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Collision With Earth's "Little Sister" Created the Moon
astroengine writes The primordial planet believed to have smashed into baby Earth, creating a cloud of debris that eventually formed into the moon, was chemically a near-match to Earth, a new study shows. The finding, reported in this week's Nature, helps resolve a long-standing puzzle about why Earth and the moon are nearly twins in terms of composition. Computer models show that most of the material that formed the moon would have come from the shattered impactor, a planetary body referred to as Theia, which should have a slightly different isotopic makeup than Earth.
Orbit is too circular for that solution to work.
Our substantial magnetic field may be due to the merging of the iron cores of the Proto-Earth and Theia. Earth is the most dense planet in the solar system, and from what we know of Mars and Venus, we suspect that our iron core is far larger than the other terrestrial planets.
Venus has a super-thick poisonous atmosphere; it's at least possible that our large Moon has, over a period of 4+ billion years, "skimmed away" enough of our atmosphere to have protected the Earth from a similar fate.
Of course, we only think that our atmosphere is right because we evolved here, in this atmosphere; if the atmosphere had been different, we would have evolved differently, and (had intelligent life developed at all) we'd think that THAT was the right sort of atmosphere.
Causing an environmental catastrophe at the time.
Yep, oxygen build up was a disaster for the cyanobacteria that created it and had reigned Earth for 3+ billion years. On the plus side the free oxygen enabled collagen to form, which is the substance that holds single cells together in multi-cellular organisms. We call that transition "The Cambrian explosion". The collision we are talking about occurred 3.5 billions years before the Cambrian explosion.
And did you exchange a walk on part in the war for a lead role in a cage? - Pink Floyd.