The first person to create a system of rules to classify living organisms was a Greek scientist and philosopher named Aristotle. He pointed out that classification requires two steps. First, organisms have to be carefully described and second to identify and describe their functions. For example in an electronics store, one way to classify items is by function. But before store owners can put an item in an aisle, they need to identify and then describe the function of the item. Earlier, living organisms were just given names that described where they were found. The same organism might have twenty different names, which led to confusion. Hence, living organisms are classified mainly to avoid confusion, to make study of organisms easy and learn how various organisms are related to each other. Scientists classified living organisms into different kingdoms, phylum, class, etc and are based on different criteria. Earth is a home for millions of living species including microbe, insects, birds, reptiles, plants, animals and more. Based on certain specialized features, these living species have been classified into their respective categories. Their names also differ from place to place, even within a country. To avoid this kind of confusion, scientists have classified and named them according to their characters.
Classification
On the basis of their similarities and differences, living organisms are arranged into different groups and sub groups, which are termed classification of living organisms. This scientific process of classification is termed Biological classification. Science that deals with the classification and nomenclature of all living organisms are termed as Taxonomy. Carolus Linnaeus, a Swedish botanist, devised the present taxonomy system. Classification enables us to better comprehend variety. It aids in the identification of living entities and in comprehending their variety. It enables us to get knowledge about the many species of plants and animals, their characteristics, similarities, and differences. It allows us to comprehend the evolution of complex species from simpler organisms. Classification of a living organism involves a hierarchy of steps where every step indicates a particular category or rank. The level of hierarchy in the system of classifying organisms is:
Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species
Domains
1. Archaea
The Archaea domain includes all prokaryotic cells, they lack nuclear membrane; have distinct biochemistry; contain RNA markers from bacterial cells. The Archaea are considered as the oldest species of organisms on Earth. They can survive in extreme, harsh environments that differentiated them from other domains. The cell wall of archaea lacks peptidoglycan. Archae also contains ether leakage in their membranes. Archae has three phyla;
Crenarchaeota: They can survive at extremely high temperatures and extremely low temperatures.
Euryarchaeota: Some of them are known as extremely halophiles, which can prevent inhibit highly saline environments.
Konarchaeota: It includes all those species were found in a single hot spring, Obsidian Pool, present in Yellowstone National Park (USA).
2. Bacteria
These are also prokaryotic cells with bacterial rRNA and contain diacyl glycerol diester lipids in their membrane. They are also called eubacteria or “true bacteria”. Their cell membrane contains ester linkage between unbranched fatty acid chains and glycerol. Their cell wall made up of peptidoglycan. There are present 5 phyla of the bacterial domain;
Proteobacteria: The example of proteobacteria is E. coli, Salmonella typhus, Legionella, Heliobacter pylori (cause of many ulcers), Neisseria gonorrhea (cause of gonorrhea).
Cyanobacteria: The example of Cyanobacteria is Photosynthetic ‘blue-green’ bacteria which produces O2 gas.
Eubacteria: The example of Eubacteria is Clostridium (tetanus, botulism), Bacillus, Mycoplasma (walking pneumonia).
Chlamydias: The example of Chlamydias is Giardia, Chlamydia (STD), etc.
Spirochaetes: The example of Spirochaetes is Spiral bacteria that cause syphilis, Lyme disease.
3. Eukarya
These are eukaryotic cells with a membrane-bound nucleus. Their membranes contain their membranes between unbranched fatty acids chains and glycerol. They lack peptidoglycans on their cell wall. Eukarya inhibit the antibacterial antibiotics but sensitive to antibiotics that affect eukaryotic cells.
There are presently four Kingdoms of Eukarya;
Protista: These are slime molds, euglenoids, algae, and protozoans.
Fungi: These are sac fungi, club fungi, yeasts, and molds.
Plantae: These are mosses, ferns, conifers, and flowering plants.
Animalia: These are sponges, worms, insects, and vertebrates.
Kingdom
The basic taxonomy of living things includes six kingdoms. Each of the three domains 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 molds)
Protists (domain Eukarya) - microorganisms that don’t fall into one of the other kingdoms (algae and slime mold)
Phylum
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 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
Class
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)
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
Family
Each order has multiple families. A few examples of 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)
Genus
Each family in the taxonomy of living things can be identified by genus. For the family Hominidae, the genus divisions are as listed below. pan (chimpanzees)
- Gorilla
(gorillas)
- Pongo (orangutans)
- Homo (humans)
Species
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).