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This is why I have a real problem with people who claim that 'Earth like' planets are common - they are not. When they state a 'large object' struck the Earth, they are talking about a planet size object the size of Mars!

Without the moon conditions on the Earth would not have been stable enough for complex life to form. Sure there may still have been some sorts of simple life, such as a soup or maybe even bacteria, but without a large object to stabilize rotation and axis conditions would not be stable enough for complex life to form. You need not only conditions that are stable (such as a longer day) to allow development, but also complex enough (like the seasons) to force variation. Without the Moon these conditions would not exist.

So when they calculate the likelihood of 'Earth like' planets they really need to throw in a variable for the likelihood of two planetary objects, in the Goldilocks zone, of colliding to form a planet/moon system with a moon of comparable size. The size of the moon is what makes it special, it's relative size to the Earth is what gives it it's stabilizing ability. No other known moon is as relatively large to it's host planet as the Moon. The normal way that moons form will not allow the creation of such a moon, every other moon in the solar system is tiny to it's host, the Moon is not.

When people claim scientists keep finding 'Earth like' planets, I turn away and shake my head, because it's simply not true, the likelihood of another planets having the conditions that allowed complex life on Earth to exist is extremely rare.