<p>If any amino acid chain on Mars is the same or similar to any amino acid chain on Earth, then we know with final certainty that they have the same origin. Simple combinatorics proves it within as many sigmas you wish. Do the math. A median protein on Earth is about 300 amino acids long. That combination has never occurred by chance anywhere in the visible universe ever. Evolution has no chance to converge them, because evolution can only test that which occurs by chance.</p>

CC BY-SA 3.0
2016-09-30T21:21:44.283Z
I would think that it could be possible that life never originated on Mars, but we could find evidence of life that came from Earth. What I would say to that is that such a find would be very localized whereas, if we are all Martians, then evidence would be all over Mars (albeit buried in the ground).
Jack R. Woods
CC BY-SA 3.0
2016-10-01T06:42:42.440Z
@zibadawatimmy But it is not about likeliness. Evolution can only try that which exists. Evolution is only applicable on those amino acids and genes which happened to have formed here for unknown reasons. Evolution doesn't care about what would've been possible but never materialized. Maybe a simple mutation would cure cancer. But if it never happens evolution is totally blind for that opportunity. And obviously almost none of the possible life forms ever happened. The universe is too small and temporary.
LocalFluff
CC BY-SA 3.0
2016-10-01T17:01:45.627Z
@LocalFluff The objection is that you cannot reasonably assert what is "possible" beyond pointing at what has already happened.  The earth constitutes a single data point of how amino-acid using life can happen.  It's conceivable that every other such event will develop an identical set of amino acids, unless you can point to clear, scientific evidence to prove otherwise.  Just because you can arrange a bunch of other acids in a lab or in theory doesn't mean they can arise in actual practice, on a planet, or how likely they are to arise.
zibadawa timmy
CC BY-SA 3.0
2015-10-09T18:34:07.707Z
Of course, knowing that two things share a single origin does not tell us where that origin is.
Wayfaring Stranger
CC BY-SA 3.0
2015-10-09T18:55:25.260Z
@WayfaringStranger Indeed, finding life on Mars might very well leave us none the wiser.
LocalFluff
CC BY-SA 3.0
2015-10-09T23:40:44.770Z
This seems to make certain assumptions about the probabilities of particular amino acid chains arising, given that any arise at all.  These sound unprovable to me.  I get the impression that you are considering all possible amino acid chains--especially ones that could arise in whatever we might call "life"--as equally likely.  Do you have a justification for this?
zibadawa timmy
CC BY-SA 3.0
2015-10-10T02:57:05.583Z
@zibadawatimmy Just combinatorics. Evolution can only select what occurs by chance mutation. The average length protein is a sequence of 300 out of 22 amino acids. The shortest is longer than 100, the longest is over 30 000. And 22^100 is a number larger than what can occur twice by chance in the history of the visible universe. There cannot independently exist two similar proteins on Earth and Mars. For example, there are only about 10^31 virus particles in the biosphere. They could never twice by chance mutate the same protein if they didn't have the same origin.
LocalFluff
CC BY-SA 3.0
2015-10-10T08:28:44.653Z
The really fundamental test would be if they used RNA the same way.  It would be too much coincidence for the replicating molecules to end up the same on both planets.  Probably RNA is even more fundamental than protein or DNA (look under 'RNA world').
AlaskaRon
CC BY-SA 3.0
2015-10-10T13:32:43.660Z
@LocalFluff I feel like that still presumes knowledge about the chances of various mutations at any given stage (is absolutely every one of the $22^{100}$ chains equally likely? or are the ones we know inordinately more likely?), but that's at least sounding more plausible.
zibadawa timmy
CC BY-SA 3.0
2015-10-10T14:28:59.007Z
It'd be cool to see differences in codon table usage between earth and mars, but a couple billion years is too long to have any reasonable expectation of recovering such info. To get *good* data, we'll need to recover some perchlorate loving slime growing next to a martian water seep. Likely have to do analysis on site too because folks'd worry about Space Plague: http://www.goodreads.com/book/show/2103647.Plague_from_Space
Wayfaring Stranger
CC BY-SA 3.0
2015-10-09T18:04:40.227Z
2015-10-09T18:04:40.227Z
LocalFluff
<p>We'd analyze the DNA and RNA sequences (assuming they had any) and the coding of amino acids, and compare them with the sequencing and coding on Earth. As far as we are evolutionary from Archaea and Bacteria, we're still a lot closer than an alien life form would be.</p>

CC BY-SA 3.0
2017-09-13T01:42:45.213Z
2017-09-13T01:42:45.213Z
Syd Henderson
<p>In addition to already present good answers, I would say that if we found life on Mars on a century there is a noticeably probability of it being made from <a href="https://en.wikipedia.org/wiki/Planetary_protection" rel="nofollow noreferrer">Earth microbes accidentally taken there by one of our probes</a> or their descendants. Risk of contamination of other planets is addressed by carefully sterilization of probes but no sterilization is complete, so there is always a small risk.  </p>

