At least one more factor in the Drake  Equation should be introduced. And that's a magnetic field of the right intensity early on. 4 Billion to 3.5 Billion years ago the Moon was very close to Earth and was still spinning on it's axis. It had it's own inner dynamo and had a very strong magnetic field. Being close, it combined with Earth's magnetic field forming one big, strong magnetic field. This may have allowed for early life to flourish where it otherwise would not have. So a stablizing sattelite also might be a key factor in the Drake equation.,(reply>), That falls under habitable