[kgp4death]

Well lots of ways world will end....
1) Super virus, super bacteria, or plague (last time the bubonic plague wiped out 1/3 of the planet.
2) Asteroid (destroyed pretty much all life on earth a few times - trilobites and dinosaurs)
3) Aliens of course I think it is very possible with literally millions of alien species likely to exist. (actually I saw a UFO once it was one of the spherical metallic ones it was hovering 250ft above the highway and was visibly spinning it suddenly it made a large spiral in the sky as if it wanted to go right but because it was spinning it was faster to make a spiral. Then about 10 seconds later 3 fighter jets past chasing it and let me tell you the UFO sent from hovering to way faster the the fighter jets in about half a second. I am not saying it was aliens it was probably a military project but no one can tell me that the objects (whatever they are do not exist...I am actually serious, and am not joking)
4) Humans completely destroy environment
5) Sudden radiation spike from a star exploding or pulsar
6) a hundred other things our immature 1000 old science can't even imagine

Solution:
Colonize as many planets/solar systems as possible as fast as possible. With little to no inter planetary movement after the initial colony drop (in order to stop spread of new disease)