This diagram attempts to show the modern understanding of evolutionary relationships and groupings of organisms on Earth. Caveat emptor: The diagram was drafted by a geologist as a summary for a geology class. For a more authoritative treatment, see the University of Arizona's Tree of Life or other more authoritative biological sources. Two other diagrams, generated similarly and subject to the same caveats, deal with
prokaryotic and eukaryotic cells
relationships among chordates. An older version is also still available on the Web.
The lines on this diagram represent evolutionary lineages indicated by studies of RNA sequences. Those lines suggest that the first lifeforms on earth were members of the Archaea, which today are largely thermophiles inhabiting hot springs on land and in the sea. The lines suggest that later Archaea, the Bacteria, and the Eukarya evolved from those early Archaea, and that eukayotes are more closely related to, and descended from, Archaea rather than Bacteria. Curves show commonly accepted symbiotic pathways of cellular development.
This diagram is essentially a Venn diagram showing sets and subsets. For example, humans are chordates (organisms with a notochord, the predecessor of a backbone, sometime in their lives) among deuterostome ("second-mouthed") coelomate (gut-bearing) bilaterate (having bilateral symmetry) animals (multicellular ingesting heterotrophs) among the mitochondriates (things having mitochondria) in the domain Eukarya (the eukaryotes, organisms with cells with nuclei). Cynaobacteria, on the other hand, are oxygenic photosynthesizers in the domain Bacteria and so are prokaryotes.
Life is presently divided in three domains (Archaea, Bacteria, and Eukarya), replacing the five kingdoms of traditional twentieth-century biology. These three domains are distinguished technically by the chemistry and structure of their cell walls and cell membranes. Archaea and bacteria differ in that the Archaea commonly live in extreme high-temperature or high-salinity environments but are less common in "normal" environments because they are out-competed by bacteria. Bacteria, on the other, are widespread, and "there are more bacteria in a person's mouth than there are people in the world". Most human diseases attributed to "bacteria" in the broad sense are caused by the Bacteria, rather than by the Archaea.
Chloroplasts (photosynthesizing DNA-bearing organelles in higher plants) are shown among the Eubacteria, where they are genetically similar to the cyanobacteria. This has led to the conclusion that chloroplasts developed as eubacteria that were symbiotically integrated into plant cells. Mitochondria simlarly are DNA-bearing organelles in most eukaryotes. They are thought to have developed as a strain of purple bacteria (among the proteobacteria) that were symbiotically integrated into early (but not the earliest) eukaryotes. Thus, although eukaryotes are thought to be descended from archaea rather than bacteria, some bacteria have become essential symbionts in eukaryotic cells, as suggested by the curving paths.
Viruses are shown on this diagram but are not included in or with any of the other kinds of life. That is because viruses are only sheathed molecules of DNA or RNA and are not cellular. They are active and reproduce only when they have invaded the cell or cells of another organism. In fact, some biologists don't include viruses among forms of life. They nonetheless wreak havoc in lifeforms by causing things like colds, HIV, and Ebola.
Prions are another not-really-life form shown. Prions are protein strings that likewise invade cells of organisms and cause proteins in those cells to conform to the structure or folding-pattern of the prions. The result is disruption of biological activity recognized as diseases like scrapie in sheep, bovine spongiform encephalopathy (BSE or Mad Cow Disease) in cattle, and Kuru and Creutzfeld-Jakob disease (CJD) in humans.
The placement of the microsporidia on this diagram represents recent advances indicating that they evolved from the fungi and have lost mitochondria, rather than having evolved from the eukaryotic line prior to the inclusion of mitochondria (see Keeling, P.J., 2001, Nature, v. 414, no. 6862, p. 401). They thus present the paradox of a eukaryote with relatively late evolutionary origins but no mitochondria.
As discussed above, the branching lines on the diagram suggest that later Archaea, the Bacteria, and the Eukarya evolved from early Archaea. However, because plasmids allow lateral transfer of DNA among prokaryotes, single lines in the prokaryote field may be inappropriate. That means that the diagrammatic tree of life may not be an oak with branches converging downward onto a single trunk, but a more convoluted "tree" in which the main stem is an intergrown network, rather than one trunk.
Sources of information used in compiling this diagram include
This diagram was constructed to provide Railsback's GEOL 1122 students with context for material covered in lecture. Railsback's GEOL 1122 students are not expected to reproduce this diagram on exams.
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