Man can only have a certain quantity of hair, teeth and ideas. There comes a time when he necessarily loses his teeth, his hair and his ideas. -Voltaire.
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...When we look at living species (A and B) that we know shared a common ancestor resembling one of them (A), we can guess that the features seen in A evolved in steps more or less linearly to eventually resemble the corresponding features seen in B.
One of the first things I was ever told about what makes reptiles different from mammals was that reptile teeth were the same all throughout their jaws (called a homodont condition) and that mammal teeth were different throughout their jaws (called a heterodont condition).