> you would have no choice but to assume contamination from human probes as the most likely explanation.

You would have considerable choice; it would be quite obvious whether they'd been on a separate evolutionary trajectory for billions of years or not. That's still the boring outcome, though; not related to us at all would be far more interesting.

I mean, I wouldn't call support for panspermia /boring/ exactly, but I agree that a wholly separate abiogenesis event would be pretty astounding.

It would mean primitive life must be everywhere in the universe of it happened twice on adjacent planets independently.

Very, very unlikely outcome.

Well, we don't know how unlikely it is. It increasingly looks like life showed up on earth almost as soon as it was geologically possible for it to show up, which, if true, would indicate either that abiogenesis is pretty easy or that panspermia is a thing.

It would indicate that, but it's also somewhat expected if we're around to make that observation, which tempers how much information we can take away from it.

However, happening on two adjacent planets would really affect our estimations of the likelihood of abiogenesis.

> It would indicate that, but it's also somewhat expected if we're around to make that observation, which tempers how much information we can take away from it.

Life could have arisen on Earth much later than it did and we could still have been around to see it (or if not us, something like us) so the anthropic principle doesn't apply here. The fact that it arose so early is meaningful.

It is, but it's also sample size of 1. So yes, it indicates something but the confidence interval is really wide. Still, like you point out it's not meaningless as some people do try to argue.

If you take a bayesian approach to the problem it's far more likely that abiogenesis is common. This youtube video does a great job of explaining it: https://youtu.be/iLbbpRYRW5Y

We have basically zero data to tell us whether this outcome is "unlikely" or not. It could very easily turn out that this outcome is overwhelmingly likely.

No need to invoke panspermia. The inner planets have exchanged material such as rocks since the beginning of their existence.

I would assume it is DNA-based life. It would be a major surprise if it would be RNA-based life or something not resembling our tree of life.

If they matched, another possibility is that a stable life most easily forms a specific way, or this way only. Knowing for sure wouldn't be possible until more samples are found in the universe.

Nobody is sure, some parts are so nice that perhaps are inevitable, some parts are too random.

DNA and RNA: Perhaps they are inevitable, and any lifeforms have the same molecules and bases. Perhaps some lifeform can use only the methylated versions? Is ribose inevitable? Is 3 bases per codon inevitable? But I think there is a chance for inevitability.

The list of amino acids is somewhat random. We use 20, Some bacteria use a 1 or 2 more. And we can transform in place the amino acid inside a protein into another amino acid. I expect a similar list of 20+-5 amino acids. A perfect coincidence would be too hard, but it is not even true for terrestrial life.

The most interesting part is the Genetic Code https://en.wikipedia.org/wiki/Genetic_code that translate the 4^3=64 combinations of bases in the DNA to the 20 amino acids. As far as we know it is random. It has some internal structure, for example the code is mostly like 16 blocks to encode 15 amino acids and a stop signal, and a few tweaks here and there to break some block to encode the other 5 amino acids. But as far as we know there are no reasons to choose each block as us.

Some bacteria use versions with small tweaks, with one or two changes, so we know it if possible to change it, but very difficult to make a total rewrite.

If the life out there use the same Genetic Code (with a few tweaks) it is almost sure it is related to us.

Along those lines, I had a college professor who had just published a paper (now 25 years old?) talking about why "the 20 amino acids." He was equal parts bombastic and brilliant, and for some reason this paper of his stuck with me all these years.

https://www.sciencedirect.com/science/article/abs/pii/S00225...

Now, I'm not an expert or even an advanced layperson in this area, so I won't attempt to speak to the validity of the idea. I merely found the argument interesting at the time.

Did anyone advanced in this research line? It looks like a nice mathematical idea, but it is forced too much to fit into the genetic code.

First they propose a genetic code that is totally unrelated to the current one. It can be perfectly the table of a totally different lifeform. The transition they propose looks too difficult, almost impossible.

Probably we had a transition from RNA to DNA a loooong time ago, but they have a 1 to 1 translation, and the intermediate steps look useful. But no one is sure anyway.

They propose a change where almost all the table changes and almost all the internal structure of the table change.

It is very strange that they have 20 amino acids and no stop code. A proposal with 19 amino acids and a stop code looks more sensible. We know that it is possible to add new amino acids to the table (with enough time). I think the main problem is that using 19 amino acids and a stop code breaks the magical part of predicting the number 20 with pencil and paper.

To make the table work, you need the correct transfer-RNA. To get all the symmetries in their model you need to be really lucky. In the block model of 15+1 you any random selection of tRNA is fine if it ignore the last base.

Another big problem is that they predict that half of the proteins would have been constructed backward. But as they say this make proteins not fold correctly. They try to avoid the problem using palindromic proteins, that is not supported by evidence.

I wish I knew! I'm not in the field, and while I was close with this professor back in school we haven't kept in touch. The paper doesn't seem well cited, but I'm not sure what well cited looks like in this case.

This is about as likely as guessing a 1024 bit encrypted key.

“Match” means “match”. Not “similar structure and behavior”. If it is the same DNA then the only compelling explanation is that it is from the same origin.

If it's DNA at all then it would be from the same origin?

How likely is it that alien life would also use DNA?

Even if it is DNA, if it encodes the same proteins in the same way than the life here would a be very improbable event. Think in an alien alphabet looking like ours, but also making the same sounds and some common words and meanings with the similar looking symbols.

“Origin” in this case would be something in the very physically close and near term. Something that excludes the possibility of serious evolutionary change. Even relatively static species undergo mutations in the long term.

So if something is an exact match it is evident that they came from the same time and place (within some reasonable window). The origin would implicitly be Earth in the 1960s.

I meant match, similarly to saying curtains and carpets match.

“Matching DNA” has only one meaning that I am aware of and it is an exact 1:1 sequence. The goal is to identify a living organism from DNA that has fallen off it.

https://en.wikipedia.org/wiki/DNA_profiling

my apologies on not using terminology that suits you

There is an old line of thought on this, that views DNA/RNA as crystals. Crystals branch out/reproduce in predictable or unpredictable ways depending on scale. At large scales a dry lakebed may seem a mass of randomly-aligned crystals but at the smallest scale each crystal is identical. They each formed from the same starting conditions from which random precursors settled into identical patterns. Such logic would suggest that DNA/RNA, if beginning in near-identical conditions, might also settle into nearly-identical patterns. At large scales the crystals would appear different and random but at the smallest scales life could well be similar or even identical... if one sees life as a form of crystal.

Even if there's a "good reason" that life forms in a specific way (e.g. water + carbon based, RNA & DNA encoding information, etc.) there's very little reason that DNA should encode proteins in exactly the way ours does (on Earth-based life, AFAIK the code is nearly universal, indicating common origin).

Absolutely - let me retrospectively adjust the panspermia 'a few billion years ago' to include potentially quite recent cross-fertilisation.

In any case it makes it much less interesting, as it implies same origin.

Implies but does not prove, I should add, of course.