Martian

According to the theory of evolution if we trace back far enough, all land based animals share a common ancestor. That still doesn't mean I can interbreed cats and rabbits.

...I want my cabbit, dammit.

To elaborate, it all has to do with genetic makeup, specifically chromosomes. Creatures of the same genus, such as lions and tigers (both panthera) are able to cross breed, even though they're different species (Panthera leo and Panthera tigris, respectively). They're classified as the same genus because they have the same number of chromosomes, which is what makes their genetic material compatible. Dogs and wolves are the same species,canus lupis and thus interbreed easily (and are able to have viable (fertile) offspring) and are also able to mate with coyotes or jackals (which are the same genus but different species), although in that case, as in most interspecies breeding, the offspring are not likely to be viable.

And that's where we run into problems with the idea of interbreeding humans and greys. In order for that to work the greys would need to be of the genus homo, of which homo sapiens are the only known surviving species (more specifically the subspecies homo sapiens sapiens, which is of course what you and I are). This is highly unlikely if they evolved apart from us; their genetic material would simply not be compatible with ours and if an attempt were made to interbreed it, the zygote would shortly die. It would be getting conflicting instructions from the two disparate genetic strains which would make it unable to divide and form into a foetus as it's meant to. In order for this to occur, one would theoretically need an understanding of genetics strong enough to decode the two seperate genomes and then resquence them to be compatible, at which point the resultant creature would no longer be related to either human or grey, but be something else entirely. If that's even possible, which it may well not be; I don't profess to be a biology major and when we get into the nitty gritty of microbiology I tend to get lost.

So there's your crash course on genetics that in my mind at least disproves the human/grey interbreeding theory. Take it or leave it, I'm still not convinced the damn things even exist.

EDIT - For Menoman - Saying those things are highly likely, even in the context of that theory, is a bit of a stretch. They are a possibility and I'm certainly not denying that life can and probably has developed on other planets. The universe, hell, even our specific galaxy is huge beyond comprehension and amino acids are relatively simply formed from a chemical perspective. It's very possible for them to have formed and combined in sequences that create life. It's also possible that said amino acids hitched an interstellar ride on a space rock and didn't actually form terrestrially. It's worth noting, however, that life developed and evolved here on Earth. We are terrestrial, we are not aliens in any sense, technical or otherwise. We evolved and adapted due to the unique evolutionary pressures on our planet all the way up from the single celled organisms in the so-called primordial soup. That's what makes us of this Earth, regardless of where the first building blocks came from. Further to that, saying that we could be related to aliens is true in only the most remote and unimportant sense. We're also related to Komodo dragons and lemurs, but that doesn't make us the same as them at all.

__________________
I wake up in the morning more tired than before I slept
I get through cryin' and I'm sadder than before I wept
I get through thinkin' now, and the thoughts have left my head
I get through speakin' and I can't remember, not a word that I said

- Ben Harper, Show Me A Little Shame