If they weren't sure if it was poisonous or not.
Personally I have a belief in creationism, but I am a bit skeptical
about the family record in Genesis. The Chinese apparently have no
records of extraordinarily long lived people in their history, which
goes back a long way.
However since it was often Jewish custom only to regard the male line
as important, the writers of Genesis may not have thought it necessary
to include female daughters to Eve because the culture of the time
would not have considered the matter important, and readers / hearers
would have assumed Enoch came from Seth and one of Adam's daughters.
As pointed out above genetic load would not have been so important in a
near perfect world (this same genetic load is another argument against
evolution, since species tend to increase in negative mutations as time
goes by, not positive).
And as pointed out above, incest or marrying a close relative was not
forbidden in the very beginning. After all if we all do have only one
female descendant, then obviously there must have been some "incest",
since all the currently alive males have descended from "Eve" as well
as all the females.
I believe examinations of mitochondrial DNA are supposed to indicate
that all humans today seem to have descended from one female. The
reason for mitochondrial DNA being used for this is that we are
supposed to get our mitochondrial DNA from our mother only, whereas our
nuclear DNA comes from both parents. That is what I read somewhere (I
think).
It is a bit uncanny of the ancient Biblical writers, who knew nothing
of genetics, to include in their "myth" that we all have one "mother".
It is also uncanny when Marian apparitions come about nearly 2000 years
after the angel Gabriel declared "Hail Mary, full of grace The Lord is
with you." (The opening lines of the "Hail Mary" prayer for
non-Catholics is nothing more than the words used by Gabriel himself.
The next bit is Elizabeth's cry to Mary, "Blessed are you among women
and blessed is the fruit fof your womb".)
It would seem we all have one spiritual mother as well as well as one
earthly genetic mother. It was Christ who said, "Behold, your
mother!".
And there is of course her declaration to Elizabeth, "From this day
forward all generations will call me blessed".
Bob Crowley.