Mars lost all of its (surface) water presumably due to solar wind, and thus never had time to evolve complex life. Earth would have faced the same fate were it not for the fact that the Moon collided with us at some distant point in time, surrendering its iron core to ours. Now, our Rare Earth has an unnaturally large active iron core, resulting in an unnaturally efficient magnetic field which is able to protect our waters from being blown away by Sun.
– https://news.ycombinator.com/item?id=33370397
Note: In my current understanding, it wasn't the Moon colliding with the early Earth, but another planet.