Regarding the issue of 'alien DNA' it seems to me that if 'aliens have DNA' then they aren't 'aliens' at all. I know there are those in the UFO field who don't have a problem with it. Even in the movie "ET" there is a seminal line, "He's got DNA!" Well, I do have a problem with it. I can understand how general characteristics such as bipedal walking could be a natural form of reaction to gravity, meaning the hominid form, roughly, could be prevalent in the Universe, the forelegs evolving into 'hands' with opposable thumbs, leading to civilization.
But I don't buy the idea that a completely alien life form born on a completely different ecosystem would develop the same exact form of replication, including the exact same four amino acids tied together in helical form with exactly the same number of genes to transmit heredity. After several courses in applied statistics I still don't know how to compute the odds for this, but they are, if you will excuse the term, astronomically high against this possibility. Can we exclude the possibility altogether? Nope. That's not how statistics works, but for all practical purposes, it's impossible.
There are also those who figure the explanation is in pre-contact, i.e.: We were seeded here by an alien race and are actually from somewhere else, and/or that the human race is much older than we suppose. The evidence for our longer tenure on Earth is usually anecdotal and consists of mysterious and unexplainable artifacts found by a guy who told a guy who told another guy, but the actual artifacts themselves, unearthed in 1850, have gone missing, but there is this here newspaper article written about it, so it must be true and here's the proof. This is the Cremo/von Daniken approach to research. Even if you tabulate thousands of mysterious artifacts, if you can't come up with any today, they're still missing.
The fact is, from everything we have ever learned about our origins, we are from here. We evolved on earth and Homo sapiens is no more than a couple hundred thousand years old. The genus Homo (upright walking), including aferensis, is about three million years old. That's the time frame we are working with. Prior to that were some sort of apes, monkey like creatures without tails. Do we know everything? Of course not. Do we still espouse untruths? Probably. But in the final analysis it doesn't matter if Homo sapiens evolved from Homo erectus or the marginally earlier Homo ergastus. (BTW, we've got Neanderthal DNA now. From what we've learned so far, they were way different than us--distant cousins, not close ones.)
We evolved from an ape like creature, probably in a more complex way than we have delineated, and the fact is, there is not one whit of evidence to the contrary. Everything we've discovered amounts to filling in pieces of the general puzzle that we already have the outlines of. This includes fossil evidence, stratigraphic evidence, and dating technologies. Then along comes genetics, completely different in every respect to anthropology, and verifies everything we've discovered. None of these techniques has thrown up a red flag. Every one of them fits. We are all Africans. No matter what came before and how widespread it was, all of us, every last one of us, came out of Africa 150,000-250,000 years ago. Genetics has traced to the very tribe of Bushman we came from, which is still there today. If there were any other hominins (new term for hominid--same thing) already out of Africa at the time of this migration (there probably were), our ancestors wiped them out as they expanded into the rest of the world.
The thing is, we get confused in matters of scale. There is plenty of time within the Homo sapiens species as we know it for there to have been ancient civilizations, whether it was a pre-Egyptian 'higher' civilization with some odd technology or an 'Atlantis' that has now disappeared. I am firmly convinced, for example, that there was a near-Renaissance era culture on the shores of India 12,000 years ago which disappeared when the sea level rose 60 feet after an ice dam broke by Hudson Bay. We were coming off an ice age. It all fits. It also acounts for the flood stories and ancient Vedic texts. This is all well-within the time frame of Homo sapiens. You needn't stretch anything.
If there is time for a star traveling civilization from here within these parameters, I don't know. I kind of doubt it. There is no evidence of infrastructure to support it. If it did happen it would have to be based on entirely different technologies. As for humans contemporaneous with dinosaurs as shown by footprints. OK, WHAT footprints? Where are they? Where can I see them? Show them to me in situ, undisturbed, with provenance. This "he told a guy who told a guy" stuff does not cut it.
We don't want to get too far afield here because any sentence here is fodder for another argument and thread. The issue for purposes here is that we are from here and have not been 'seeded,' therefore if 'aliens' have DNA like ours, they are from here, not from out there. They could very well have 'tweaked' us genetically. The brain size of Homo sapiens developed very rapidly and 'off the curve' of other primates. You could view that as a tweak--or a positive feedback loop.
IF we can get hold of this elusive DNA of 'hybrids' and take a look at it, IF this idea can be confirmed absolutely by the technology we already have in place to do it, then the conclusion must be that the 'aliens' are, in some way, us. Whether the answer lies in some sort of inter dimensional aspect of reality we don't quite get yet, some sort of mind-boggling seemingly impossible time travel, or whether the answer is even more bizarre than that, we simply do not know yet.
But so far, 'alien DNA' is as elusive as implants or the skeleton of a giant seen by a guy in a cave who told a guy who told a guy.....