Hawking's comments were part of a lecture at George Washington University on Monday in honor of NASA's 50th anniversary. He theorized that there are possible answers to whether there is extraterrestrial life.
One option is that there likely isn't life elsewhere. Or maybe there is intelligent life elsewhere, but when it gets smart enough to send signals into space, it also is smart enough to make destructive nuclear weapons.
Hawking said he prefers the third option:
"Primitive life is very common and intelligent life is fairly rare," he then quickly added: "Some would say it has yet to occur on earth."