We've commented on the talk swirling around these days about the search for life on Mars. We don't think there is any good evidence that there is any, or was any, such life. But of course we have no way to know that....yet.
In part, NASA talks up the life-on-Mars scenario because it wants support for extended, extensive Mars missions, a big budget even to include sending people to explore, and eventually colonizing the Red Planet. It's an understandable desire if you work for NASA, of course. Nobody will support all this if the place is just a big rock with some polar ice caps.But it goes beyond that. It has to do with the truly interesting question: Are we alone in the universe?
If life is a commonplace occurrence, a chemical inevitability that occurs all over the place, it will heat up space exploration, and the public's willingness to pay for it. It will be driven by its interplay with philosophy, science, religion, and even the arts.
What might be found?
Maybe the NASA hype machine will turn its very Disney-like graphical promotions and, yes, its extremely compelling actual footage from Curiosity and prior vehicles, onto some real, wriggling little green worm, at least. If it does that, we will of course be mesmerized as will everyone else.
The search initially will be for molecules compatible with having been part of life at that time. That means the expectation that life there is like life here. Either that's a kind of inevitability belief, or a belief that chemistry has proven that that's the only way life could exist, or it's just a total gullibility on the part of scientists for science fantasy. If the latter, the only reason we don't still expect complex creatures like little green men is that we have already explored Mars, on the ground and all over the planet by satellite, and haven't seen any, nor even the tracks of their cars and houses (nor their spoors or footprints).
Scientists who seek organic molecules know (and sometimes, when being candid, acknowledge) that these kinds of molecules can arise in many ways and, indeed, have always rained down on Mars (and Earth) from space, having nothing at all to do with life. But at least one scientist who should know better has suggested in an interview that the similarity to these atoms and molecules and the ingredients in DNA is a relevant fact. As we noted, that makes the humongous leap of faith in parallelism to believe that Mars life would be based on a polymer coding system related to proteins as the basis for its organization.
So, let's suspend judgment and ask what we might find. Of course, on Earth at least, DNA degrades too rapidly to be conserved for more than a few tens of thousands of years, which is why mammoth and Neandertal, but not dinosaur, DNA can be found. So of course one can find some carbon and fantasize that into DNA, but let's pretend we core down into the Mars ice (as part of mining it for water to pipeline out to support our space colony there, as is being suggested by NASA's PR office), and do find some DNA. What might it show?
Well, normally, DNA sequences are basically random strings of nucleotides. That means there's no formula from the sequence itself by which knowing the nucleotide in one position can predict what the nucleotide will be at any other position. Yet, bioinformatics is a highly sophisticated science that does identify very non-random aspects of DNA sequence--the location of genes, for example, and their regulatory regions along chromosomes. We can do this because we can compare species' DNA, and we have a century of experiments that have revealed what bits of sequence do, and why they do it.
Part of this, for example, is the nucleotide code for amino acids, by which sets of 3 successive nucleotides in DNA code for specific amino acids and hence for the structure of proteins. That is a primary function of DNA. But on Mars, leaping to the assumption that DNA is to be expected is the assumption that it will be a protein code as well, could we read that code? Is the code, and the set of available amino acids, the same as on Earth? How much inevitability or parallelism would that imply?
In fact, some analysis has suggested that, to some extent at least, the evolution of the code can be related to primitive RNA/amino acid interactions. That has been used to explain the specific aspects of the code, like which triplet would code for which amino acid, and why the redundancy of the code is as it is. But such ideas, even if correct, don't show that our system is inevitable. That makes what are almost anti-evolutionary assumptions: that evolution must follow a path that is predictable from the outset, down to its very specifics.
One way we relate species to each other, and infer genetic function here on Earth is to align DNA sequences from different species to find corresponding regions to show, for example, that we are closely related to mice, less so to alligators, and so on. We can reconstruct the general phylogeny of all life that way, step by step. On Mars, if we don't know the coding system, and if it were different from Earth's, it would be very difficult to make any sense of fragmentary ancient DNA we would find there. In fact, unless we struck some kind of miraculously preserved long stretches, and maybe also some corresponding RNA (which doesn't preserve well and is unlikely to be there), it would be nigh impossible to make sense of them--in terms of Earth life.
Suppose it's really there?
But, hell, let's be generous and suppose we found, for example, that Mars microbial DNA had very similar functions to Earth microbes' DNA. Maybe they used similar respiration and metabolism and so on. Maybe they had little ring DNA, called plasmids here on Earth, that protect from viruses! What would the explanation be?
By far, the most likely explanation is not high parallelism or inevitability of Earth-like life!
The most likely explanation will be that Earth and Mars life share the same common ancestry, not that they arose independently. We could compare Earth and Mars microbial DNA to estimate the date of that common origin, and perhaps from geological and cosmological evidence make a guess as to whether life started here, or there, and was transported by meteorites splattering off one planet and ending up on the other. Or it might represent stuff raining down from space that, as some have long suggested rather fancifully, could survive the harsh space environment, with some origin 'out there' somewhere in inter-stellar space that we don't know about. Geez, maybe the newly found planet around Alpha Centauri that has everyone so excited!
The same would be true if we inferred from Mars DNA that it used the same protein code, but we couldn't show species similarities to Earth microbes. That's because, in 3.5 billion years Earth life has diverged from its primordial forms to the point that one could not argue that the difference from Mars life showed independent origin.
Finding DNA-driven life on Mars would be very interesting, of course, but would not contribute to theories about its inevitability, or that life had to be based on DNA as a protein code, and such finding could work the other way--suggesting that the only way we get life is by being seeded from some unique source. It won't answer questions about whether life as we know it is a predictable chemical phenomenon. Or how common it is in the cosmos.
So as we said the other day, in our music analogy to start this little series of comments, the way ideas about extra-terrestrial life are embedded in our current science culture, often subtly and perhaps not in our awareness, is profound. We mix wishful thinking with limited thinking. We do it to advance interest and knowledge, and science, based on how we know it today. In that sense, we don't show enough respect for what we don't know. Of course, if you did that, perhaps you simply couldn't get anyone to pay for the exploring you want to do. Because the paying public is at least as deeply rooted in our current culture as scientists are.