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Comment by interfixus | Hacker News Reader

If life - present or past - is one day found on Mars, I shall be hugely surprised if it doesn't share our basic DNA. Interplanetary pollution - pollination, if you will - or common influx from somewhere entirely else.

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On the other hand we haven't been able to establish a firm minimum amount of time from when Earth got oceans to when it got life. It took a long, long time to add photosynthesis to cemosynthesis, a long long time to create eukaryotes, and a long, long time to create multicellular life. I made a timeline:

http://hopefullyintersting.blogspot.com/2018/03/the-drake-eq...

Interplanetary pollution seems to happen pretty quickly compared to the history of life but I'm not sure we can say that it happens quickly compared to the start of life. Of course for all we know the reason Earth developed life so quickly was that it was seeded by the quicker-cooling Mars which had had Oceans before Earth did. More research is needed!

I would not presume to have any opinion on the direction such traffic might have taken. All I know is the one certain, observed fact in this case is that pieces of Mars have indeed fallen to the surface of Earth. There is of course no known reason why the reverse would not have happened.

From your writings (added to feed, thank you) I notice we share a belief: That life may well be quite common out there, but human level intelligence exceedingly rare. I somehow believed this from a very young age, long before I ever heard of Drake or his equation or any such calculation.

History repeats itself. So many times a group of people on earth thought they were alone (a tribe, a region, an island, a continent), they were wrong. Earth is our current island, scale it up. Chances are when we find other life, it will share a common source with us. There's likely other life that doesn't but the closer ones probably do.

Maybe we are the "uncontacted tribe". I wonder at what point when you're observing an isolated tribe do they demonstrate enough advances that it's OK to contact them? Or do you just wait until they discover the observer?

I doubt we are observed. Space is really really big - even bigger than that... Are first radio signals have barely reached 100 light years by now (you a great antenna to received them). All other intelligent beings know about earth is what they can infer from how our gravity changes the sun, and other such indirect measures.

If we're not currently odds are we will be--given N tribes, we're only 1.

I was just reading about how far out into space the ejecta from the Chicxulub impact went and it really brought home how possible it is that life from Earth migrated elsewhere, Mars being an obvious possible destination. But as one gets further out, it seems less and less likely that any life found has terrestrial origins. I am very much interested in knowing if there is any independently evolved life in the oceans of Europa or any of the other ice moons.

Space is pretty inhospitable though. Is it possible for living organisms to survive ejection, a space journey and re-entry?

It's an open question, but there is some reason to believe that bacteria can survive a journey through space. Multicellular life would be much less likely to survive, although I'm constantly amazed at how hard it is to kill a tardigrade!

Long journey to Mars in hard vacuum and under UV radiation makes it very unlikely, but bacteria spores are very resilient to harsh conditions. I think it is still theoretically possible. There are a few papers on this.

https://www.ncbi.nlm.nih.gov/m/pubmed/11539977/

A tardigrade must first complete cryptobiosis, otherwise they are fragile[1]. Given that it takes time to enter cryptobiosis (maybe 1 hour), it is unlikely that any active tardigrade would survive a meteor impact.

Suppose a tardigrade was in complete cryptobiosis and survived a journey to the surface of Mars. Conditions would need to be favorable if it were to ever regain an active metabolic state.

[1]https://www.quora.com/What-does-it-take-to-kill-a-waterbear-...

It's called panspermia, for the curious.

> Space is pretty inhospitable

Is it? Predation, old age (wear and tear), disease (pollution) are the primary reasons that what we consider "living things" die. Space has very limited vectors of death for something like the tardigrade. To some bug dessicated hibernating in a rock, it's not too bad.

> Predation, old age (wear and tear), disease (pollution) are the primary reasons that what we consider "living things" die.

...on Earth, where the multitude of things that make space inhospitable - both objectively, like no atmosphere to protect from cosmic radiation; and subjectively, like vastly different conditions that terrestrial life hasn't evolved for - aren't an issue.

Eventually, cosmic ray damage will even get the tardigrade.

If life is discovered on Mars, I'm going to be waiting to see if it is obviously Earth-based DNA. If so, then as excited as people will be, it won't actually be any particular sort of evidence for wide-spread life. It'd still probably be a reasonable candidate for discovery of the century, but not the millenium.

(We will be able to determine if it is has the same source as Earth-based life, because Earth-based life has a lot of things that seem like they ought to be free parameters, but are highly conserved across our entire ecosystem. This includes certain basic genes and processes and their exact "spelling" in genes, the mapping of DNA letters to proteins, and probably quite a few other things. Can't disprove a common third source that way, of course.)

Or ... do we have Mars-based DNA?

I've seen the theory tossed out there, but it's hard to do anything about it. We'd need paleontologists on Mars to really be sure (to prove life that predates Earth, as a step 1), and we're a long way away from that no matter how you slice it. But as I understand it, the time scales work; for the same reason that Mars is basically a dried-up, used up planet now while Earth is flourishing with life, Mars was theoretically possibly habitable much earlier than Earth.

We're aware of meteorites from Mars coming to Earth (a quick search brings up [0]) - who says they can't go the other way!

[0] https://www.space.com/14268-rare-mars-meteorite-rocks-tissin...

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