Panspermia is the idea that life on earth was 'seeded' by the arrival of organisms or compounds from space. Not necessarily little green men with a sinister plan, but maybe primitive microbes that then evolved into all life on earth. An interesting idea that is usually rarely mentioned in biology courses because, for me at least, the more interesting question is could life have once spontaneously arisen on earth? If so, under what conditions and how do these conditions relate to what we know about the early earth?
However if you want to read more about panspermia there are some fanatics on the web (just google it) or you can read this 2005 Scientific American paper - Did Life Come From Another World? for a more balanced view.