|MadSci Network: Evolution|
The very oldest species on Earth are the cyanobacteria, which seem to have been around virtually unchanged for nearly 4 billion years. This is based on fossils of that age that have been compared under high-powered microscopes to existing species of these bacteria. Other bacteria that are about as old are the archaea, which thrive in acidic, high- temperature, or high-radiation environments.
Another remnant of ancient organisms includes the stromatolites, mounds of algae and bacteria that today grow in the Bahamas, Shark's Bay Australia, and a few other limited environments. They have been around, also largely unchanged, for over 3 billion years and at one time were the Earth's dominant form of life.
On the animal side of things, the brachiopod Lingulaa is probably the oldest, having existed nearly unchanged for over 500 million years, and the horseshoe crab (limulus) has been on the planet for several hundred million years, too. Since the average "lifespan" of a species is only a few million years, having such ancient species around is very impressive.
A good book about the earliest life is "Cradle of Life" by J. William Schopf. He is one of the premier scientists doing this sort of research. At a more advanced level, he has edited two scientific books, "Earth's Earliest Biosphere" and "The Proterozoic Biosphere", which should be in your university's geology or science library.
Try the links in the MadSci Library for more information on Evolution.