Hi Honkytntmn ( great user name )
USincognito explained it better than I could have done.
Species only evolve into modifications of themselves they can't become something entirely different.
But over billions of years, or even tens of millions of years, that can produce massive diversity.
Species carry their heritage with them, we are humans but we carry our history in our genes, we are mammals, tetrapods, chordates etc.
And if you go back far enough we too are just modified worms, we share a common ancestor with modern worms that was probably pretty worm like and lived around 600-700 million years ago.
But whilst modern worms still look pretty much the same we have been on a much more interesting evolutionary journey.
The usual creationist question to this is why, in that case there are still worms, why didn't they evolve so much.
To which the answer is; because being a worm is a perfectly good answer to passing on your DNA, the ecological niches that worms inhabit have always been there, it is just that some worms evolved to fill different niches and we are the end product of hundreds of millions of years of such evolution as is pretty much every other multicellular animal on Earth.
The is a name for the branch of biology that characterises everything in terms of its evolutionary heritage is cladistics
http://en.wikipedia.org/wiki/Cladistics