Comets and asteroids may have brought water to the Earth. This has largely been disproved for comets as a ratio called the deuterium hydrogen ratio of the water on comets (which has actually been measured by probes) is different to the water that exists in the oceans of Earth. These differences shows that almost definitely the water does not come from comets. It could be that different comets brought water to the Earth from the Kuiper belt at the edges of the solar system or from the Oort cloud. Asteroids may have brought water to the Earth. Recently evidence of water ice and organic material on the asteroid 24 Themis was found. This was a bit of a surprise as the surface of the asteroid was too hot to support water ice as the asteroid was close to the Sun. The water is thought to come from the interior of the asteroid and this would support a theory that the asteroid added to water on the Earth.
Interesting... so, it's a real long shot that comets or asteroids brought over a majority of Earth's water. More than likely it was a fraction, if any. (See Article
for more details)
As for extraterrestrial life, I'm in agreement with TJ... the "coincidence" of sentient life with sufficient technology that is able to communicate at just the right timing so that other sentient life may receive it is up against extremely poor odds. The fact that radio waves must travel hundreds or thousands of years due to the distance and the fact that just 100 years ago we weren't even capable of picking up any transmissions just goes to show you how remote it really is.
Of course, this is all with respect to the start of technologically capable societies... the other side of the coin is the longevity of sentient life. If other sentient species also have to struggle with the incompatibility between civility and primal instincts the way we are presently struggling, the odds of success are not very good. While we can all chat here in extreme comfort, our lives are far more tenuous than most people realize. The infrastructure for distributing essential life sustaining supplies is very weak. Something like the "Dust Bowl
" of the 1930's could easily happen again and with a significantly larger population dependent upon a steady food supply (and aging power grids susceptible to failure) , our societies could easily break down into mass chaos, setting us back technologically for many decades to come... assuming we are even able to recover.