In earth's infancy, billions of years ago, aquatic life forms were the first to inhabit our planet. A milieu of primitive organisms occupied the earth, which at the time was largely comprised of water. Approximately two billions years ago, these unicellular organisms began to become increasingly complex. These organisms, in turn, gave rise to primitive multicellular organisms, all within an aquatic environment.
The most primitive life forms are known as protozoans ("first animals"). Six hundred million years ago, protozoans began to give rise to organisms that have one of three different types of body symmetry: radial, bilateral, and asymmetrical. The type of body plan an animal possesses demonstrates whether it is primitive (seen early on in evolutionary history) or advanced (more recent).
Asymmetrical organisms are the most primitive invertebrates, and are represented by the sponges which show up in the fossil record about 590 million years ago. Radially symmetrical animals show up later in evolutionary time, and include marine forms such as the jellyfish and comb jellies. Bilaterally symmetrical organisms are the most advanced, and include nearly every other marine creatures. These have given rise to the modern-day mollusks (e.g., octopus, clams), arthropods (e.g., lobsters, horseshoe crabs), annelids (segmented worms), and echinoderms (e.g., sea cucumbers, starfish) as well as the group that is thought to have given rise to modern day vertebrates.
While most aquatic organisms are invertebrates, vertebrates (fish) are much larger and garner more attention. The most primitive type of fish were the jawless fish, or agnathans. These have given rise to modern day lampreys and hagfish, which also have no jaws.
The two major groups of fish we know today, the chondrichthyes (cartilaginous fishes) and osteichthyes (bony fishes) also arose from agnathans. The chondrichthian fish are represented today by the sharks and rays. The osteichthians gave rise to most of the other fish that are familiar to us (e.g., tuna, perch) as well as the group that is thought to have given rise to the amphibians 360 million years ago.
While the evolutionary histories of fishes are diverse, most fishes have a few structures in common. Gills are a means of withdrawing gaseous oxygen out of the water. Water coming into contact with blood-filled gills passes oxygen molecules onto the surface of the gill and removes carbon dioxide, much in the same manner breathing does for terrestrial animals. Bony fishes possess structures known as swim bladders, which are small organs used for buoyancy control. Because cartilaginous fish do not have swim bladders, they must continually move through the water to avoid sinking.
Although species of fishes can look very different from one another, they generally have a similar, fusiform shape that allows them to glide easily through the water and fins to help propel them. A phenomenon known as convergent evolution has brought a similar shape to other aquatic life, including whales, sea turtles, and dolphins. These non-fish vertebrates have the same hydrodynamic shape as many fishes, as well as modified fins known as flippers used for propulsion.
With current environmental crises such as global warming, and increased pollution of aquatic habitats, it is uncertain what the evolution of aquatic life will bring. The evolution of major aquatic life forms took hundreds of millions of years. Most scientists doubt that the many evolutionary changes necessary to adapt to current environmental degradation will occur in time to save many species.