A common topic when I was in high school was that a lot of the kids felt DNA or the building blocks of DNA landed on earth as cosmic dust that may have traveled through the universe for many centuries before it landed.
If particles of organic matter were to land on other planets hospitable to life would they actually have any chance at all of developing into some living thing?Maybe. We already know that bacteria can survive for years exposed to hard vacuum. Tardigrades in particular are well documented. If a microorganism somehow flew through space, survived entering the atmosphere, and then landed in a place conducive to bearing life then, sure, why not?
The problem is that the origins of life have already been explained through far simpler means, and there’s no good way of testing the theory. Also, saying some random schoolchildren “felt” it might be the case is hardly a ringing endorsement.
DNA by itself isn’t useful for much. RNA polymerese (an enzyme within cells) is necessary to transcript codon sequences forming genes into RNA from which a ribosome (an intracellular ‘machine’ for building proteins) can make actual structures. The amino acid precursors not only can but do exist in interplanetary (and very likely interstellar) space. They can also be synthesized in the conditions of early Earth by readily available chemical precursors, so there is nothing special about the basic chemical components of life.
The theory of panspermia–life from extraterrstrial sources–once held some degree of sway by conveniently dismissing the problem of abiogenesis (the formation of life from ‘inert’ matter) but regardless of where it occurred the event had to occur, and there is no particular reason it couldn’t have occurred on Earth multiple times (although all extant life appears to have a single source, which is probably a result of DNA-based life having outcompeted any other pretenders).
There is no particular evidence for, or need to invoke, panspermia in order to accept the emergence into sufficienlty complex adapative chemical systems that form life as we know it. Life is certainly interesting, but because of its complexity and variety, not its origins or any mysterium involving how it functions.
Stranger
I have a related question.
The transcription of RNA triplets into amino acids is often described as a somewhat arbitrary code, implemented by enzymes. (And in a chicken-and-egg twist, those enzymes are themselves specified in RNA/DNA using the same code they implement!)
Does this mean that RNA cannot produce proteins without those enzymes, and that if it did (by waiting a long time for “good luck”) a completely different genetic code might develop?
Or, instead, is there some geometric/chemical relationship between, e.g. an AUG triplet and Methionine, that makes codes very similar to the actual Genetic Code the only likely codes?
(I do see that, if the alien DNA were associated with a protein-based life form, it wouldn’t spawn living organisms until the “correct” genetic code were chanced upon!)
I believe the answer to your first question is yes; at least, we don’t know of any reason they should be special, although there are ribozymes which can catalyze certain biochemical operations. Whether an alien form of life would also use DNA and RNA to synthesize proteins is an open question; these appear to be the most stable structures that Earth life uses to encode replication information, and there are specific reasons why this srtructure is so robust, but it is possible that some other form of life could be based on a novel polymer construction using a different coding scheme, or perhaps using different materials entirely, though no one has been able to come up with a construction that would serve as well as DNA. Then again, it has been less than a couple of decades since we’ve even been able to crudely synthesize DNA, a molecule with a well characterized structure, and we can only construct very simple proteins artificially, so we really are like infants when it comes to biology.
Even if an alien species uses DNA and RNA transcription, the probability of any infectious or biologically compatible organism or enzyme being shared between them is vanishingly small unless there is some common origin. There is such a massive variety of potential protein structures and all of Earth life is based on a very small subset of potential proteins (on the order of 100,000 proteins characterized out of essentially infinite possible combinations) and no reason to believe that the proteins we use are in any way special or unique.
“Living” organisms aren’t some special sequence of proteins per se; they’re just a combination of individual enzymes, proteins, lipids, and other elements at the cellular level that have emerged to be a cohesive self-supporting system. The process to get from basic catalytic replication to such a complex emergence may appear dubious if you think of it in linear terms, but once you realize that it results from billions upon billions of reactions and combinations per second per square inch of organic material in an energy rich environment, it seems nearly inevitable that sooner or later the the right confluence of reactions and chemical compounds will come together for robust replication, and from there it is like turning the dial on a safe; after enough combinations finding the cominbation is actually unavoidable.
Stranger
In the short term, any genetic code will work as well as any other.
However, in the longer term, some might work better than others, for things like mutation resistance. In the genetic code we have, there are a number of places where a change of a single nucleotide will result in the same amino acid, with no net result on the protein produced, and a number of others where a single change will result in a similar amino acid, which might still result in a functional protein.
