If a scientist tells you that he has almost perfected his time-travel machine, run for cover. You'll know that mankind is about to be wiped out by a freak meteorite or something once we get to the point when scientists have almost invented a time-travel. Why? 1) People from the future don't keep popping up in time machines all over the place throughout history. 2) Therefore man never discovers the time machine. 3) Therefore mankind must get wiped out before the invention of the time machine.