Although most paleontologists wouldn't recognize them as true fish, the first fish-like creatures to leave an impression on the fossil record appeared during the middle Cambrian period, about 530 million years ago. The most famous of these, Pikaia, looked more like a worm than a fish, but it had four features crucial to later fish (and vertebrate) evolution: a head distinct from its tail, bilateral symmetry (the left side of its body looked like the right side), V-shaped muscles, and most importantly, a nerve cord running down the length of its body. Because this cord wasn't protected by a tube of bone or cartilage, Pikaia was technically a "chordate" rather than a vertebrate, but it still lay at the root of the vertebrate family tree.
Two other Cambrian proto-fish were a bit more robust than Pikaia. Haikouichthys is considered by some experts--at least those not overly concerned by its lack of a calcified backbone — to be the earliest jawless fish, and this inch-long creature had rudimentary fins running along the top and bottom of its body. The similar Myllokunmingia was slightly less elongated than either Pikaia or Haikouichthys, and it also had pouched gills and (possibly) a skull made of cartilage. (Other fish-like creatures may have predated these three genera by tens of millions of years; unfortunately, they haven't left any fossil remains.)
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