The oldest undisputed fossils, stromatolites, are about three and a half billion years old. As I said, extremely early fossils are very rare because the older the rocks are the more likely they are to have been destroyed by the rock cycle. It isn't beyond imagination that even older life existed - evidence for it, for the same reason, would be pretty tough to come by. We're already, with the fossils we have, only just over a billion years from the formation of the Earth, and who's to say there wasn't life much earlier. Mrrzy's point, that "life began on this planet before it (the planet) had finished forming", isn't too far-fetched. It's also worth pointing out that life has existed on Earth through some extreme environmental changes, which you could argue means that life on other planets is all the more likely. Life is resilient and not too fussy (as long as it has liquid water, on the whole). For billions of years it even got by very well without free oxygen. There are plenty of opportunities for it out there. All we need now is the evidence. I can wait for that.