Work published in 2008 from Israel claims detection of element #122. If true the implications are stunning. The only possible method of generating element 122 is by artificial nucleosynthesis, a technical feat beyond the capability of modern day humans. While subsequent study may have disproven the theory this opens up a new method of proving the existence of aliens. Even a single atom of element 122 (or higher) would be an artifact proving the existence of alien life...
Or time travel, it's always possible that in the future human will make the element and then send back in time either coincidentally or as an actual component of the time machine.
Regular readers of my blog know that I propose there are no aliens anywhere and never have been. So if element 122 is really discovered I claim it's proof of time travel, not aliens.