This is because I currently exist. If you showed me a newly formed planet orbiting in the habitable zone of a G type star and asked me the odds of me turning up and walking around after 4 and a half billion years I would rate them as very very small.

Anyway, my point was that gene changes are fairly common, you know some mutants. And most of these mutations are not fatal. One reason why is because we have a double set of chromosones. But new mutations are constantly entering the genepool. I was under the impression you might have thought that our genetics had to be "just right" for us to survive when really it's more a matter of whatever works. We are rather shoddily put together. I produce more earwax than I could ever concievably need.