The classification of living things involves organizing life forms based on shared characteristics. The word taxonomy is the term used to describe the classification of living things. The taxonomy of living things starts at the broadest level and moves to the most specific level. The basic taxonomy of living things involves eight distinct levels. Each level of the taxonomy is referred to as an individual taxon.
There are three domains: Eukarya, Bacteria and Archaea. The broadest way to classify a living thing is to decide which of these three domains it falls within.
- Living things within the Eukarya domain are the ones most likely to be covered in general science or basic biology classes. This domain includes plants, animals, protists and fungi.
- Bacteria and Archaea are single-celled organisms that differ with regards to their cellular structure. These are likely to be covered in advanced science courses.
Domain was not always included in the taxonomy of living things. It was taught as a seven-level taxonomy for many years, but an eighth level (domain) was added in 1990.
The basic taxonomy of living things includes six kingdoms. Each of the three domains (above) is associated with particular kingdoms.
- Eubacteria (domain Bacteria) - common bacteria like the good bacteria found in yogurt and the bad bacteria that cause bacterial infections
- Archaebacteria (domain Archaea) - uncommon bacteria such as those found in environments that are devoid of oxygen or are extremely acidic
- Plantae (domain Eukarya) - all plants
- Animalia (domain Eukarya) - all animals
- Fungi (domain Eukarya) - spore producing organisms (mushrooms, tree, yeast, and most mold)
- Protists (domain Eukarya) - microorganisms that don’t fall into one of the other kingdoms (algae and slime mold)
Example: For each level, review how humans are classified. Humans belong to the kingdom Animalia.
Once a decision has been made about which kingdom a living organism should be classified in, the next step is to determine which phylum it belongs in. There are multiple phyla (plural of phylum) for each kingdom. Examples of some of the most commonly known phyla include:
- Anthophyta (kingdom Plantae) - flowering plants, including vegetables, fruit, beans, and nuts
- Arthropoda (kingdom Animalia) - invertebrate animals with an exoskeleton, segmented bodies, jointed limbs, and a vented central nervous system
- Chordata (kingdom Animalia) - animals that have a notochord (like the umbilical cord on a baby), a dorsal hollow nerve cord (spinal cord), pharyngeal slits, and a functional or vestigial tail
- Coniferophyta (kingdom Plantae) - plants that stay green year-round, such as fir and juniper trees
Example: Humans are in the phylum Chordata.
Once an organism has been assigned to a phylum, assigning it to a class is the next step in classification. Many classes are assigned to each phylum. Examples of several classes in the phylum Chordata include:
- Amphibia - animals born with gills that later develop lungs and spend parts of their lives in water and parts on land; they must reproduce in water
- Aves - warm blooded animals with wings; they reproduce through internal fertilization and lay eggs
- Mammalia - vertebrate animals that have specialized teeth, strong jaws, are conceived and born via the mother’s reproductive tract, and nurse from their mothers
- Reptilia - cold-blooded animals with dry, rough skin; all except for snakes are tetrapods (which means they have four legs)
Example: Humans are in the class Mammalia.
Once an organism has been assigned to a class, the next step in classification would be to use a taxonomic key to assign it to an order. Each class includes several orders. For example, there are 19 orders within class Mammalia. A few examples include:
- Carnivora - mammals that have canine teeth beneficial to their primarily meat-centric diet; most are fur covered and tend to be small to medium in size
- Chiroptera - consists solely of bats, which are the only mammals that have the ability to fly
- Primates - mammals with flat nails on theirs hands (instead of claws), relatively large brains, ability to sit and stand upright, requires care for an extended time after birth
Example: Humans are in the order Primates.
After identifying an organism's order, the next step in classification is to determine which family it is in. Each order has multiple families. A few of the dozen primate families include:
- Callitrichidae - the smallest primates (marmosets and tamarins)
- Hylobatidae - the lesser apes (gibbons and siamangs)
- Hominidae - the great apes (chimpanzees, gorillas, orangutans, and humans)
Example: Humans are in the family Hominidae.
Each family in the taxonomy of living things can be identified by genus. For the family Hominidae, the genus divisions are as listed below. Please note that, unlike the previous levels, genus and species names should not be capitalized and should be italicized.
- pan (chimpanzees)
- gorilla (gorillas)
- pongo (orangutans)
- homo (humans)
Example: Humans are in the genus homo.
The final level of classification is species. In some cases, there is only one species per genus, while there are multiple species for others.
- The genus homo has only one species (sapiens).
- The genus gorilla has two species: gorilla beringei (eastern gorillas) and gorilla gorilla (western gorillas).
Example: Humans are in the species sapiens.
If you’re looking for an easy way to help you remember the levels of the taxonomy, there is a mnemonic device that can help you remember the category names in the proper sequence. The first letter in each word of the following sentence is the same as the first letter of each level, in order: “Do kings play chess on fine gold stands?”
- do - Domain
- kings - kingdom
- play - play
- chess - class
- on - order
- fine - family
- gold -genus
- stands - species
This taxonomy is grounded in the work of Carl Linnaeus, who in the 1700s introduced the first system for classifying living organisms in a consistent manner. Prior to the introduction of the Linnaean system, there wasn’t an agreed-upon system for categorizing living organisms. The system he introduced centuries ago has been refined by scientists over hundreds of years, yet still forms the basis of the system used today.
Now that you have a basic understanding of how living organisms are classified, you should understand what it means when humans are referred to as homo sapiens. That is simply the proper terminology for the genus and species of human beings. Further expand your scientific knowledge by exploring the key characteristics of living things.