One of the goals of biological science is to arrange species living and extinct into an evolutionary tree. Each branch on the tree, a taxa, can be named. What identifies these branches are certain characters (forms) which exist in that branch alone, features which evolve once and then stick around indefinitely (though not necessarily in the same form). Those characters used to locate species on the evolutionary tree are called synapomorphies, meaning shared (syn) derived (apo) form (morph). For example, breasts are a synapomorphy of mammals.

A cat is a mammal (having hair, mammary glands, single jawbones, and three tiny bones in the ear). All mammals are synapsids, having one temporal fenestra (a hole in the skull for jaw muscles). All synapsids (and all reptiles) are amniotes, and all amniotes (along with amphibians) are tetrapods, having four limbs. Tetrapods, including you and I, belong to a larger category called sarcopterygians, the lobe-finned fishes, with muscles in their fins. You and I call our fins "arms" and "legs", and have made a few changes to them, but they're still fins. The coelacanth, another lobe-finned fish, never saw the need to upgrade.

Let's change tactics and go up the tree instead of down. CVA begins with the chordates, a vast group of land and sea animals which are distinguished by four big inventions. The notochord, a simple backbone made of cartilage-like material, made it possible for a chordate to use an efficient undulating motion (ultimately the basis for all locomotion) to swim without squishing itself. The dorsal hollow nerve cord came about to control all those new and improved muscles. The endostyle was a simple groove that secreted mucus to trap food particles. The post-anal tail provided more muscles for better swimming. Other animals had tails, of course, but they all pooped out the end of them (think earthworms). In case you're wondering where you're hiding all of these marvelous synapomorphies, they are the pulp of your backbone's intervertebral discs, your spinal cord, your thyroid gland, and your tailbone, respectively. The occasional baby born with a vestigial tail is a reminder of our heritage.

Branching from the chordates are the craniates, which have a more advanced five-part brain, new sense organs, a semicircular canal to determine position, and other innovations. In particular, the craniates brought along a development in, well, development -- an embryonic feature called the neural crest. It's so important it occasionally gets called the "fourth germ layer", along with ectoderm, endoderm, and mesoderm (trust me, you'll need to know those). Without leftover cells from the neural crest, you would be missing skin pigment, peripheral nerve ganglia, teeth, and the better part of your face.

The sister group to craniates (i.e., the chordate group closest to craniates that's not a craniate) is, depending on what research you believe, either cephalochordates or urochordates. The lancet (amphioxus) is a cephalochordate; the sea squirt is a urochordate or tunicate. You'll want to get familiar with both of them, especially the sea squirt larva, because it's probably the closest thing you'll find to an "original" chordate.

Within the craniates are the vertebrates, with a second semicircular canal, and electrical and pressure wave sensors. We probably still have the genes for those sensors somewhere, although they wouldn't do us much good unless we go the way of the whale and the dolphin. Vertebrates are also remarkable for having bone, which likely evolved not for armor or support, but to insulate those new electrical sensors. Things that evolve to do one job, and then later do others, are a common motif in evolution.

Hagfish are the sister group to the vertebrates. Hagfish burrow into dead things through their assholes, eat out the guts, and then crawl back out again leaving a bag of bones. In addition to their marvelous table manners, hagfish are notable for release huge quantities of incredibly slippery slime when frightened, giving them the name slime eels. Eelskin wallets are actually made of hagfish. They're considered a delicacy in some areas, and are supposedly quite tasty. According to recent research hagfish slime makes excellent egg substitute for scones.

Within vertebrates are gnathostomes, whose jaws, pelvic fins, movable head, and third semicircular canal would have made them killing machines in comparison to the competition. The sister group of gnathostomes is the lamprey. From our vantage point, they're on a distant branch of the evolutionary tree, but nonetheless lampreys are just as evolved as we are. It found its niche -- clamping onto living fish and sucking their innards out -- long before you and I came along, and has stayed there happily ever since. It doesn't take very many lamprey to wipe out an entire lake of "higher" species.

Finally, within the gnathostomes are the osteichthians, or bony fish, with their air sacs (we like to use them to breathe, since our gills aren't doing us much good) and advanced use of bone. The chondrichthians are the "cartilage fish", including Squalus acanthias (dogfish) whose branchial basket is featured above.

And that brings us back to lobe-fin fishes and Gizmo's arm and leg muscles.