why you're most probably not part alien

It's not completely impossible that you could have some alien DNA. But the odds are literally astronomical.

As anyone able to pass a college level biology course can tell you, the chances of humans interbreeding with any alien species are quite literally astronomical. Even if we embrace the notion of panspermia and assume that life on multiple planets has a common origin, the window for hybridization is under two million years and by the time the colonizing microbes start to evolve, creating a branch of their evolutionary tree endowed with a certain intelligence, if this would even happen at all, the gulf between them would be measured in billions of years.

And if we were to assume the more standard approach, that life arises on a planet where the chemistry can sustain the necessary biochemical reactions, then there's pretty much no way alien life forms can ever be similar enough to even genetically engineer themselves to be remotely compatible with each others' biology.

And yet, not only do we have people who believe that aliens could've interbred with humans, they think there is actual, tangible proof for the idea, with some going as far as dismissing evolution as an inadequate way to explain the rise of our species. And if you think that ancient astronaut proponents make some strange or specious arguments from their very liberal interpretation of astrobiology, it gets even more bizarre when they start using religious scriptures to justify a scientific investigation into the notion…

The Bible says the daughters of man had relations with fallen angels, and that their offspring were giants. Scoff if you want, but why don't they (scientist etc.) do DNA samples of all the known giant fossils? Maybe something shows up.

Sure, scientists can test fossils for alien DNA until the cows come home, but what exactly are they supposed to find? Are they just going to do some magical sciencey stuff and cough up a string of extraterrestrial genes? Of course not. We have no idea what real alien hereditary material would be like, much less what four or five, or six billion years of evolution could do to it. Aliens could use quadruplet codons, have ten nucleobases, and cells with no nuclei.

Without having an example of an intelligent extraterrestrial's genome to study its evolution and biochemistry, looking for an alien genetic code in human fossils would basically be an exercise in blindly stabbing in the dark, hoping to find something on the off chance it's there. And there are much better uses for the very limited amount of money scientists get than trying to prove fanciful tales from holy books or fulfill an emotional need to spice up our lives with an alien conspiracy theory. If and when we get a hold of some old, wise alien's genome, we could certainly run a few tests. But until then, it's just not going to happen.