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Evolution of skeleton and locomotion in vertebrates
The evolution of different groups of multicellular animals reflects the evolution of skeleton. The earliest vertebrates were fish like having different organs for swimming. The jaw-less fishes swam with undulations of their body. Usually there were no paired fins and the body moved by alternating waves or ripples passing throughout the body from head to tail of the organism. These waves passed through the muscles on either side of the body. This caused the body to undulate from side to side. These undulations were transmitted through the tail and the back of the body as a series of backward pushing against the denser water and as a reaction to theses series of backward pushes the animals of that time achieved a very effective method of moving efficiently. This is the primary type of locomotion in the vertebrates. For such swimming animals the backbone is generally flexible. This character is found in fishes where the vertebral column lacks the articulation for locking the individual bones.
Among the fishes there was a kind of lobe finned fish in which the paired fins were supported by a series of bones arranged in particular way. At the base of each fin was a single long bone. On the upper side was another bone called the shoulder bone and towards the lower side was two bones long in nature lying side by side. On the top of each long bone were many short bones. This was the beginning of the development of four legged or tetrapod vertebrate skeleton. The long bone was that of the upper part of the arm or leg and two lower long bones were that of the middle part of the arm or leg. The several bones were of the feet or hands. Thus the lobe finned fishes can be regarded as the ancestors of amphibians. As the amphibians evolved and left the water they had to face the force of gravity of earth and had to tolerate tense water conditions on land. Hence the vertebral column or back none become stronger for the support of the body and the vertebrae were therefore linked by interlocking segments. In the amphibians the legs sprawled out from the body so the animals dragged themselves on the ground. This type of locomotion also characterized the first reptiles. There was a trend among reptiles of bringing feet beneath the body and raising the body off the ground. Some of the reptiles adopted this method which was efficient for walking.
From the reptiles two divergent lines appeared. In some reptiles forelimbs were changed into wings and became the ancestors of birds because they adopted the habit of flight. Other reptiles became bipedal in course of time. In mammals the bones of limbs have undergone modifications; they walk on hind limbs or legs and the forelimbs are used for feeding. One very special type of locomotion found among the primates is brachiation. They swing through the trees by using their grasping hands and long arms. Few mammals can also glide on tree and hence gliding membrane is formed between front and hind legs as in flying squirrels.