What Percent of animals have gone extinct over the history of earths existence?

Countless thousands of species went extinct before man every showed up on earth. The difference is: Since we weren't here to screw anything up, it was a natural part of nature. Now even today, the great majority of species facing extinction are there due to no fault of man's - nature is continually extincting species and replacing them with new ones, man or not. But some species are definitely near the brink because of man, and those are the ones we are working hardest to save. For if they become extinct, it won't be natures doing - it will be our fault.