Domain (biology)

In biological taxonomy, a domain is the highest possible rank for all living organisms. It is the broadest way to categorize life on Earth. Scientists use domains to organize every living thing into massive groups. This system helps us understand the relationships between all life forms.

By using domains, researchers can see how different species are connected through history. This classification is essential for studying the entire tree of life.

To build this system, scientists look at the tiny molecular structures inside a cell. One of the most important tools is the study of ribosomal RNA. This is a specific type of RNA found in ribosomes, which are parts of the cell. Each domain has its own unique ribosomal RNA sequences. Scientists also examine the cell membrane, the outer layer of the cell. The biochemistry of these membranes differs greatly between the different domains. These molecular markers act like biological fingerprints that reveal how organisms are related.

The three-domain system divides life into Archaea, Bacteria, and Eukarya. The first two, Archaea and Bacteria, are classified as prokaryotes. Prokaryotes are single-celled microorganisms that lack a membrane-bound nucleus. This means they do not have a protective envelope around their genetic material. Eukarya is the third domain, consisting of eukaryotes. Eukaryotes are defined by having a cell nucleus and other membrane-bound organelles.

These organelles are specialized structures that perform specific jobs within the cell.

Classification history has changed as our tools have improved. In the mid-eighteenth century, Carl Linnaeus created a famous taxonomy system. Later, Charles Darwin provided more information to improve these groupings. However, early scientists struggled to classify bacteria because they had few observable features. In 1974, Royall T. Moore proposed using the term "dominion," but it was not widely used. A major breakthrough occurred in 1977 when Carl Woese compared nucleotide sequences of 16s ribosomal RNA. Along with Otto Kandler and Mark Wheelis, he introduced the three-domain system in 1990.

Each domain has very specific physical characteristics and sizes. Archaea are prokaryotic cells with unique membrane lipids. These lipids consist of branched hydrocarbon chains attached to glycerol by ether linkages. These chemical bonds help Archaea survive in extreme heat or high acidity. Some examples include halophiles, which love salt, and hyperthermophiles, which love heat. Archaea range from 0.1 μm to 15 μm in diameter. They can grow up to 200 μm in length. The genus Thermoplasma contains some of the smallest known Archaea.

Bacteria are also prokaryotes, but they are chemically distinct from Archaea. Bacteria use phospholipid bilayers for their cell membranes instead of ether linkages. They also possess different internal RNA structures in their ribosomes. Bacteria show an incredible amount of diversity. This diversity is complicated by the exchange of genes between different lineages. This gene exchange can make it hard to organize bacteria into a perfect tree. Instead, some scientists suggest their relationships look more like a network. Examples of bacteria include cyanobacteria and mycoplasmas.

The domain Eukarya is massive and includes four main kingdoms. These kingdoms are Plantae, Protista, Animalia, and Fungi. Eukaryotes are much more complex than the other two domains. Some scientific theories, like the eocyte hypothesis, suggest Eukarya evolved from Archaea. This hypothesis proposes that Eukarya is actually a branch within the Archaea domain. This would result in a two-domain system of Bacteria and Archaea.

It is important to note what is left out of this system. Most domain models do not include non-cellular life, such as viruses. Because viruses do not have cells, they do not fit the standard definition. In 2012, Stefan Luketa proposed a five-dominion system to include them. His system adds Virusobiota, which contains viruses, and Prionobiota, which contains prions. Prions are acellular and do not have nucleic acids. Other scientists have also suggested different ways to group life, such as the two-empire system. These debates show that our understanding of life is always evolving.