I will start by saying I am by no means an evolutionary biologist, so I will answer the question the best I can. Since your question assumes evolution does exist (for the sake of argument) I will answer your question as such and not debate whether it does or noes not exist (as that is not your question).
To put it very simply, Life started as single cell organisms, those eventually evolved into more complex multi-cellular organism. For instance, blue green algae appear to be plant, photosynthesize, but are actually bacteria. Fungi have characteristics of both plant and animal. Sponges are basically a bunch of single cell organisms living in a commune so to speak. Even single cell organisms have other single celled organisms living inside them. Viruses can invade and replicate a cell's DNA. This is how we think organelles (and other multi-cellular characteristics) evolved. So eventually these things formed into invertebrate, chordates, vertebrates. Those vertebrates may or may not evolve into organisms that can move out of the water (via wings, legs, lungs, etc.). This may happen for various reasons (competition, disaster, limited resources, "it was a fluke and happened to work."). Its survival of whoever passes along their genes and produces viable offspring who do the same.
With the study of molecular genetics we are understanding how similar we are (and how vastly different) and how small molecular changes can have large morphological changes. Whales' skeletal structure possess "hands" and pelvic girdles similar to humans (relatively speaking). Embryos of many animals look very similar. Evolution is much more like a tree than a strait line. It is not a simple X-> Y-> Z, things branch off and die. Branches branch off of other branches, etc. This happens over Millions of years, not in a couple of life times. Although viruses can mutate very quickly (and I believe have been shown to "evolve" very quickly- I'm not a virologist either). To say we evolved from rocks, fish or even chimps is vastly oversimplifying the concept.
I hope this answers your question. Maybe someone more adept in evolutionary biology will respond.