The “tree of life” describes the evolution of life and the evolutionary relationships between organisms. The root of the tree is the common ancestor to all life on Earth. All other species radiate from this point, much like the branches of a tree. The numerous tips of these branches on the tree of life represent every living, or extant, species. Extinct species, which are species that no longer exist, can be found towards the center of the tree. Currently, these organisms, both extant and extinct, are organized into three domains: bacteria, archaea, and eukaryotes. However, the distinctions between these domains and the classification of organisms is debated continuously, resulting in a ‘living’ tree of life that changes as more information is uncovered.

The History of the Tree of Life

The tree of life, itself, has evolved, much like the organisms it contains. Shortly after Charles Darwin published On the Origin of Species in 1859, Ernst Haeckel proposed three “kingdoms” of life: plants, animals, and protists - which were simply organisms that were neither plant nor animal. The distinctions between these kingdoms were based entirely on easily observable phenotypes.

In 1937Edouard Chatton divided the tree of life into two vast ‘empires’ instead: eukaryotes and prokaryotes. Simply put, the eukaryotes possessed membrane-bound nuclei, and the prokaryotes did not. During the 1970’s, Woese found that a group under the bacteria classification, called archaea, differed from the bacteria in several ways: an absence of peptidoglycan in their cell walls, differences in coenzymes, and differences in their 16S rRNA gene sequence. All of these differences suggested that archaea originated from a line of descent separate from bacteria.

The Three Domains of Life

The modern tree of life consists of three ‘branches’: bacteria, archaea, and eukaryotes. The domain bacteria consists of unicellular, prokaryotic organisms that lack a membrane-bound nucleus. Bacteria can take many shapes, from spherical cocci to rod-shaped bacilli. Members of this domain are found in almost every place on Earth, including on and in the human body.

Similar to bacteria, archaea are also unicellular, prokaryotic organisms. Phenotypically, archaea are very difficult to differentiate from bacteria. At the molecular level, archaea resemble eukaryotes in terms of their machinery for replication, transcription, and translation. However, archaea resemble bacteria in terms of metabolism and energy conversion. Archaea are often found in the most inhospitable of habitats: bogs, wastewater treatment plants, the deepest parts of the ocean, and hot acid springs.