Abstract
<jats:title>Abstract</jats:title> <jats:p>Although the concept of ‘ancestor’ is familiar, because we all have parents and grandparents, it is difficult to know how the ancestor of a modern animal can be recognized as a fossil. This is essential to form natural groups, that is, to group animals by common descent, as Darwinism demands. In practice, animals are placed in the same group when they share derived characters. Animals which have similar ways of life tend to evolve similar adaptations, and might be included in the same group because of their similarity despite having different ancestors. In general, the characters that most reliably indicate descent are usually those that appear earlier in development. This generalization led to an emphasis on embryos to chart the divergence of different kinds of animal.</jats:p>