Why life is on earth

I personally have no idea for why theere are people on this planet and on no other.  A millieum ago there was this drawf planet that the human race was living on.  The planet was called htrae.  This had been the 100th planet the humans had tried to settle on. The half of the planet that had been destroyed had been for natural cause but the others were from human problems.  htrae could have been the twin sister to the planet we call earth. the humans had been on htrae for quite a few generations and had not seen anything wrong with this planet until the storm happened.  A clear summer day was just about to start when a nuclear bomb came out of the sky and exploded killing many people.  The survivor although there were not very many climbed on to their space ship and few away.  The ship was set on auto pilot to go to the nearest planet that could support life.   The ship took off at light speed causing the passengers to have their memory wiped. The ship then landed on earth all dazed and confused.  That is also why all the dinosaurs died because the humans brought a new disease to the world making the dinosaurs sick and then they die.