CC BY-SA 3.0
2017-09-13T01:47:04.987Z
Could they not simply carbon date the fossil and show it was older than all the fossils on earth?
Tim
CC BY-SA 3.0
2016-09-30T14:55:55.217Z
2017-09-13T07:40:02.460Z
2017-09-13T07:40:02.460Z
Pere
<blockquote>
  <p>Why would we assume that the early martian life originated on Mars,
  rather than Earth?</p>
</blockquote>

<p>There's still a whole lot we don't know.  As Wayfaring stranger points out in the comments, Origin is a whole different question.  It's possible that life originated outside our solar system and came to either Mars and/or Earth from outside the solar-system.   I don't think anyone who studies this idea is "Assuming" life originated on Mars, only that the idea has a chance of being true.</p>

<blockquote>
  <p>Is it not likely that somewhere between 2.5-3 billion years ago, a
  major volcanic eruption or meteor impact could have hit an earth
  teeming with simple-life and sent it hurdling through space to Mars?</p>
</blockquote>

<p>As I understand it, volcanic eruptions are unlikely to send anything into space unless it's a smaller sized moon.   The escape velocity (young earth, maybe 10 km/s, young mars, maybe 3-4), volcanic eruptions, as far as I know, don't shoot things out at 10,000 - 20,000 MPH.   But meteor impacts of sufficient size can do that.  </p>

<p>Mars is a better meteor debris making target than the Earth cause it's smaller, so the gravity is lower and presumably it's mostly had a thinner atmosphere too.  We've found martial meteors on Earth.   We might not find any Earth meteors on mars cause it takes a much bigger impact to knock bits of rock off Earth and because the Atmosphere slows objects down, both coming in and going out.</p>

<blockquote>
  <p>I understand that my theory has the gravity of the sun working against
  it, but I also see it not being entirely impossible.</p>
</blockquote>

<p>The sun isn't as big a factor as you might think.  Once something is knocked off a planet and it gets into orbit around the sun, gravitational assists can move it around further out or further inside the solar system.  What presumably happens is that, with a big enough impact, many thousands if not millions of bits of debris get into solar-system orbit and from there, some of them land on other planets - probably much less than 1% of those hit Earth, but if it carries life that can survive the trip, all you need is one rock.</p>

<blockquote>
  <p>I mean, 100 years ago, one could assume that life originating on Mars
  and then being sent this was was ridiculous and impossible but now
  it's a rational, viable theory.</p>
</blockquote>

<p>While that's true, the "People used to think this was impossible" isn't a scientific approach for what might be true.   We should determine what is possible and/or thought to be likely, based on physical evidence, not what wasn't understood 100 years ago.   Your example is a good point on why it's important to keep an open mind about the unknown.   You can still make theories based on evidence, and keep an open mind on the unknown.  There's really no conflict between the two.</p>

<blockquote>
  <p>Considering such a scenario, how would scientist verify that these
  life-forms didn't originate from Earth, rather than Mars? It seems to
  me that this question is the first question that would need to be
  asked and answered after the fossil-records were found and verified.</p>
</blockquote>

<p>It's a good question.   </p>

<p>The simple answer is that Mars cooled first and Mars (likely) had oceans first, so it's a better candidate to have developed life first though extremophiles can live in hot oceans, so . . . time will tell.</p>

CC BY-SA 3.0
2015-10-09T21:40:50.017Z
2015-10-09T21:40:50.017Z
userLTK
<p>Consider this scenario.  One of the probes sent to Mars either very soon, or within the next century, finds conclusive micro-fossil evidence that life once existed on Mars.</p>

<p>One theory today that is quite common is that life might that have arisen on Mars was then ejected into space, floated to Earth and then seeded life on our planet.</p>

<p>I'm tracking with you and that could make sense, assuming we found the conclusive evidence for life on Mars.  However, why would we assume that the early martian life originated on Mars, rather than Earth?  Is it not likely that somewhere between 2.5-3 billion years ago, a major volcanic eruption or meteor impact could have hit an earth teeming with simple-life and sent it hurdling through space to Mars?</p>

<p>I understand that my theory has the gravity of the sun working against it, but I also see it not being entirely impossible.  I mean, 100 years ago, one could assume that life originating on Mars and then being sent this was was ridiculous and impossible but now it's a rational, viable theory.</p>

<p>Considering such a scenario, how would scientist verify that these life-forms didn't originate from Earth, rather than Mars?  It seems to me that this question is the first question that would need to be asked and answered after the fossil-records were found and verified.</p>

