structures had visible complex structure I'd buy life (clearly a skeleton, no matter how mineralized by fossilization, was not produced by a purely unliving process), but I'm logically concluding they don't have any more than what we can see in the pictures or the scientists would have (correctly) already announced it was life.
Small amounts of biological material can't conclusively prove origin from Earth, but might conclusively prove non-terrestrial origin if for instance all the chirality were flipped. (That would both prove biological processes were at work, since randomly created amino acids have random chirality, and that it was non-terrestrial since our entire ecosystem works on only one of the two forms; anything that exclusively used the other would satisfy me as alien.)
