The fossil record and modern studies on genetics have shown how animals have evolved from the simplest worm-like organisms.
The development and relationship of animals can be drawn like a tree, with branches representing each of the major forms of life. This shows that vertebrates probably split off from primitive worm-like ancestors many millions of years ago. Invertebrates, such as insects and spiders, developed along quite a different line of evolution. Many branches of the evolutionary tree were unsuccessful and have died out, for example the dinosaurs.
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