Bacteria. From Mars. Transported on meteorites

I reckon it's a good word and a good theory.
[Edit: actually I just discovered a website dedicated to panspermia
http://www.panspermia.org/! Lots of fascinating stuff.]

The life doesn't have to come from Mars on meteorites, of course - it could arrive on comets from distant nebulae as per Fred Hoyle's theories (once sneerily poo-pooed, now right back in fashion).

Actually Fred Hoyle is credited as a pioneer in this area, though he is also known for "his seminal contributions to the theory of the structure of stars"

http://www.cf.ac.uk/maths/wickramasinghe/hoyle.html

(advance apologies, this page has a background that really screws your eyes up)