CC BY-SA 3.0
2015-10-09T17:22:37.873Z
What is your astronomy or astrophysics question? Actually, it is generally thought that it would work the other way around, since Mars was "habitable" before Earth. We might all be Martians.
ProfRob
CC BY-SA 3.0
2015-10-09T17:55:26.480Z
Question seems more appropriate for a biology forum or something. I'm kind of curious to hear the answer from someone knowledgeable in that field.
Florin Andrei
CC BY-SA 3.0
2015-10-09T18:42:56.003Z
@FlorinAndrei I asked this hear, because this is where it seemed most appropriate.  This is a question that would likely span astronomy, physics, geology and biology.
RLH
<earth><mars><life><origin-of-life>
If we found evidence of life on Mars, how would we know that it originated on Mars rather than Earth?
RLH
# If we found evidence of life on Mars, how would we know that it originated on Mars rather than Earth?

(Asked by: username_0 on 2015-10-09)

Consider this scenario. One of the probes sent to Mars either very soon, or within the next century, finds conclusive micro-fossil evidence that life once existed on Mars.

One theory today that is quite common is that life might that have arisen on Mars was then ejected into space, floated to Earth and then seeded life on our planet.

I'm tracking with you and that could make sense, assuming we found the conclusive evidence for life on Mars. However, why would we assume that the early martian life originated on Mars, rather than Earth? Is it not likely that somewhere between 2.5-3 billion years ago, a major volcanic eruption or meteor impact could have hit an earth teeming with simple-life and sent it hurdling through space to Mars?

I understand that my theory has the gravity of the sun working against it, but I also see it not being entirely impossible. I mean, 100 years ago, one could assume that life originating on Mars and then being sent this was was ridiculous and impossible but now it's a rational, viable theory.

Considering such a scenario, how would scientist verify that these life-forms didn't originate from Earth, rather than Mars? It seems to me that this question is the first question that would need to be asked and answered after the fossil-records were found and verified.

**Tags:** earth, mars, life, origin-of-life
> **Comments:**
> **username_1**: What is your astronomy or astrophysics question? Actually, it is generally thought that it would work the other way around, since Mars was "habitable" before Earth. We might all be Martians.  
> **username_2**: Question seems more appropriate for a biology forum or something. I'm kind of curious to hear the answer from someone knowledgeable in that field.  
> **username_0**: @FlorinAndrei I asked this hear, because this is where it seemed most appropriate.  This is a question that would likely span astronomy, physics, geology and biology.  

---

## 4 answers

### Answer by username_11 on 2015-10-09. Score: 9 | Accepted answer

> Why would we assume that the early martian life originated on Mars,
> rather than Earth?

There's still a whole lot we don't know. As Wayfaring stranger points out in the comments, Origin is a whole different question. It's possible that life originated outside our solar system and came to either Mars and/or Earth from outside the solar-system. I don't think anyone who studies this idea is "Assuming" life originated on Mars, only that the idea has a chance of being true.

> Is it not likely that somewhere between 2.5-3 billion years ago, a
> major volcanic eruption or meteor impact could have hit an earth
> teeming with simple-life and sent it hurdling through space to Mars?

As I understand it, volcanic eruptions are unlikely to send anything into space unless it's a smaller sized moon. The escape velocity (young earth, maybe 10 km/s, young mars, maybe 3-4), volcanic eruptions, as far as I know, don't shoot things out at 10,000 - 20,000 MPH. But meteor impacts of sufficient size can do that.

Mars is a better meteor debris making target than the Earth cause it's smaller, so the gravity is lower and presumably it's mostly had a thinner atmosphere too. We've found martial meteors on Earth. We might not find any Earth meteors on mars cause it takes a much bigger impact to knock bits of rock off Earth and because the Atmosphere slows objects down, both coming in and going out.

> I understand that my theory has the gravity of the sun working against
> it, but I also see it not being entirely impossible.

The sun isn't as big a factor as you might think. Once something is knocked off a planet and it gets into orbit around the sun, gravitational assists can move it around further out or further inside the solar system. What presumably happens is that, with a big enough impact, many thousands if not millions of bits of debris get into solar-system orbit and from there, some of them land on other planets - probably much less than 1% of those hit Earth, but if it carries life that can survive the trip, all you need is one rock.

> I mean, 100 years ago, one could assume that life originating on Mars
> and then being sent this was was ridiculous and impossible but now
> it's a rational, viable theory.

While that's true, the "People used to think this was impossible" isn't a scientific approach for what might be true. We should determine what is possible and/or thought to be likely, based on physical evidence, not what wasn't understood 100 years ago. Your example is a good point on why it's important to keep an open mind about the unknown. You can still make theories based on evidence, and keep an open mind on the unknown. There's really no conflict between the two.

