Biological organisation, or the hierarchy of life, is the hierarchy of complex biological structures and systems that define life using a reductionistic approach.[1] The traditional hierarchy, as detailed below, extends from atoms (or lower) to biospheres. The higher levels of this scheme are often referred to as ecological organisation.
Each level in the hierarchy represents an increase in organisational complexity, with each "object" being primarily composed of the previous level's basic unit.[2] The basic principle behind the organisation is the concept of emergence—the properties and functions found at a hierarchical level are not present and irrelevant at the lower levels.[3]
Organisation furthermore refers to the high degree of order of an organism (in comparison to general objects).[4] Ideally, individual organisms of the same species have the same arrangement of the same structures. For example, the typical human has a torso with two legs at the bottom and two arms on the sides and a head on top. It is extremely rare (and usually impossible, due to physiological and biomechanical factors) to find a human that has all of these structures but in a different arrangement.
The biological organisation of life is a fundamental premise for numerous areas of scientific research, particularly in the medical sciences.[3] Without this necessary degree of organisation, it would be much more difficult—and likely impossible—to apply the study of the effects of various physical and chemical phenomena to diseases and body function. For example, fields such as cognitive and behavioural neuroscience could not exist if the brain was not composed of specific types of cells, and the basic concepts of pharmacology could not exist if it was not known that a change at the cellular level can affect an entire organism. These applications extend into the ecological levels as well. For example, DDT's direct effect occurs at the subcellular level, but affects higher levels up to and including multiple ecosystems. Theoretically, a change in one atom change the entire biosphere.
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The simple standard biological organisation scheme, from the lowest level to the highest level, is as follows:[1]
More complex schemes incorporate many more levels. For example, a molecule can be viewed as a grouping of elements, and an atom can be further divided into subatomic particles (these levels are outside the scope of biological organisation). Each level can also be broken down into its own hierarchy, and specific types of these biological objects can have their own hierarchical scheme. For example, genomes can be further subdivided into a hierarchy of genes.[5]
Each level in the hierarchy can be described by its lower levels. For example, the organism may be described at any of its component levels, including the atomic, molecular, cellular, histological (tissue), organ and organ system levels. Furthermore, at every level of the hierarchy, new functions necessary for the control of life appear. These new roles are not functions that the lower level components are capable of; they are referred to emergent properties (see Philosophy below).
Every organism is organised, though not necessarily to the same degree.[6] An organism can not be organised at the histological (tissue) level if it is not composed of tissues in the first place.
This hierarchical scheme can be viewed as an implementation of the epistemological theory of reductionism, that a complex structure is nothing more than the sum of its parts.[1] This philosophy does allow for the observed emergent properties, characteristics not seen at the previous level. In this way, the organisation is often erroneously viewed through the theory of holism, that the complex structure can not be described by the sum of its parts. However, these emergent properties of life still fall within the realm of reductionism because, while a different function may emerge at each level of complexity, these functions can be directly explained by the processes by which the object is formed and the arrangements and interactions of the units. For example, the dissected brain is simply a network of billions of neurons that come together to produce phenomena such as cognition, sensation and regulation. The reductionist approach, and the purpose of this hierarchy, views these phenomena as a direct result of how these neurons are arranged and interact.
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