Two groups that are geographically separate will adapt uniquely to their surroundings and after time will form separate species. Variation comes from mutations, and this variability is acted on by natural selection. The sediments harden to form sedimentary rock and minerals replace the remains, thus preserving details Teeth and bones are common, and can be used to deduce size, gait, where muscles attached, etc. Rock layers have oldest on bottom, but the layers can be disturbed, so geologists age by mineral content and index fossils, which are common in rocks of a certain age. Can also compare layers that are in different areas.
Radioisotopes decay over time.
Rocks can be dated by comparing the ratios of different isotope forms. Comparative anatomy - basic structure is modified by evolutionary pressures. Bird wing, dolphin flipper, human hand have similar bones, but no mechanical need for structural similarity. Homologous structures - features in different species that are similar in underlying structure because of common evolutionary origin. Analogous structures don t have a common evolutionary origin.