Time travel stories often focus on the need to avoid changing history if you traveled into the past, but if you went further back than just a few thousand years ago, you couldn't avoid it. The moment you arrived you'd be breathing out bacteria and viruses which didn't belong in that era and to which nothing at the time had any natural resistance. Just by doing that, you'd probably trigger ecological changes which would have discernible effects on all of subsequent history. ~ Infidel753