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Three years motrin 500 para que sirve Next Yu and his colleagues turned to a computer program to draw a new family tree for the jawed vertebrates, animals with both jaws and backbones. Early in its history, this group included only fish. But from these humble beginnings arose two important branches of animals. One group was the sharks and rays, whose skeletons are built mostly of cartilage. The second was the bony fishes and their descendants, including amphibians, reptiles, birds and mammals.