They are thought to date from the very early days of the solar system. (sciencemag.org)
Some asteroids are known to be rich in water, and some of these would inevitably have crashed into Earth during the chaotic early days of the solar system. (newscientist.com)
Most researchers think that the Moon formed in the very early days of the Solar System, 4.5 billion years ago, when a large protoplanet smashed into the embryonic Earth. (scientificamerican.com)