The occurrence of diversity of animals on earth does not suggest their diverse ancestry. There are number of types of animals on this earth. On the basis of certain common characters, these animals are grouped into different group and subgroups. For example- On the basis of presence or absence of notochord, all animals are grouped into two groups- Chordata (with notochord) and Non-Chordata (without notochord).
Because of common characters, we draw a conclusion about common ancestry.
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