The odds of a terrestrial planet being in the habitable zone of a star long enough for complex life to emerge is not paticularly impressive, especially when the number of worlds available are considered.
It gets more complex when you consider the type of planet (size and mass) the generation of the star is formed around (it needs to be at least third for the right elements to be available) and it needs to evade astronomical disaster for extended periods (a Jovian gs-giant might help in this respect). A moon would be good, too, and any form of atmosphere (except maybe an inert one). This only helps life as we know it form on a planet. They may not be the only options.
So it happens, there are about thirty understood possible chemical pathways to life on Earth, and probably over a hundred more that no one has figured out yet, any one of which could have been the process that actually did lead to life (here, anyway). Or maybe some combination.
But without another independant example of life in the universe, all of our estimates are stabs in the dark.
As for us being really lucky that our planet is perfect for life, that is backwards logic. We would only exist on a planet suitable for life, so the odds of the Earth being suitable for us is 1. Any suitable planet would do, and we would call it Earth. It is perfect because early life has changed the environment into something suitable for large organisms, who in turn have evolved to be perfect for the environment. And are pretty hardy, as far as giant rocks falling out of the sky are concerned.
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