Estimates of 5% may be misleading because they are based on the "Goldilocks zone" for
this planet as it is now. Such a zone for any planet may be much much wider.
Remember that the long term carbon cycle of a planet has a major effect. Mars is no longer geologically active and is now much colder than it once was.
Remember too that without the natural Greenhouse effect the average global temperature would be 25 degrees below.
James Lovelock's Gaia hypothesis (
http://en.wikipedia.org/wiki/Gaia_hypothesis)
suggests that organisms can have an active role in maintaining the climate of a planet to be suitable for themselves.
This originally met with a lot of skepticism not least from Richard Dawkins. Lovelock then came up with the "Daisy World" mode showing how in principal a feedback mechanism of this type might work.
http://en.wikipedia.org/wiki/Gaia_hypothesis#D aisyworld_simulations
Which satisfied these objections.
Because organisms evolve as the planet's climate evolved an Earth in a different position might have resulted in very different life forms, or different life forms that affected the climate in a different manner to bring about a climate similar to the one we have now