How do you form a habitable planet?

The longer one looks at the requirements for the development of higher order life on Earth, including us, the more extraordinary it seems. For this discussion I express appreciation to Ward & Brownlee for their book "Rare Earth" and to Gonzalez and Richards for their book "Privileged Planet".

"Rare Earth" is an extraordinary adventure story with lots of scenarios where the conditions for life are in precarious balance. Neither of these sets of authors would claim that their books are the final word on these things, and certainly this treatment is but a brief and tentative overview. Some of the relevant conditions or scenarios are collected below.