Now then, let us discuss the flaws in these theories. First a civilization which may have been transplanted to this planet thru necessity or exploration outpost or colony would already have complete control of their DNA and bio-systems and thus have no need to introduce the human DNA into their lines as it would be somewhat weaker and less evolved. An advanced species which had control of their DNA would not have innate needs for procreation or need sex and thus would have figured out a way to clone without DNA replication loss issues. Additionally a species, which was far advanced would not be worried about DNA damage and would simply fix those issues themselves without having to splice genes from human beings. They would be able to genetically modify their DNA to over come any loses or replication issues from long-term space flight due to radiation or any issues with in-breading from their small population of alien space colony travelers.
Indeed whether you believe in aliens, UFOs or extra terrestrials is irrelevant to the question of if such a story is plausible, as it does not stand to reason. The legends, ancient writings and theories do not seem to hold water. As science learns more about DNA and as our own species becomes transhuman we will be able to better debate these alien urban legends and their flaws.
Source: unexplored-earth.blogspot.com