I don't think any search for life on other planets begins with looking for and sequencing DNA so the time between getting a false positive and disconfirming it could be a decade or more and be extremely costly to send unneeded equipment to Mars or Europa or wherever.
That's a very good point, I didn't think of this lag. Now considering we just did this thought experiment, maybe it wouldn't be a bad idea for any mission to Mars in search of life to have some cheap way of identifying Earth-based microbes, by, for example, coupling a cheap microscope with some machine learning. It wouldn't replace DNA sequencing, but it could be 95% there. Oh, and by the way, assuming we do find some extraterrestrial life form, wouldn't we want to find the way it encodes information as soon as possible, rather than wait for a decade or more for a follow up mission?
An extremely expensive although effective approach would be to "seed" the spacecraft with isotopically enriched and documented solvents and food sources such that all the oxygen or carbon or both on the spacecraft, if any contamination DOES exist, would be a peculiar and documented isotope ratio that is different than Mars ratios.
I think we know the natural ratios for Mars in bulk material, so anything immediately found to be growing that matched the documented "salted" isotope ratios obviously was a spacecraft contaminant whereas anything mars ratio was local. Of course bacteria can reproduce fast, so Earth bacteria eating Mars material would rapidly revert to Mars isotope ratios. Still for the first hour on the ground this would likely be usable for life detection experiments. Although it would probably be unimaginably expensive.
Are there machines that can sift through a sample of dirt, detect cells and isolate them for library prep and sequencing? Or are you thinking of human missions to Mars?
It seems likely that there will be such machines within a few years. There is nothing in this procedure which can't be automated using existing technology.
There's an open hypothesis that life may have evolved elsewhere in the universe and then brought to Earth by some means. DNA would probably have diverged by now, but who knows?
As I understand it DNA and RNA store their information in the form of certain amino acids. If other life uses amino acids in a similar fashion they may not use the same ones that life on Earth uses. Life here uses I believe 20 different amino acids and there are several hundred known.
So there's potential there I would imagine in regards to testing that hypothesis.
However what mainly bothers me with it is that it doesn't explain the origin of life and why Earth couldn't have original life begin on it.
Unless we can rule out that early Earth conditions would have been too inhospitable for life to start but was sufficient to sustain it I see no reason to think an extraterrestrial origin to Earth life is more likely.
> As I understand it DNA and RNA store their information in the form of certain amino acids.
Actually, information is stored as RNA, with DNA as long-term storage (mnemonic: RAM nucleic acid vs disk nucleic acid) and then amino acids are assembled into proteins (hardware) based on that information. Proteins only function as information storage in pathological cases like prions.
But yes, if 'alien' life used the same 20 amino acids, and especially if it has the same mapping from 64 nucleic acid triplets to 20 amino acids, then that's confirmation that it's actually just a earth microbe that escaped or was left behind.
Agreed that the extraterrestrial origin hypothesis doesn't actually explain anything. It just merely adds a factor to the Drake equation of how likely it is for life to get started.
You can identify statistical differences after some generations of bacterial life in the lab, as far as I recall, i.e. a very short period of time at human-scale.
Therefore, yes, life would have diverged sufficiently that scientists should be able to tell the difference.
Why do you assume so? It's possible a sattelite in the Solar System was contaminated by Earthling bacterias via asteroids and somehow miracalously adapted that environment. If this is true, scientists would find a group of bacteria branched off of bacteria here on Earth a billion year ago or so.
