Your points about the unlikelihood of hominids evolving separately are
valid.  But the solution to the mystery is much simpler.

UFO stories are made up by people whose chief contact with science fiction
is via comic-books.  In comic-books, the vision of "Man: 10,000,000 A.D." is
of a diminutive character with an oversized head and a thin, frail body.
After a while, comic-book artists started depicting aliens in this manner to
convey that they were "more evolved" than humans.  And so we have the modern
image of the Grey.


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Regards,
Mike Combs
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Member of the National Non-sequitur Society.  We may not make
much sense, but we do like pizza.