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Some general principles as to how living things work are studied in biochemistry and genetics. To find out more about specific organisms we investigate their anatomy, physiology, behavior? and ontogeny?. To find out about how organisms relate we investigate their ecology. At the microscopic level, we study their [cell biology]?.
One of the central concepts in biology is the principle of evolution. The descent of an organism is called its phylogeny?; these are studied using all sorts of evidence, including the study of ancient life, paleontology. Various methodologies have developed, including phylogenetics?, phenetics?, and cladistics.
The classification of living things is called taxonomy, and ideally should reflect the evolutionary trees of the different creatures. Historically, a system known as Linnaean taxonomy was used, where living things were divided into a hierarchy starting with five kingdoms:
This five kingdom system is now considered by many to be outdated and the data better reflected by a model starting with the three domains:
The distinction between life and non-life never being a hard and fast one, there are also a series of parasites that are progressively less alive: viruses, viroids, and prions.
See also http://phylogeny.arizona.edu/.