Oliver Morton in More Intelligent Life:
The idea that the Earth and Moon might be made of the same stuff seems sensible at first glance; compared with most objects in the solar system, they are remarkably near each other. In the 19th and early 20th centuries there were various theories that they were formed together and subsequently separated. Unfortunately such arguments had a problem. The conservation of angular momentum means that spinning things go slower when pushed apart, faster when closer together (this explains why ice skaters spin so fast when they tuck their elbows in). The Earth-Moon system has a great deal of angular momentum, so if its two parts had been joined when they were created they would have had to spin as fast as an ice skater turning into a drill bit and boring her way down through the rink.
Hence suggestions that the Moon was created elsewhere and captured by the Earth’s gravity. But once the Apollo programme had shown how chemically earthlike the moon was, they seemed implausible too. In 1974 scientists hit on a new idea: that a wandering planet the size of Mars had struck the Earth a glancing blow. This, they suggested, caused the cores of each body to merge and part of their rocky mantles to be thrown into orbit, where the molten mess eventually condensed into the Moon. The catastrophic arrival of the impactor explained both why Moon rocks look earthly and why the Earth-Moon system has a lot of angular momentum—if one skater grabs another’s outstretched arm as he passes, the two will be set a-spinning. It was a simple idea, but radical. And it had profound implications. The fact that the Earth has a big moon stops its axis from wobbling about too much—Mars, which has titchy moons, swings its poles around like a drunken drum majorette. A stable alignment of the poles may have kept the Earth’s climate stable too, and thus hospitable towards life. So an accidental collision in the early solar system could have been crucial to the emergence of complex life on Earth. We could be the result of a planetary fluke.