text: None of the ideas suggested in the answers so far would result in an end of ALL life on earth. As humans we think of ourselves as important, but in the greater scheme of things we are not. A biog comet impact might wipe out humans, but it would not wipe out life, in fact for many of the surviving species it would be a very poitive outcome, getting rid of homo sapiens, the planets biggest poluters and parasites. The end of lie on Earht is likely to happen when the sun reaches the end of it current stage of hydrogen fusion, this will happen in about 5 billion yeras, so we are hal way there. the sun will at first contract, then as it gets hot enough for Helium to fuse it will expand and become a REd Giant. THIS will end life on earth, effectively. The earth may even be swallowed up as the sun swells to a diameter of around 300,000,000 km which is about the same as the earths orbit (so the earth would be at the surface of the sun).