Time Travel in Science Fiction has multiple variants to cope with its paradoxes such as the Grandfather Paradox, the most famous of them being
- the closed time loop, where anything that seems to modify the past turns out to already have been part of the past
- the parallel universe theory, where there basically is no paradox since the time travelling creates a new timeline existing in parallel with the original one
- the Back To The Future vanishing act, where the time travellers are basically slowly replaced by the new futures travellers, should they still be existing and travelling back in time, otherwise simply vanishing (this sentence should actually require some new grammar to better express the "will have would happening"-ish stuff)
To my knowledge, Star Trek's time travel usually obeys the multiverse theory, and another question's answer lists some post-TOS episodes where the conservation/loss of memories is treated. But my question concerns the Temporal Prime Directive (TPD):
When the TPD was established, were there any precedence cases known where the past was modified, and therefore, is the (pre-2009) star-trek universe a future that only came to be due to some time travelling?