If anyone where to present me with an animal, from an alien planet, in an alien solar system, sharing no evolutionary history, that where able to breed with any complex life in any way other than as a parasitic being and produce viable offspring, I would start believing in creationism.

@Clearer If that happened, I'd expect evolution to be as discarded a theory as Newton's gravitation was by measurements of the solar system. Useful at a planetary scale, useless for everything else, but there existing no theory that predicts what is observed.

I agree with that. But: Universe is infinite... there are infinite possibilities, right? Anyway, even if there is another place within this infinite universe where life is just like it is here (which I don't believe), the distances are too huge for them to ever get here

Is there not an outlier/exception in known biology where two cladistically distal species could produce an offspring?

The biological differences between an alien dog and an Earth dog are likely to be greater than the difference between an Earth dog and an Earth computer.

@uwnojpjm... no. We only know cases of species closely related crossbreeding... Like species from the same genus and, sometimes, species from different genera, but within the same family and same subtribe or tribe

@Thai: Just because two things are both (claimed) infinite does not mean they're in any way comparably so.

Yeah.. I know... but... who knows, really? We know too little. Who knows what is there out there?

The only exception I can think of to life using DNA is an RNA virus; and that's still pretty close. I can include isolated life like thermophiles, halophiles, and psychrophiles too. Stochastically it seems very likely that alien life uses DNA as well.

@MichaelEricOberlin - Problem is, that’s likely an issue of common descent. Even if alien life were carbon-based (not even that’s guaranteed), I’d suspect a large state space of possible information-carrying molecules (though of course quite small compared to utterly useful molecules).

A dog would have a better chance of breeding with a tree than an alien. At least the dog would share ~half its genes with the tree.

@Obie 2.0 Respectfully that's a reach that requires a counterexample. There's nothing wrong with reaching in would building, but chemistry follows a unique and strict set of physical laws, and for the traits we recognize as life, DNA is the most probable medium. Additionally, not all life on this planet is guaranteed to have the same ancestor when you factor in unreachable and toxic environments, as I did above.

@MichaelEricOberlin “DNA is the most probable medium”. How can we know that when we only have one example of our biosphere to go on? There are many possibilities for alternative DNA theguardian.com/science/2012/apr/19/… worldbuilding.stackexchange.com/questions/36222/… And different pentose sugars en.wikipedia.org/wiki/Pentose and so on

@MichaelEricOberlin - I mean, it’s not a terribly controversial hypothesis that everything on Earth - yes, even those organisms that now live in isolated environments - is descended from a single common ancestor. And that leads to problems with declaring DNA “inevitable” or even “probable.” It would be like looking only at the population of Western Europe and declaring lactose tolerance to be the default.

I’d look here for a convincing demonstration of common ancestry, though there are others. Personally, I find sequence similarity to be a priori strong evidence of kinship - it is in most cases, after all - but there are tests that don’t assume this. While it is impossible to rule out undiscovered extremophiles that do not share common ancestry with all other living organisms, I remain convinced that all currently known organisms likely have a UCA, which means we should assume nothing about the most probable carrier.

I mean, we know - or have strong reason to believe - that we could create a sentient organism with the right configuration of silicon chips. That alone should call into question our assumptions about the necessary molecules for life. There have even been researchers who have succeeded in making boring old Earth organisms produce carbon-silicon bonds. I think we should be very careful indeed in assuming which molecules be probable for life, let alone inevitable.

Feel obliged to point out alongside these comments that we also have no way of knowing that the different possibilities are, in fact, possible. Our sample-set is limited, but a lack of evidence is not evidence one way or another. It's more than conceivable that DNA is one of only a few viable evolutionary maxima, due to reasons temporarily out of our reach...and for fiction, oftentimes "not knowing" is more than good enough for a story.

Mating requires the transfer of, and mixing of some pretty 'intimate' details about the parties doing the mating. We see even relatively close looking strains of animals unable to mate with each other, and on occasion there are humans pairs that just can't mate too. As such, I'd say the "not a chance" summary is likely accurate - unless two species happen to have evolved to the same point right now, but being light years apart for millennia. That sort of assumes they both started evolving at the same time, or else if one is evolving faster, then it may be able to mate today, but not next year.

@Thai "Universe is infinite... there are infinite possibilities" No to both parts. The observable universe is only 93 billion light years across. There might be some more universe past that, but we can never interact with it. And even if it was truly infinite, that doesn't mean that anything is possible. If I flip a coin an infinite number of times, It'll land on heads an infinite number of times, tails an infinite number of times, and even land balanced on its edge an infinite number of times. But it'll never land as the 3 of spades.

@Ray thanks for ruining my awesome poker strategy...

@Onyz Yes it’s true we can’t know either way, but DNA as a basic structure is just the start of the problem, let’s assume that DNA is the only viable media, what are the chances that the protein coding system will be the same, that the same amino acids would be used or coded for in the same way? Especially as we know that one important feature of evolution is random mutation.

@Slarty Yes, I agree on that point for certain. At the very least it seems easy enough to say that the chances of an alien species being able to create viable offspring with a Earth species is several orders of magnitude less likely than, for example, a random species on Earth being able to produce viable offspring with another random species on Earth... which is already incredibly unlikely, statistically speaking.