Several different theories have emerged in recent decades about the exact relationship between each domain. Scientists are constantly debating and reorganizing the tree of life, based on new evidence. Relationships between the Three Domain Groups The other domains, Bacteria and Archaea, contain largely biochemical variability, as their physiological variability is limited by being unicellular. While it is easy to demonstrate within this domain because the differences are largely physiological, each domain has this much variability within it.
This may give a scale for how variable a domain can be. This means that the microscopic amoeba and the largest animal on the planet, the blue whale, are both in this domain. The eukaryotes include everything with organelles and a nucleus. As such, the organisms in the domain Eukarya have expanded to a large number of forms. This allows them to process glucose, a necessary sugar for life, much more efficiently. For instance, mitochondria are energy-transferring organelles which enable organisms in this domain to undergo aerobic respiration. In separating different tasks of life into these membrane-bound chambers, eukaryotes are able to increase their efficiency and host a different set of chemical reactions.
The organisms in this domain are eukaryotic, meaning they have a membrane bound nucleus and organelles. The domain Eukarya represents, well, everything else. Even the plague, which has killed off massive amounts of the human population throughout history, was caused by organisms in this domain. However there are several species, such as the bacteria which cause cholera or meningitis, which are highly detrimental to human health. We need many bacteria for the vitamins and nutrients they produce. For instance, many of the organisms living in your gut helping you digest you food come from this domain. While there are millions, if not billions of species of bacteria, there are a handful that are significant to humans. Each bacterium in the image below is roughly 1 micrometer long. As seen in the scanning electron micrograph below, these bacteria can be quite small. They contain their DNA in a small ring, and carry out all the functions of life within their single cytoplasmic space. These small, single-celled organisms live almost everywhere, and are the size of most eukaryotic organelles. The domain Bacteria is also possibly the largest domain, containing possibly millions of unknown and unrecorded specimens. Organisms in the domain Bacteria are also prokaryotes, having no distinguished organelles or membrane-bound nucleus. While organisms in the domain Archaea were often considered distantly related to bacteria because of their weird tendencies, other scientists have hypothesized that they are more closely related to the organisms in the domain Eukarya. This could be a highly salty environment, one of high or low temperature, or even one with chemicals that are toxic to other organisms. These organisms thrive in environments which other organisms find hostile. The organisms found in the domain Archaea are often extremophiles. Their cell walls, and the RNA they produce, are significantly different from that found in the domain Bacteria. While they still have bacteria-like DNA and no organelles, they also have several differences which put them into a completely different domain. The Archaea are a domain of bacteria-like organisms, but they have a distinct biochemistry which distinguishes them from bacteria. Each of these kingdoms is then broken down into smaller groups, all the way down to individual species. In the domain Eukarya, for instance, there are four kingdoms: Animalia, Plantae, Fungi, and Protista. A domain is then further broken up into kingdoms.