The one reference I cited as arguing for pre-Adam humans (1656, A Discourse
upon...) bases his ideas on Romans, not on scientific evidence, as far as
can be discerned from the summary at hand (and from the title).
I think one can make a strong case for lack of consensus among early
commentators on 24 hour versus some other amount of time for the days of
Gen. 1. In contrast, some YEC's claim that everyone believed that until
folks began to turn from the true faith to accommodate scientific claims.
Given that the Biblical genealogies were the only line of evidence they had
as to the age of the earth, a young estimate would not be surprising.
However, as most modern young-earthers roughly triple the amount of time
before Abraham relative to the genealogies, the high ground of extreme
literalisticism on these dates seems vacant.