Why didn't the author of Mark know about the virgin birth?

The gospel of Mark is reproduced almost in entirety in the gospel of Matthew with original greek misspellings and grammatical errors corrected. Luke, also, contains verbatim much of what is written in Mark. The conclusion that the authors of Matthew and Luke had Mark in front of them when they compiled their gospels is inescapable.

It has thus been deduced that Mark is the earliest synoptic gospel written (probably around the year 75).

So Mark was written first and, to the point, contains no mention of the virgin birth. It seems likely that the author must not have know of it's existence otherwise he/they surely would have included it in the gospel.

Is this suggestive that the virgin birth is a later addition to what became Christian theology and that the people alive in the time of Jesus knew nothing of it?
 
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