All right. Here’s my guess.
Imagine that a space ship appears over Washington DC, and by some miracle is not shot down (this is fiction, right?). Imagine this ship is carrying intelligent beings that claim to be from another planet very very far away. Their technology is far more advanced than ours, and they can verify their ability to travel between the stars. Perhaps we saw their ship coming towards us from outside the solar system.
Now, imagine that these intelligent aliens (and they are aliens) look somewhat like humans, at least eerily like us. We notice strange similarities between our biology and theirs. They have DNA too, and the same genetic code between nucleic and amino acids. Then, to everyone’s amazement, we find that they are reproductively compatible with us. We can bear hybrid offspring with them, that end up being “normal,” growing up to be non-diseased individuals.
What should we conclude about this? How could this possibly have happened?
There are two cases we need to consider here: either (a) they have animals and plants on their planet which are reproductively compatible with ours, or (b) they don’t.
In case (a), I can think of only two conclusions one could reach. The similarities clearly cannot be a coincidence. To explain the seeming coincidence without resorting to colonization (too far-fetched, as it would have had to have been going on for billions of years, with regular visitations) or time travel (which I consider to be metaphysically impossible), one must either (i) deny that living things evolved independently on Earth and the alien planet or (ii) suppose that evolution takes place along predetermined lines - rather like the thesis propounded by Simon Conway Morris, only much more extreme: one would have to claim that there is no contingency in evolution.
Re (i): one could suppose that the planet which the aliens came from originally orbited around our Sun, but got snatched out of the solar system a few hundred thousand years ago by a passing star, which then acquired it. One would also have to suppose that while it was still in our solar system, this planet’s orbit passed very near to Earth’s on regular occasions, and that DNA was exchanged between the two planets via volcanic debris, collisions with meteorites or asteroids, or something like that. For instance, maybe, just maybe, animal eggs might, on rare occasions, get swept between the two planets when they passed within a million miles of each other, say, keeping their evolution running in tandem with each other. (I know, it’s a big stretch, but it’s conceivable.) There would still be a mystery as to why the aliens’ technology was so far ahead of ours, but one could suppose that maybe their planet, after being snatched away, never had to pass through our Ice Ages, so the humans on the alien planet managed to discover agriculture and established cities hundreds of thousands of years before we did.
Re (ii): evolutionary hyper-determinism is out of fashion these days, as the neutral (or nearly neutral) theory of evolution is very much in vogue. I suppose one could argue that while most mutations are neutral, there are certain underlying “laws of form” which constrain evolutionary pathways and make it inevitable that intelligent humanoids appear. (I believe Michael Denton and Richard Sternberg think along these lines.) I’m rather doubtful, however, that this model could explain why the aliens were able to interbreed with us. That would be an unexpected bonus.
In case (b), the aliens come from a planet which either (i) has no animals and plants at all, or (ii) whose animals and plants are not like ours, genetically. Re (i): the aliens would still need to eat, so one might suppose that they periodically “raid” Earth for food. However, their technologically primitive ancestors (assuming the aliens weren’t always an advanced race) would have needed to eat, too, but they wouldn’t have had spaceships, so this scenario appears to be a non-starter.
Re (ii): if the animals and plants on the alien planet had a different genetic code and lacked DNA, then it would appear that the aliens themselves had been “transplanted” onto their planet, at some point in the past. But who would have done such a thing? The only possibility I can think of is that another race of aliens abducted some Earthlings a few hundred thousand years ago and deposited them on the alien planet, and then proceeded to give them a crash course in science, which is why they now have a technological advantage over us. I don’t think the aliens’ ability to eat and digest animals and plants on their own planet which lacked our DNA and had a different genetic code poses a particular problem, as their bodies would be able to break down the protein in these animals and plants into its constituent amino acids. (Mind you, I never studied biochemistry, so maybe there’s something I’m missing here.)
I suppose another possibility I haven’t considered is that God Himself (or His angels) did a bit of alien “transplanting,” but I don’t think that was what you had in mind, @swamidass. Or was it? My two cents, anyway. Cheers.