In November 2010, comet Hartley 2, a dislodged comet from the Kuiper belt (a region of the solar-system beyond Neptune's orbit). It was at this time the spacecraft, Deep Impact/EPOXI photographed earth-like water - suggesting the original source of Earth's water.
Comets are usually covered in ice, making them an obvious candidate for the origin of Earth's water. Although, some scientists say that comets would account for no more than 10% of water's origin, and that computer modules indicates asteroids as the main origin of water.
Scientists compared isotopes of hydrogen, in water from Earth and water from asteroids and comets. They found that all isotopes have the same amount of protons, but each have different numbers of neutrons.
Either way, science is pointing towards the fact that live on earth was introduced by collisions of extraterrestrial substances. Considering that water is crucial to life on earth - everywhere life virtually is found on our planet everywhere there is water - it's a fascinating concept that it was a foreign introduction.