All living things (or organisms) on the Earth are classified into
six five (biological) kingdoms. The five kingdoms make up the biological tree of life, and the concept of kingdoms and biological classification were made up by Swedish botanist Carolus Linnaeus. The five kingdoms of biology, defined by Robert Whittaker (1969) are:
- Animalia, for all animals. All organisms in the Animalia kingdom rely on other organisms for their food and are made out of many cells.
- Plantae, for all plants. All organisms in the Plantae kingdom make their own food and are made out of many cells.
- Fungi, for mushrooms and mould. They absorb food from decaying material.
- Protista, for single-celled organisms with a nucleus. Some are like, but not technically, plants and animals.
- Monera, for bacteria. All monerans consist of one cell with no nucleus, but still have DNA.
Carolus Linnaeus originally defined three kingdoms: Animalia (same), Vegetabile (for plants and fungi), and a third one for minerals and gemstones.
There is ongoing debate as to whether viruses, obligate intracellular parasites that lack metabolism and are not capable of replication outside of a host, can be included in a kingdom. If they must be included in one, the decision would be problematic since it is suspected that viruses have arisen multiple times, and they have a penchant for accumulating nucleotide sequences from their hosts.