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Some say life appeared on earth 2 billion years ago. Scientists say "living" things must have been molecules possessing properties that above all distinguishing the living from the nonliving, reproduction. A blanket of cloud that kept the earth warm and atmosphere of stinking methane and ammonia let an oceanic vat of dissolved minerals where large molecule carbon compounds combine and recombine until the right combination was reached.
Fish splashed into the oceans 500 million years ago, but they had no scales nor fins. They had flexible skeletons which were firm to support their swimming muscles. They had round mouths and fed by sucking at their meals like vaccuum cleaners. A large bony sheild protected their heads and gills. Later early fish were shaped something like an arrow about 4 inches long. These new fish were grouped as OSTRACODERMS, or "shell-skinned." The hag and the lamphrey closely resemble these fish.
These fish eventually developed harder backbones and jaws more like mouths. This gave them more versatility and easy adaptability in different habitats. Jaw fish appeared roughly 435 million years ago and were called PLACODERMS, or "plate-skinned." They are now believed to be completely extinct.
CHONDRICHTHYES, which means "cartilage fish," came next. This group is the sharks, rays, and chimeras. They have highly developed jaws and their fins and bodies resemble fans. Rays more closely resemble their ancestors.
OSTEICHTHYES, or "bony fish," came after cartilaginous fishes. These fish have complete skeletons made of bone, rather than cartilage, leaving more fossil records.
Modern cetaceans arrived 55 million years ago. Fossil records indicate a relationship to even-toed ungulates, hoofed animals such as cows and deer. The animals began their transition in the ancient Thethys Sea, which is now a region of the Mediterranean and the Asian Sea. These fossil records are of ARCHAEOCETI, or "ancient tooth" and are the first sub-Order of whales. They lived in estuaries, not far from shore, and still had four stout limbs.
