Cosmic coincidences part 2

Four and a half billion years ago, when the earth was just a wee bairn, it was struck by an object the size of Mars. From what physicists can make out collisions of this magnitude are extremely rare in the universe. When the dust of that collision had settled there were two key outcomes. Firstly the earth had grown a bit bigger, and secondly the debris that was kicked into space coagulated to form the unusually large moon.

Typically moons are very small compared to the planet they orbit, but for earth's satellite body things are very different. This is crucial to maintaining our climate. The size of the moon means that it acts as a gravitational steadying force, stabilising the earth's tilt, and in doing so preventing wild fluctuations in the heat coming from the sun.

Conditions might be bad for complex land-based life if there were no moon and obliquity varied significantly.