Time Travel
The ability to travel back and forth between the distant future and the forgotten past has been a dream of many people since H.G. Wells published his novel The Time Machine way back in 1895.
As of the moment, a scientific way to travel at will through time remains beyond our capability as a species, and if a non-scientific way exists, no one seems to be telling... but, of course, we're talking about time travel. So if anyone ever discovers how to do it, at any time in our future or past, it can still effect us directly!
Having said that, the real problem is that ideas of 'evidence' of time travel really relies on the idea that time travelers will make mistakes that we can discover; and, frankly, I'd hope anyone traveling to the past would avoid mistakes as much as possible; though it's not like anyone would notice if their own personal past history was changed...