Oh now, PM... open your mind up. Maybe they won't be green! There's another theory: Are the aliens us? UFOs may be piloted by time-traveling humans, book argues

Yeah, yeah, I know... The Vulcan Science Directorate says that time travel isn't possible. Maybe they aren't as smart as they logically conclude they are!

<snicker>

In an odd sort of way, the time traveler thesis seems slightly more sensible than the aliens thesis. We know we are here, and technology continues to advance. We (well, some of us) humans are curious about our ancestors... it seems reasonable that our descendants will be too. If they were to eventually figure out time travel, I can see them avoiding contact in an effort to avoid contaminating the time line. I can't explain the claimed abductions and (ahem) probing though... Maybe proctology advances (or not) at a very different rate.