Some of the most interesting homologous structures are vestigial organs, structures that have marginal, if any, importance to a living organism, but which had important functions in the organism's ancestors.
For example, the skeletons of some snakes and of fossil whales retain vestiges of the pelvis and leg bones of walking ancestors.
The succession of fossil forms is consistent with what is known from other types of evidence about the major branches of descent in the tree of life.
For example, considerable evidence suggests that prokaryotes are the ancestors of all life and should precede all eukaryotes in the fossil record. In fact, the oldest known fossils are prokaryotes.
Fossil fishes predate all other vertebrates, with amphibians next, followed by reptiles, then mammals and birds.
This is consistent with the history of vertebrate descent supported by many other types of evidence.
The Darwinian view of life also predicts that evolutionary transitions should leave signs in the fossil record.
Paleontologists have discovered fossils of many such transitional forms that link ancient organisms to modern species.
For example, fossil evidence documents the origin of birds from one branch of dinosaurs.
Recent discoveries include fossilized whales that link these aquatic mammals to their terrestrial ancestors.
Kingdom: Animal, Plant, Fungi, Protista, and Monera, phylum, class, order, family, genus, species
Each taxonomic level is more comprehensive than the previous one.
As an example, all species of cats are mammals, but not all mammals are cats.
The named taxonomic unit at any level is called a taxon.
Example: Panthera is a taxon at the genus level, and Mammalia is a taxon at the class level that includes all of the many orders of mammals.
Higher classification levels are not defined by some measurable characteristic, such as the reproductive isolation that separates biological species.
As a result, the larger categories are not comparable between lineages.
An order of snails does not necessarily exhibit the same degree of morphological or genetic diversity as an order of mammals.
Phylogeny: the evolutionary history of a species or a group of species
Taxonomy: division of organisms into categories; naming and classifying the diversity=Binomial nomenclature and hierarchical classification