Progenitors going up and down plain (8)
I believe the answer is:
apparent
'plain' is the definition.
(I know that plain can be written as apparent)
I don't understand how the rest of the clue works.
(Other definitions for apparent that I've seen before include "Quite obvious and clear" , "Noticeable" , "as such an heir" , "Ostensible, seeming" , "See 1" .)