Panspermia
Basically the theory of Panspermia states that life on this planet was originally brought here by asteroids that crashed on the earth in it's early history. The thinking is that the asteroids that brought this life were the same ones that originally brought water here as well. These life forms are thought to have been very basic single celled organisms that were able to adapt and change quickly to suit different environments. These asteroids worked as nature's sperm, traveling the galaxy and impregnating different planets. When the asteroids fell to the planets that housed the correct combinations to promote life (such as heat), the organisms spread and changed the environment themselves to allow for more advanced forms of life.
Many variations on the panspermia idea have been proposed. William Thomson proposed that spores might travel aboard meteorites, giving them better protection from high-energy radiation in space. It is possible that large violent events (volcanic explosions, or maybe planetary collisions) would throw rocks from biologically active planets into space and far enough away from the planets gravity allowing them to travel great distances. There is now overwhelming evidence that ballistic panspermia occasionally operates between worlds of the same planetary system. This follows the discovery of meteorites on Earth that have almost certainly come from the surface of Mars and the Moon.
In recent years there has been mounting
evidence of at least some extraterrestrial input to the formative
stages of planet-based biology. Organic chemicals have been detected
in
interstellar clouds
(similar to that from which the Solar System formed),
comets, and meteorites. It has been estimated
that 37,000-78,000 tons of raw material falls to earth each year. So
that if there is life forms in cosmic debris it would not be
farfetched to say that was how life originated on this planet
(perhaps many others as well) or even that life forms may still be
getting deposited hear.
Sources
http://leiwenwu.tripod.com/panspermia.htm
A few others on the Reference Links Page