Soon after the development of a centralized brain & sensory system, this delicate complex of nerves & organs was given a dual layer of protection from damage by a novel new kind of tissue. This new tissue took relatively common calcium out of the water & bound it together with the comparatively rare, but essential, phosphorus into a hard crystal network now called bone. A bony covering of the anterior part of the body protected sense organs (and, quite possibly, kept electrical receptors from shorting out), while another form of this tissue formed a series of protective struts over the surface of the spinal cord & enlarged brain. These bony plates later expanded somewhat to form the skeletal basis for modern vertebrate animals: the backbone.
Up to the late Cambrian, chordates gathered food in a manner that didn't differ much from the way a vacuum cleaner picks up dust. Water was pulled into the mouth and with it any loose particles. The water was strained in the region of the gills and the edible stuff quickly swallowed. The sources of the edible "food" varied, from tiny animals and plants floating in the water, to bits that are stuck to something else and forcibly scraped away by a mobile "tongue". In all cases the trapped food particles needed to be swallowed quickly before they could get out through the mouth. For most vertebrates, all thatchanged when the first gill arch became hinged in such a way that the lower and upper parts could touch. For the first time, these animals with backbones could stop its food from escaping simply by blocking its exit! With the later addition of teeth to these newly formed jaws, food could be grasped, larger food items bitten, and "Jaws" was born.
The evolution of jaws unleashed an incredible array of vertebrate forms. Not only did jaws enable vertebrates to keep food inside their mouths, but with stronger bones and associated muscles, jaws could be used to bite off pieces of food, crush the hard coverings of other kinds of animals, or daintily pluck a tidy morsel with surgeon like precision. As a number of novel ways to use jaws increased, so did the diversity of body forms. Among the many novel forms were the early sharks, the bony headed placoderms, and strange fishes that had their fins at the ends of stout, muscular stumps. This later group, the lobe-fins, also held on to a pair of lungs in addition to their perfectly good gills.With lungs and limbs, lobe-fins could go where no other fish could, shallow stagnant ponds and bays. It was just one small step from there to a whole new world: land!
Once lobe-fins started extracting oxygen from the air, there was no looking back. Atmospheric air has lots more oxygen in it than can be found in even the best quality water. Gills may have still been present, but were not necessary for sustaining life. At about the same time, the back end of nasal cavity took a wrong turn and poked a hole inside the palate. In all earlier vertebrates, the nasal passageway was only used to detect deliscious smelling molecules in the water. Now, fluids can pass from outside the body directly into the mouth, in much the same way you get water up your nose when swimming. For the still mostly aquatic lobe-fins, life couldn't be better. By sticking their noses above the water-line, they could breath the oxygenated air without exposing themselves, or their sensitive skin, to the hot sun. When the sun wasn't a threat, they could perhaps freely roam on land with limbs that were far more flexible at the hip and shoulder joint. Land dwelling was becoming more of a reality, and some of the accessories of aquatic life, such as a dorsal and anal fin, soon vanished.

All Text © Dr. Carl Ferraris

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