For serious discussion of the controversies, approaches and enigmas surrounding the origins and development of the human species and of human civilization. (NB: for more ‘out there’ posts we point you in the direction of the ‘Paranormal & Supernatural
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I would say that the main objection to 'alien' intervention is that it devalues the skills and technical abilities of our ancient ancestors.
It suggests that they were too stupid or too inept to produce anything of value themselves, and had to rely on beings from on high to help them. The implication of this sort of thinking is that because we don't fully understand how they constructed their sites, they couldn't have done it without help.
Is that really a fair way to look at the legacy of the Ancients?