visit the Earth & do God only knows what, like carving in andesite or tweaking DNA such that a better offspring from an alien/human mating could occur? I don't claim either way. I've never seen such a being, though the Bible says that they did exist. They were called the men of renown, the men of old. Nephilim are also mentioned. & giants certainly populate the Hebrew Bible throughout.
EDIT (From the computer, the earlier part and the message before it were from my phone):
I've done some more research on Erich von Daniken, and some more reading of his book, "Twilight of the Gods". He asks good questions. Some things are not explained well, but his answers are in many cases just outright absurd. At this point, I don't know what to think.
A lot of what he writes is non sequitur. For example, he takes the Great Sphinx, and other sphinges (yes, that is the correct plural of "sphinx"), and comes up with the idea that because they exist in art, they must have existed in reality. I mean, I won't dispute, its possible. But with our current knowledge of genetics, its highly unlikely. Taking the genetics of a human and a lion and producing an actual half-Human half-Lion Sphinx seems a bit far-fetched. And that doesn't even begin to speak of all the other sphinges that there are. Half-Bird, half-Goat, half-Dog, half-Ram, etc, etc.
And what would be the point? I mean granted, some have asked the same question of why one would create a half-Lion and half-Tiger (a Liger), and these actually DO exist. I don't know the answer to that. Is there a purpose to creating such a beast? A half-Ass and half-Horse Mule obviously serves a purpose, in that they are hardy little beasts, but does a Liger serve a point, or was it just done to see if it could be?
If it were possible to make a Sphinx, would there be a point, or would some advanced alien do this just for fun? Its one thing for humans to make a Liger. We didn't have to travel halfway across the Galaxy to do it! Why would aliens travel halfway across the cosmos, stop here, and then create a Sphinx? Why, for that matter, would they mate with proto-humans (if that were possible), thus creating Homo sapiens)? Again, what would be the point?
So, although he does ask interesting questions, like, how the Hell was Puma Punku built (I for one don't believe the standard answer), I am not at all sure I accept his answers either.