Generally with the oldest and simplest fossils found on the bottom of undisturbed sedimentary rock strata. By examining these specimens we can see a gradual change in species over a long period of time. Sometimes species will remain the same for a long period of time and then change very quickly. This is usually the result of some catastrophic disaster like an earthquake or volcano. This is a fancy way of stating that how we develop as an embryo reflects the organisms from which we may have evolved.
Humans have gills and a tail early in our embryonic development.
These structures have a common origin but have evolved specific functions for their specific organism. The flipper of a whale and the wing of a bat Insects are arthropods and birds are vertebrates. The wing of a bird and the wing of a butterfly are examples of analogous structures. Similar functions but NOT structurally related. It tries to depict what the Earth was like billions of years ago.