Although various eighteenth and nineteenth-century American poets had professed an interest in Native American poetry and had pretended to imitate Native American forms in their own works, until almost 1900,scholars and critics did not begin seriously to study traditional Native American poetry in native languages.

A)until almost 1900,scholars and critics did not begin seriously to study
B)until almost 1900 scholars and critics had not begun seriously studying
C)not until almost 1900 were scholars and critics to begin seriously to study
D)it was not almost until 1900 when scholars and critics began to seriously study
E)it was not until almost 1900 that scholars and critics seriously began studying

This is an o.g question and E is the correct answer. I however do not understand what is the 'it' referring to.
I understand we make this usage in colloquial language, but can someone please explain its grammatical significance and how and when "it" can be used.

a few other examples from the o.g.

-Long before it was fashionable to be an expatriate, Josephine Baker made Paris her home

-It can hardly be said that educators are at fault for not anticipating the impact of microcomputer technology