Biology is the study of living things. To understand the diversity of life, scientists classify organisms based on shared traits and ancestry. An introduction to biology includes understanding classification. Classification makes it easier to compare observations of living things, from the simplest single-celled organisms to complex systems containing trillions of cells. Methods of classification have evolved over time as scientists continue to collect information and use advances in technology to learn more about life on the cellular level. As a result of these discoveries, scientists now categorize living things into three large divisions: Eukarya, Bacteria and Archaea.

Renowned philosopher and scientist, Aristotle was regarded as the father of biology for centuries. The areas of biology he studied were animals and the natural world, which earned him another moniker, “father of zoology.” Based on his observations, he categorized animals into two large divisions: blooded and bloodless. These groups roughly aligned with vertebrates and invertebrates, and were subdivided into smaller groups similar to the classes and orders used today: mammals, birds, fish, insects, reptiles, crustaceans, etc. Because Aristotle’s classification system was limited to organisms he could view with his unaided eyes, he did not put microorganism into any groups.

Until the 1960s, there were only two large divisions of life, and all living things were classified as either plants or animals. In 1969, the two-kingdom system was updated to include additional types of biology and separated into five kingdoms. In addition to plants and animals, kingdoms were created for bacteria (Monera), fungi and protists, thanks to advances in microbiology. Kingdom Monera contained prokaryotes while the other four kingdoms contained eukaryotes. The main difference between eukaryotic cells and prokaryotic cells is the presence of a nucleus and organelles in eukaryotes, which prokaryotes lack. The five-kingdom system held until 1990, when a University of Illinois professor named Carl Woese proposed a major change to the classification system.

Woese conducted research on a newly-identified third form of life. These organisms, termed archaebacteria, are prokaryotic cells that are sufficiently different from bacteria to warrant their own classification. The discovery of archaebacteria resulted in the creation of a level of classification higher than kingdom: domain. The kingdoms of eukaryotic organisms – Animalia, Plantae, Monera, Fungi and Protista – now fall under Eukarya. Bacteria belong to their own, self-named domain. Archaebacteria share some characteristics with both eukaryotes and bacteria. They also have some unique traits all their own, which puts them in their own domain: Archaea.

Four kingdoms of life make up Domain Eukarya: animals, plants, fungi and protists. This domain encompasses single-celled organisms such as algae and protozoan; fungi such as molds, yeast and mushrooms; and more complex, multicellular organisms such as plants and animals. The cells of these organisms have a nucleus and distinct organelle structures encased in membranes.

This domain includes single-celled prokaryotic organisms that are distinct from Eukarya and Archaea. The cell walls of bacteria contain peptidoglycan, which is absent from the cell walls of archaebacteria and eukaryotes. Some bacteria can be helpful to humans and other types are harmful. Common bacteria include cyanobacteria, lactobacilli – beneficial gut bacteria – and pathogenic species that cause illness, such as streptococcus.

Some species of archaebacteria live in soil, water or other common locations. Other types of archaebacteria can live in the most inhospitable places on Earth. Organisms from this domain have been found living in high concentrations of salt, methane and other chemicals. Some organisms can survive extremely high temperatures. A feature unique to Archaea is the composition of their cell membranes, which allows them to withstand conditions too harsh for bacteria or eukaryotes.