The inner workings of terrestrial organisms -- from microbes to men -- are so similar in their biochemical details as to make it highly likely that all organisms of the earth have evolved from a single instance of the origin of life. -- Sagan Intelligent Life in the Universe
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Certainly, similarities in organisms can indicate closeness of relationship, but this is not a dependable guide because there are many cases of close similarity that could not be due to inheritance from a recent common ancestor. If similarities show evolutionary relationship, then dissimilarities should conversely show a lack of relationship.
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Whenever new types of organisms are occasionally discovered they never turn out to be ancestral to known groups but stand related only as sister groups in keeping with the thesis that nature's basic order is circumferential rather than sequential.
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We now know, for instance, that man in his prenatal stages does not go through the complete evolution of life -- from primitive single cell to fishlike water creature to man. Today it is known that every step in the fetal development process is specifically human.
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