# Would the discovery of DNA based life on Mars be a minor disapointment? **URL:** \ **Category:** Miscellaneous and Personal Stuff I Must Share\ **Created:** [April 21, 2015, 10:36am UTC](https://boards.straightdope.com/t/would-the-discovery-of-dna-based-life-on-mars-be-a-minor-disapointment/718171 "2015-04-21T10:36:50Z")\ **Posts on this page:** 6\ **Page:** 2
**Author:** ![panache45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/panache45/32/64_2.png) [@panache45](https://boards.straightdope.com/u/panache45)\ **Post date:** [April 22, 2015, 6:39am UTC](https://boards.straightdope.com/t/would-the-discovery-of-dna-based-life-on-mars-be-a-minor-disapointment/718171/21 "2015-04-22T06:39:26Z")
> [@Ludovic](#): > > It might even be exciting to find decades-old contamination, if it’s been able to mutate enough to be a wildly divergent strain. “DECADES-old”??? Do you understand how short a decade is? Do you understand how long things take to mutate that much? It’s not like contamination that occurred back in the 80s has become some freakish monster. ---
**Author:** ![MEBuckner](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mebuckner/32/2896_2.png) [@MEBuckner](https://boards.straightdope.com/u/MEBuckner)\ **Post date:** [April 22, 2015, 7:27am UTC](https://boards.straightdope.com/t/would-the-discovery-of-dna-based-life-on-mars-be-a-minor-disapointment/718171/22 "2015-04-22T07:27:22Z")
> [@TimeWinder](#): > > The chances against as complex and specific a molecule as DNA arising twice in two planets next to each other without a common origin can be safely stated as zero. It’s not just improbable, it’s macroscopic-quantum-underwear-removal improbable. IANA molecular biologist, but as I understand it, there might be DNA-based life which is nonetheless not related to Earth life. First off, while DNA and RNA are massive and complex molecules, much of that complexity arises from the fact that they are made of very long strings of relatively simple subunits ([nucleotides](https://en.wikipedia.org/wiki/Nucleotide)); because those nucleotides can vary (guanine, adenine, thymine, or cytosine for DNA; guanine, adenine, uracil and cytosine for RNA), you wind up with something a bit like the (relatively) simple 26-letter English alphabet which can nonetheless produce an essentially infinite number of different sentences. Nucleotides are apparently relatively easy to synthesize and [might have arisen spontaneously](https://en.wikipedia.org/wiki/Miller%E2%80%93Urey_experiment#Other_experiments). The nucleotides are (roughly) like letters, which all life on Earth uses to spell out “words” (mostly proteins) in [the same “language”](https://en.wikipedia.org/wiki/Genetic_code); so [cytosine-adenine-thymine codes for the amino acid histidine](https://en.wikipedia.org/wiki/DNA_codon_table). There are only minor variations in the genetic code used by humans, whales, trees, or bacteria. The “letters” (guanine, adenine, thymine, or cytosine) spell out “words”–amino acids–but as I understand it, which nucleotide triplet codes for which amino acid is probably essentially arbitrary. On some other planet, cytosine-adenine-thymine might code for glutamic acid, or for some [amino acid that isn’t even used to make proteins by life on Earth](https://en.wikipedia.org/wiki/Non-proteinogenic_amino_acids). Loosely, one can analogize this to the way the letters of the same alphabet (the nucleotides) can be used to spell out words in Turkish, even though Turkish is not known to be related to English at all. If Martian life uses DNA _and_ uses the same genetic code, it’s probably genetically related to Earth life (panspermia), but if Martian life used DNA but with a different genetic code, life may well have arisen independently on the two planets. ---
**Author:** ![Ludovic](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@Ludovic](https://boards.straightdope.com/u/Ludovic)\ **Post date:** [April 22, 2015, 10:32am UTC](https://boards.straightdope.com/t/would-the-discovery-of-dna-based-life-on-mars-be-a-minor-disapointment/718171/23 "2015-04-22T10:32:33Z")
> [@panache45](#): > > “DECADES-old”??? Do you understand how short a decade is? Do you understand how long things take to mutate that much? It’s not like contamination that occurred back in the 80s has become some freakish monster. I didn’t say they would become a freakish monster. I said they may have mutated enough to be interesting (and most likely would need to in order to survive). Microbes could have gone through thousands of generations, with the stress of a new environment _potentially_ driving new adaptations. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [April 22, 2015, 3:35pm UTC](https://boards.straightdope.com/t/would-the-discovery-of-dna-based-life-on-mars-be-a-minor-disapointment/718171/24 "2015-04-22T15:35:21Z")
> [@lazybratsche](#): > > … > 2. The DNA sequence appears to have a common ancestor with something that exists on earth. That implies a relatively simple one-way panspermia scenario, where some bit of ejecta from Earth happened to carry an organism that then survived and thrived on Mars. That’d be interesting enough to keep a few scientists busy for their entire career. > 3. The DNA includes multiple sequences with different common ancestors on Earth. An expansion of scenario 2, but demonstrating frequent and sustained colonization of Mars by something from Earth. > … Overall I agree with your breakdown and with your conclusions. But I question how you can say the causality of #s 2 & 3 necessarily flows from Earth to Mars. IOW, #2 says “There very likely existed a common Earth/Mars ancestor at some point long ago.” How does that force Earth to be the donor and Mars the recipient? ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [April 22, 2015, 4:08pm UTC](https://boards.straightdope.com/t/would-the-discovery-of-dna-based-life-on-mars-be-a-minor-disapointment/718171/25 "2015-04-22T16:08:28Z")
If we discovered non-DNA based life, we could be quite, quite certain that it descended from a non-Earth abiogenesis event; or at least a different abiogenesis event that led to modern Earth life. Very very cool. If we discovered DNA-based life, we’d need to do a lot of studying to figure out whether or not it’s related to us. If it turned out to be unrelated, that would tell us that life, for some reason, really likes to use DNA, which would also be interesting. ---
**Author:** ![lazybratsche](https://avatars.discourse-cdn.com/v4/letter/l/ba8739/32.png) [@lazybratsche](https://boards.straightdope.com/u/lazybratsche)\ **Post date:** [April 22, 2015, 7:08pm UTC](https://boards.straightdope.com/t/would-the-discovery-of-dna-based-life-on-mars-be-a-minor-disapointment/718171/26 "2015-04-22T19:08:37Z")
> [@LSLGuy](#): > > Overall I agree with your breakdown and with your conclusions. > > But I question how you can say the causality of #s 2 & 3 necessarily flows from Earth to Mars. > > IOW, #2 says “There very likely existed a common Earth/Mars ancestor at some point long ago.” How does that force Earth to be the donor and Mars the recipient? Sorry, I wasn’t clearly describing the scenarios I had in mind. I was imagining something closer to “There very likely existed a common Earth/Mars ancestor at a very specific time in the history of life on Earth.” E.g. we sequence the critters and find that they diverged from, say, a specific clade of sulfur-reducing proteobacteria that arose on Earth 300-400 MYA. (I’m completely making up that example. I’m sure someone else has done a thorough study of which extremophiles on Earth might have the best chance of surviving on Mars.) [Previous page](https://boards.straightdope.com/t/would-the-discovery-of-dna-based-life-on-mars-be-a-minor-disapointment/718171.md?page=1)