I watched National Geographic's Earth: The Biography Sunday evening. The narrator kept saying over and over the Earth may be the only planet in the universe to hold complex life. Several reasons were given and for each one he stated it was a matter of luck. According to the show, it was a matter of luck that when the Earth formed it was much smaller and it had a twin. The two collided and the Earth gained more mass and the moon was formed from the debris.
Also, at that time, the Earth began to revolve and the Earth's iron core was formed. Both things were necessary for life to begin. The Earth's rotation keeps it from experiencing extreme temperature ranges. The core's massive gravitational pull keeps the atmosphere from leaking out into space. According to the narrator, it was a matter of luck.
Also, it was a matter of luck, according to the narrator, that the Earth is placed just right. It's not too far away from the Sun that water will freeze over like Mars. It's not too close so that water will boil off into space like with Venus. Water is the key ingredient for life to exist.
According to the narrator, Jupiter's massive size and location was lucky for Earth as well. Jupiter's size and relative proximity to Earth draws most of the large comets and meteorites to Jupiter and therefore Earth has been spared many bombardments which would have devastated Earth. A couple did get through. Sixty-five million years ago, dinosaurs ruled the Earth. Mammals were few in number and usually lived underground. The dinosaurs were wiped out and it gave mammals the chance to evolve and for humans to come into being. The narrator also states this was just a matter of luck.
To me, it seems it was much more than luck.