Ask yourself this question:
Do you think that at ANY time in the future (whether it's 10 years from now or 10,000 years from now), time travel will be possible?
If you answer "yes" to that question, then it must be conceded that it's possible we're now (and have been in the past) being visited.
Physicists have theorized that time travel might involve the "bending" of the time line. In other words, it would part from our linear time/space way of thinking. Take a piece of paper. Draw a dot on one end of the paper and draw another dot on the other end of the paper. Instead of trying to get from one dot to the other by traveling across the paper, you would instead fold the paper in half, thus effectively bringing your destination closer to you. If I remember correctly, wormholes are involved. I'm sure there are people on GLP who are more apprised of the details than I am.
Also, it's interesting to consider our Government's response to UFO sightings since WWII. When they do investigate (Robertson Panel, Project Blue Book, Condon Report, etc.), they consistently assure us that there's "no threat to national security." If these UFOs are indeed time travelers (read: us from the future), then our government officials would not be lying when they tell us there's no threat to us.
It's also interesting to note the alleged attention our nuclear facilities (particularly our nuke weapons facilities) receive from UFOs. After dropping the atomic bomb on Hiroshima, the Enola Gay and most of the rest of the 509th Bomb Group of the Strategic Air Command was stationed at Roswell Army Air Field in Roswell, New Mexico. Could this be why ostensible time travelers had an interest in the Roswell area? Not to mention the nuke testing that went on there prior to Hiroshima and Nagasaki at White Sands, New Mexico.
Also in the 1960s, 70s, and 80s, UFOs appeared over flight lines and nuclear missile silos at several American Strategic Air Command (read: nuke) bases, including Malstrom AFB in Montana (1967), Minot AFB, Edwards AFB, and Bentwaters AFB (U.S. base) in England (1980s). Allegedly, some of the nuclear launch codes were changed, or were briefly rendered inoperable.
Once humankind learned to harness the energy of the split atom, the future of humanity (hence the time travelers from the future's very existence) would be in jeopardy for the first time in our history (as far as we know). Prior to the nuclear bomb, no other weapon that we know of has had the potential to destroy civilization.