To understand the origins of the dinosaurs, it is necessary to understand some of the basic principles of evolution...
Charles Darwin proposed the idea of evolution in his book 'On The Origin Of Species' in 1859. He called evolution 'descent with modification'. It is the process by which all life on earth has diversified from bacterial mats that existed over 3.6 billion years (3,600,000,000,000 years) ago. Evolution has had a long time.
For a long time it was mistakenly thought that evolution was a simple linear progression, with humankind at the top of the ladder. This old view was replaced long ago as new evidence came to light. We now understand that evolution proceeds in a kind of branching pattern, with species on one branch giving rise to other branches and so on.
Evolution proceeds as changes (mutations) occur in the genetic code of an organism. These mutations are 'tested' by the environment in which the organism exists. Many (most) of the changes will be harmful to the organism, and it will die before the harmful genes are passed on to its offspring. Occasionally, however, a gene combination will arise the actually improves the adaptation of the organism to its' particular environment and these genes are more likely to be passed on. By this process, called 'natural selection', all life has branched. The process of natural selection is the means by which the great diversity of dinosaurs was able to evolve.
To reconstruct the evolutionary history of a related group of animals, such as the dinosaurs, it is necessary to look at the detailed shape of the bones. Not all dinosaurs are completely different. Some share certain similarities in their bone structure. By finding out what dinosaurs share these similarities, or 'characters', it is possible to produce an evolutionary tree, or phylogeny, showing the relationships of the animals to their ancestors.
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