Scientists have worked tirelessly for centuries to accurately identify and classify living things using a taxonomy of similarities and differences. The task has been made easier by advances in technology such as the electron microscope. A shared taxonomy helps researchers collaborate and communicate their findings as they study life forms on Earth and in outer space.

The tree of life branches into three large domains that further divide into kingdoms. A kingdom is the one of the largest classification levels. The number of kingdoms has changed over the years as scientists learn more about the elusive mysteries of life at the cellular level.

Born in 1707, Carl Linnaeus will long be remembered for his work in classifying plants and animals. Inspired by Aristotle and other scholars, Linnaeaus was fascinated by similarities and differences between living things. After examining plants and animals, he assigned a Latin genus and species name to the organisms and cataloged them by type.

Systema Naturae is a timely classification manual written by Linnaeus, and it helped scientists of the day identify and categorize curious specimens gathered by explorers returning from voyages to the New World. The taxonomy of Linnaeus has been modified many times since the 1700s and will likely face continued revision as the result of ongoing research into the amazing biodiversity of life.

Taxonomy is any system of classification – such as the one used by natural scientists – to group similar living things. A taxonomy moves from broad categories to narrower ones.

Levels of classification include: domain, kingdom, phylum, class, order, family, genus, species. Family, genus and species names are italicized, and species names are lowercased.

For example:

• Domain: Eukarya
  
• Kingdom: Animalia
  
• Phylum: Chordata
  
• Class: Mammalia
  
• Order: Primates
  
• Family: Hominidae
  
• Genus: Homo
  
• Species: sapiens
  

Humans like to organize, group and classify to make sense of the world around them. At an early age, school children learn that fish, birds, bears and tigers are classified as animals because of shared characteristics like needing food to live and move about in their environment. By contrast, plants capture energy from the sun, produce their own food and remain stationary unless moved by an external force like wind or water.

Students also observe that animals come in all shapes, sizes and colors, but most plants are green due to photosynthetic pigments, particularly chlorophyll. Beyond obvious morphological differences, organisms reveal stark differences at the cellular level that help them adapt to even the most inhospitable environments.

New technologies and laboratory techniques have led to a far more nuanced system of taxonomy. One of the most important determinants of classification is whether the organism is single-celled or multicellular. From there, many other questions must be asked and answered to determine appropriate taxonomic placement.

To be classified under one of the six kingdoms of life, a specimen being analyzed must first meet all the criteria of a living organism. The six kingdoms’ characteristics of all living things include an ability to breathe, metabolize, grow, change, move, maintain homeostasis, respond to environmental triggers, reproduce and pass on traits. All conditions must be met.

For instance, a virus is actually considered non-living because it doesn’t require food and can’t replicate without a host.

New species are continually being discovered in all the kingdoms. Disagreements between scientists can occur over how a particular organism should be categorized when lines are blurry between two or more kingdoms, such as the Protista kingdom. New findings might lead to an expansion or modification of the current six-kingdom system of classification.

Animals are multicellular organisms embodying certain abilities and characteristics such as unaided mobility, growth, change, dependence on an outside food source and the capacity for reproduction of the species. Animals are heterotrophs that must eat other organisms to survive.

Animals possessing a backbone in their skeletal structure are classified as vertebrates. Animals without backbones are invertebrates. Animals are further divided into smaller subgroups that share a recent common ancestor.

Examples:

• Primates: monkeys, apes, lemurs
  
• Marsupials (animals with pouches): kangaroo, opossums, wombats
  
• Monotremes (mammals that lay eggs): spiny anteaters, duck-billed platypus
  
• Rodents: rats, mice, squirrels
  

Plants are complex, multicellular organisms. The plant kingdom contains thousands of remarkably diverse species adapted to their climate and environment. Plants are autotrophs, meaning they produce their own food and supply the rest of the food chain. Flowering plants, ferns and mosses may look very different, but they are all part of the plant kingdom.

Classification of organisms within the plant kingdom has changed substantially since the days of Linnaeus. Following the lead of Linnaeus, early botanists based classification on whether a plant had male organs (stamens) or female organs (pistils).

Plants seemingly lacking so-called sex organs were put in the Class Cryptogamia. Over time plant scientists developed more refined methods of identification and classification.

Most fungi are multicellular organisms, and all lack the photosynthetic pigment chlorophyll. Common examples of fungi include mushrooms, molds, yeasts and mildew. Fungi are different enough from plants to have their own separate kingdom. Most notably, fungi are heterotrophs that cannot produce their own food unlike plants that may live in the same environment.

Fungi are classified as decomposers that use enzymes to break down dead organisms. Digested nutrients can be absorbed as an energy source for the fungus.

Fungi fulfill a vital link in the food chain. If fungi went extinct, dead and decaying matter would blanket the Earth.

Like plants, animals, and fungi, the protists are eukaryotes. Protists are single-cell organisms that have a cell membrane, nucleus and organelles. They live in many environments, including fresh water, soil and the human body. Amoebas, paramecia, algae and slime molds are some of the more common organisms in the Protista kingdom.

Classification is not made on the basis of a protist's fuel source. In other words, protists can be autotrophs, heterotrophs or decomposers. In the human body, some protists are even parasitic and cause illness and disease. Some protists like the amoeba are able to change their shape.

Most bacteria known today are single-celled, complex organisms belonging to the Eubacteria kingdom. (Note that many sources still lump Eubacteria and Archaeobacteria into the Kingdom Monera.)

Bacteria may be helpful or harmful, depending on the type and environmental conditions. For example, Streptococci can cause strep throat, but not everyone who harbors the bacteria will get sick. Pathogenic bacteria can kill. Bacteria in the stomach and intestines play a key role in digestion.

The shape of a bacteria aids in classification within the vast Eubacteria kingdom. Coccus bacteria are oval, bacillus are rod-shaped and spirochetes are spiral. Other bacteria seen under an electronic microscope may be lobed, filamentous or star-shaped, for instance.

Archaebacteria are single-celled prokaryotes. These microbes live in many different environments, including the human body. The cells lack a nucleus, which may be a factor in how certain types of archaebacteria manage to survive in places where other life forms would instantly perish.

Note that the Archaebacteria kingdom should not be confused with the older Archaebacteria domain that was later renamed to Archaea.

Known as extremophiles, archaebacteria tolerate harsh environmental conditions. Archaebacteria have even been found in sewage, hot springs and volcanic vents. They can live in water that is highly acidic, oxygen depleted and extremely salty.