Organisms are classified into three domains: Bacteria, Archaea, and Eukaryota. A fourth domain, Acytota, represents viruses and pseudo-organisms, and a fifth, Nanobiota, represents small filamentary structures. The most important division is between prokaryotic and eukaryotic organisms, followed by unicellular and multicellular, and plants and animals. Aerobic and anaerobic organisms are also important. The discovery of life on another planet would represent a fundamental divide. The earliest common ancestor is called LUCA.
Organisms are generally classified according to the three-domain system, introduced by Carl Woese in 1990. It replaces the older five-kingdom system of classification of organisms. In Woese’s classification, the three domains of life are: Bacteria, Archaea, and Eukaryota.
Also under consideration is the addition of a fourth domain, Acytota, which means ‘cellless’, to represent viruses and other cellless pseudo-organisms such as prions. Even more speculative is a fifth domain organism, Nanobiota, which can represent extremely small and lifelike filamentary structures found in some minerals. More recently, in 2002, some scientists have considered Archaea and Eukaryota to be part of the same domain, Neomura.
The most important division in life is generally considered to be between prokaryotic organisms (cells with nuclei) and eukaryotic organisms (with nuclei). A slightly less significant, but still very important distinction is that between unicellular and multicellular organisms, and between plants and animals.
If viruses were eventually recognized as a viable life form, then the most important division would be whether or not a life form is cell-based. The possible admission of viruses into the realm of life began with the discovery of Mimivirus, a very large DNA virus with genes encoding nucleotide and amino acid synthesis, something that even certain bacteria lack.
Another important category difference is that between aerobic (oxygen breathing) and anaerobic (do not need oxygen) organisms. Once upon a time when the Earth’s atmosphere was mostly carbon dioxide, the Earth was covered in anaerobic organisms, but many of these died out during the Oxygen Catastrophe, a long period during which aerobic organisms rose to prominence, releasing large quantities of oxygen toxic to anaerobes. organisms. Now, anaerobic organisms are most frequently observed in compost piles.
If life were ever discovered on another planet, it would represent an even more fundamental divide in life. The term used to refer to organisms on Earth collectively is Gaeabiota. Sometimes the earliest earthly common ancestor is called LUCA, which stands for “universal last common ancestor.” LUCA is thought to have lived between 3.6 and 4.1 billion years ago
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