More than 600 million years ago, and perhaps as long as a billion, the eukaryote cell first appeared. Armed with DNA, these new forms of cells took on the ability to do something apparently never tried before: work together. Cells stayed together and divided up responsibility for the life of the multicellular organism. Some cells were specialized for gathering food, others for protection, and still others for reproduction. This theme of specialization continued further, and cells become organized into tissues and, later, tissues onto organs. It wasn't the only way to survive, and it wasn't even necessarily the best. But it was the beginning of the path that led to larger and more complex animals, including man.
Long before the first backboned species evolved, tadpole-shaped animals were wriggling around the seas with the help of a springy, flexible rod that ran down the middle of their tails. Called a notochord, it snapped the tail back as soon as the muscles that tugged at one side or the other relaxed. The tail action flung the protochordate forward in a somewhat controlled motion. Although these tadpole like animals were probably common before the beginning of the Cambrian, no living species resemble this chordate form as adults. Of living animals, only sea squirts have a developmental stage that come close in appearance to these earliest chordates. Sea squirt, or tunicate, larvae swim in the open ocean looking just like miniature chordates for a brief time before settling to the ocean floor for a sessile adult life.
A major change in the protochordate form came about either just before, or at, the beginning of the Cambrian. The muscles that pulled at the notochord became organized into bands on either side of the body. These bands of muscles, called myomeres, were controlled by a series of nerves that could stimulate the contraction of one myomere at a time. Precisely timed pulses of contractions were sent down one side of the body and then, a fraction of a second later, down the other. The result was a series of waves that bend the body from side to side, much like what we now see in snakes and eels. These early chordates, such as Pikaia, could now swim through the water, or burrow through soft sediments to feed and escape from predators.
Once they became mobile, chordates required increased amounts of information about their surroundings. Information on how the water conditions differed from place to place, where that good smelling food was located, what that dark shadow off in the distance was all about, and so on, all needed to be interpreted, and the sooner the better. The complex of nerves needed to bring information together and integrate it with other sources of information resulted in the formation of a command center, which we now call a brain, at the anterior end of the body. With the brain and sense organs mostly located up at the front end, craniates could better direct their locomotory apparatus to move them in an appropriate direction, as quickly as possible.

All Text © Dr. Carl Ferraris

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