You should also realize that in addition to the basic seven taxonomic categories, many other ranks exist (Subphyla, Superclasses, Subspecies, etc.). However, all organisms must still be assigned at a minimum to the seven obligatory taxa.
Note also, that recent molecular studies have led systematists to the conclusion that all life shares a common ancestor and that there are three major evolutionary lineages. Accordingly, each of these lineages is called a Domain, which supercedes the Kingdom as the broadest taxonomic grouping.
These Domains are:
- prokaryotic microbes that inhabit extreme, anerobic environments
- true bacteria, which are prokaryotic as well
- all eukaryotic organisms, that is, the protists, fungi, plants and animals.
Note: Eukaryotic organisms arose about 1.5 billion years ago, probably through the fusion of several specialized microbes into a larger, unspecialized one.