Technically our good friend "John Titor" is from another dimension. There is a divergence between his timeline and ours. If the divergence were great enough, over a long enough time period, you could have all sorts of deviations in the history of planetary evolution. This theory should support the concept of beings completely dissimilar from us being able to travel here, even though they are actually from here, just from an Earth based on a separate dimensional timeline. An example may be a timeline where dinosaurs still exist and the traveling beings are some sort of bird people descendants of those dinosaurs.