A complete and total one!

It is certainly true that any living sapient being that figures out the problem with traveling the vast distances between stars in a timely manner would probably have technology and understandings that far, far, far exceed our own. They'd probably have technology that we could only dream of: Everything from the just-within-reach nanotech, to the (might as well be) mystical, barely-even-talked-about pico- and femtotech, and beyond.

They could for instance change their fundamental genetic structure to mach our own, but as I said with Tiffany, if aliens did that to themselves they would cease being alien genetically and you wouldn't have a hybrid, just another normal human child who's mother or father use to be alien. A very smart, advanced alien who, for some reason, decided to become human, but, hey, I don't judge.

I'll see you're hybrid pony/monkey abomination and raise you one Hybrid eldritch abomination.

But, even if we all shared a common ancestor, unless we and the aliens have been regularly banging each other throughout the time we've been evolving as humans (not even mentioning the amount of time it takes for the materials from wherever the origin to develop into life in general), our genetic structure would drift to far away to be compatible.

It also brings us back to the problem that aliens, being alien, developed on a completely different planet then us, despite having a common origin.

Having a common ancestor doesn't automatically mean able to have a healthy child with them. True, the more recent the ancestor, the easier it is to do so, but the given the huge scale of the universe and distances between stars, any ancestor we'd have with other life in the universe would certainly not be recent. Interbreeding would certainly not be possible. Not unless one party changes their fundamental genetic structure to mach the other. But if aliens did that to themselves they would cease being alien genetically and you wouldn't have a hybrid, just another normal human child who's mother or father use to be alien. Same goes visa versa. It would still sound cool, and certainly make the headlines, but the child wouldn't be identifiable as a hybrid.

I never understood the "alien/human hybrid" thing in conspiracy theories and science fiction.

The genetic structure of an extraterrestrial (if it even had DNA and not some other undiscovered self replicating molecule) would be so radically different that no viable offspring would even be possible. You can't just have two completely different species mate and expect a hybrid child to come out of that. Genetics just doesn't work that way.

I'm not a geneticist, but I'm pretty sure that a giraffe and an elephant doesn't make a girelephant or an elephaffe. Yes, you can cross humans and chimpanzees and get an offspring. I don't know who would do that nor how healthy the child would be, but it would be possible. But that's only because of how extremely close a chimp's DNA is to us (greater then 98% similarity) due to the fact that they're our closest living relatives. Every animal hybrid that has ever been created were only possible due to the great similarities in the DNA structure of their parent's species, and even then, most of them turned out to be sterile and couldn't produce offspring of their own.

That said, I suppose you could physically take out certain specific genes of an alien and place them in a person, but that would only work if said alien had DNA to begin with. If a sapient lifeform evolved on a planet light years from Earth, It would stand to reason that everything about them would probably be different from anything you would find here on Earth. Different environments and different circumstances breed different results.

That's not to say that DNA found on other planets is impossible (I believe I've read somewhere that we have found amino acids in meteorites, which suggests that the rise of life on our planet was due to panspermia). It just means that it is highly improbable.

that's my 2 cents on that bit.