Structures absent in adults of some groups often appear in their:

1. Development retraces some of the steps that evolution took. 2. As a result, structures that are absent in some groups often appear in their embryonic forms. 3. They then disappear by the time the adult or juvenile form is reached. 4. That transient appearance is treated as evidence of shared ancestry. 5. Fossils and sequences give separate lines of evidence rather than this one. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 18 'Evolution and the Origin of Species', sections 18.1-18.3_