I’ve seen a study that suggests that our genetic code is one of the better possible codes in this way, in the top tenth of a percent of all possible codes. If true, this suggests some very interesting conclusions: It would appear that perhaps a thousand different genetic codes emerged, and that ours outcompeted all of the others. But the origin of the genetic code would have been right at the origin of life, which suggests that abiogenesis might have happened many times on Earth. This is all quite circumstantial evidence, of course, but still telling.
Cryptography is as black an art to me as genetics, but–and I’m not being silly here, I don’t think–the mathematics of the “fitness” of codes is a well-established and active area.
How have the general ideas of cryptography been applied to the study of the evolution (or the current state) of its particular biological implementation here?
…And do I have a snowball’s chance of understanding it…
This paper proposes that the genetic code is largely a ‘frozen accident,’ ie that it’s arbitrary but can’t be supplanted by a biochemically superior code due to constraints on evolution. An organism would have to replace multiple parts of the translational apparatus at the same time to use a different code.
It also notes other variations that may be possible such as the use of tRNA doublets or quadruplets instead of triplets; and the use of alternative amino acids and nucleotides.
We have no idea about the origins of life, much less an explanation.
Panspermia within the Solar System is possible, and it may be even likely. If life originated on Earth, it could have been transferred inside ejected meteorites to Mars, Europa, Venus and/or Titan among other locations, although there is no guarantee that it would have thrived in any of those locations. Alternatively life might have started in any of those locations and been transferred to Earth, though this seems less likely.
However interstellar panspermia is very unlikely. Stars are very far apart. According to H Melosh (2003) it is “very unlikely that even a single meteorite originating on a terrestrial planet in our solar system has fallen onto a terrestrial planet in another stellar system, over the entire period of our Solar System’s existence”.
So if we are only looking at panspermia within the solar system, there is a possibility; but this only moves the problem of abiogenesis to a nearby planet or object.
As far as the question in the OP is concerned, the possibility of cross-infection by space probes is taken seriously by NASA, and many precautions are taken; but there is no undisputed evidence that bacteria or DNA by itself could lead to the emergence of life on another world.
And despite that, we still have simpler explanations for it than panspermia. Terrestrial abiogenesis requires that life somehow, through some mysterious process, emerged on Earth, the most hospitable place known in the Universe for life. Panspermia requires that life emerged elsewhere, through some process which is at least as mysterious, and at a time even earlier in the Universe’s history, and then also requires that that life somehow made it here. It’s nothing but added complication, for no articulable benefit.
And to be clear, the “mysterious process” of abiogenesis refers to the fact that we don’t have a fossil record of it (and most likely never will), but there is no infusion of élan vital or Silap Inua or any other nonmaterial or special force or element that distinguished life from non-living matter. There is no particular reason for life to have started elsewhere, and given how hospitable our planet has been (both in the past and currently) to living organisms, it would be more simple to assume that life started here.
Stranger
“Silap Inua”?
If life started on Earth, it co-evolved with a sea of nutrients and a climate which, almost by definition, was just right for that life.
If life arrived from space then, even if during the heat of entry it retained enough structure to begin the processes of growth and reproduction, it would not have found the specific nutrients and climate for which it was suited. (And if the few spores then “waited” for such nutrients to arise by chance, they would likely have been destroyed in the same chemical cauldron that was developing the nutrients.)
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Thank you for linking to the excellent paper, Artfog. Now if only I had the time and brain-power to fully appreciate it! 
I think it is unlikely that alien DNA would trigger life in chemical compositions that were not already alive. But the arrival of DNA might significantly advance the primitive life in a leap forward, to emulate the life forms that had developed on the orb of origin.
However, it is also possible that the alien DNA would carry tendencies that were of no survival value in the new environment, and a more probable outcome would be that they would just die out.
Wald, George; “The Origin of Life”; Scientific American; August 1954
This is a particularly good explanation of biogenesis as first published by A.I. Oparin in 1936. If these ideas are true, then the extraterrestrial sourcing of life is completely unnecessary and profoundly uneconomical. That’s not to say it’s completely impossible, and given four billion years the odds of this occurring at least once could become near certainty. It would just be a matter of sequencing the DNA/RNA of all known lifeforms and looking for unusual patterns or unique amino acids within protein chains.
As far as I know, if life can spring up anywhere in the universe, then it can spring up here on Earth.