If you'll notice, not every planet has life popping up everywhere. The earth was extremely lucky, and humans were extremely luckier.
And I'm assuming that a life could evolve that doesn't use water, and it probably could.
And you may notice that most animals have VERY CLOSE TO THE SAME DNA CONFIGURATION. One kind of animal/cell got this configuration, and then things probably just took off, and that animal took over. Then, it changed.
Also, here's an example. There is a very strange mutation in humans that lets them have the sexual organs of both sexes. This has only been recorded to happen twice, and I wouldn't be suprised if those was the only two times that it happened. Both times, the person has undergone an operation or something (Well, I know at least one did. The other might have been killed, depending on when it was born.)
Now, if the mutation hadn't been gotten rid of, in the early days of human evolution, they would have quickly taken over due to pure reproductive speed. However, this small change would make the population EXTREMELY inbred, making them very vunerable to genitic diseases or viruses, and most likely wiping them out.
Just showing how a tiny change can make or break a species.
Poll