> Considering such a scenario, how would scientist verify that these
> life-forms didn't originate from Earth, rather than Mars? It seems to
> me that this question is the first question that would need to be
> asked and answered after the fossil-records were found and verified.

It's a good question.

The simple answer is that Mars cooled first and Mars (likely) had oceans first, so it's a better candidate to have developed life first though extremophiles can live in hot oceans, so . . . time will tell.


---

### Answer by username_3 on 2015-10-09. Score: 2

If any amino acid chain on Mars is the same or similar to any amino acid chain on Earth, then we know with final certainty that they have the same origin. Simple combinatorics proves it within as many sigmas you wish. Do the math. A median protein on Earth is about 300 amino acids long. That combination has never occurred by chance anywhere in the visible universe ever. Evolution has no chance to converge them, because evolution can only test that which occurs by chance.

> **Comments:**
> **username_6**: Of course, knowing that two things share a single origin does not tell us where that origin is.  
> **username_3**: @WayfaringStranger Indeed, finding life on Mars might very well leave us none the wiser.  
> **username_5**: This seems to make certain assumptions about the probabilities of particular amino acid chains arising, given that any arise at all.  These sound unprovable to me.  I get the impression that you are considering all possible amino acid chains--especially ones that could arise in whatever we might call "life"--as equally likely.  Do you have a justification for this?  
> **username_3**: @zibadawatimmy Just combinatorics. Evolution can only select what occurs by chance mutation. The average length protein is a sequence of 300 out of 22 amino acids. The shortest is longer than 100, the longest is over 30 000. And 22^100 is a number larger than what can occur twice by chance in the history of the visible universe. There cannot independently exist two similar proteins on Earth and Mars. For example, there are only about 10^31 virus particles in the biosphere. They could never twice by chance mutate the same protein if they didn't have the same origin.  
> **username_7**: The really fundamental test would be if they used RNA the same way.  It would be too much coincidence for the replicating molecules to end up the same on both planets.  Probably RNA is even more fundamental than protein or DNA (look under 'RNA world').  
> **username_5**: @LocalFluff I feel like that still presumes knowledge about the chances of various mutations at any given stage (is absolutely every one of the $22^{100}$ chains equally likely? or are the ones we know inordinately more likely?), but that's at least sounding more plausible.  
> **username_6**: It'd be cool to see differences in codon table usage between earth and mars, but a couple billion years is too long to have any reasonable expectation of recovering such info. To get *good* data, we'll need to recover some perchlorate loving slime growing next to a martian water seep. Likely have to do analysis on site too because folks'd worry about Space Plague: http://www.goodreads.com/book/show/2103647.Plague_from_Space  
> **username_4**: I would think that it could be possible that life never originated on Mars, but we could find evidence of life that came from Earth. What I would say to that is that such a find would be very localized whereas, if we are all Martians, then evidence would be all over Mars (albeit buried in the ground).  
> **username_3**: @zibadawatimmy But it is not about likeliness. Evolution can only try that which exists. Evolution is only applicable on those amino acids and genes which happened to have formed here for unknown reasons. Evolution doesn't care about what would've been possible but never materialized. Maybe a simple mutation would cure cancer. But if it never happens evolution is totally blind for that opportunity. And obviously almost none of the possible life forms ever happened. The universe is too small and temporary.  
> **username_5**: @LocalFluff The objection is that you cannot reasonably assert what is "possible" beyond pointing at what has already happened.  The earth constitutes a single data point of how amino-acid using life can happen.  It's conceivable that every other such event will develop an identical set of amino acids, unless you can point to clear, scientific evidence to prove otherwise.  Just because you can arrange a bunch of other acids in a lab or in theory doesn't mean they can arise in actual practice, on a planet, or how likely they are to arise.  

---

### Answer by username_8 on 2017-09-13. Score: 1

We'd analyze the DNA and RNA sequences (assuming they had any) and the coding of amino acids, and compare them with the sequencing and coding on Earth. As far as we are evolutionary from Archaea and Bacteria, we're still a lot closer than an alien life form would be.


---

### Answer by username_9 on 2016-09-30. Score: 1

In addition to already present good answers, I would say that if we found life on Mars on a century there is a noticeably probability of it being made from [Earth microbes accidentally taken there by one of our probes](https://en.wikipedia.org/wiki/Planetary_protection) or their descendants. Risk of contamination of other planets is addressed by carefully sterilization of probes but no sterilization is complete, so there is always a small risk.

> **Comments:**
> **username_10**: Could they not simply carbon date the fossil and show it was older than all the fossils on earth?  

---

