All living things evolve to fill a certain part of the environment, called a "niche." Dogs evolved to fill their niche, elephants evolved separately to fill their niche. They have absolutely different ancestors, and your dog will never evolve into an elephant. All environmental niches are filled with some living creature, be it animal or plant. It keeps our entire planet alive, if you believe in the Gaea hypothesis. Personally, I believe in evolution, but I think there's a lot more that we need to know about it.
I'm not an atheist.