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Table 1 Conceptualization of the aging process at the different hierarchical network levels

From: Network strategies to understand the aging process and help age-related drug design

Elements of the hierarchical network level Hallmarks of the aging process
Elementary particles of quantum systems Physical equations do not change in time (for example, Newton's laws have not changed in the past few centuries). However, the equations describing a system in a non-equilibrium state do change - this is called an aging process, which is a typical behavior of quantum particles embedded in a thermal bath, or of semi-ordered, glassy materials [7, 8]
Monomers of biological macromolecules (amino acids, nucleotides, and so on) Unrepaired replication, transcription and translation errors accumulate. Various forms of protein (and nucleic acid) damage become more and more prevalent (for example, in an 80-year-old human, half of all proteins are estimated to be oxidized): cross-links, occasional proteolytic cuts, amino acid truncations develop [6]
Proteins Due to energy loss and protein damage, protein-protein interactions may disappear or lose their affinity. Novel, unexpected, quasi-random protein interactions may also occur. Protein complex composition becomes 'wobbly', fuzzy. Proteins are dislocated and appear in unusual cellular compartments [9, 10]
Cells Intercellular interactions may become irreversibly tight (for example, by developing cross-links) or too loose, gradually loosing their high-affinity contacts. Since the development of intercellular contacts is costly, functional brain networks show loss of their small-world properties in age-related Alzheimer's disease [11]
Organisms The social network of aged individuals usually deteriorates and shrinks, keeping only the most important contacts for major remaining social functions. This contributes to age-related cognitive decline and to the loss of emotional support leading to increased frailty [12]. Ecosystems like forests also show the hallmarks of aging [13]
Social groups A network of social groups, such as a network of firms, may also display the signs of aging as has been shown in the declining network of the New York garment industry by Brian Uzzi and colleagues [14]
Ecosystems forming a global ecological network Aging research into the global ecosystem of Earth, Gaia, is in its infancy at the moment. However, our increasingly integrated knowledge, such as the global river network [15], gives us more and more tools to assess the rather worrisome aging of our habitat
Elements of human conceptual, cultural and technological systems Human conceptual networks (such as arguments over a complex issue; cross-references in textbooks, and so on), cultural networks (such as the network of actors in a Shakespeare drama, movie actor networks, and so on), or technological networks (such as electric power grids, computer programs, the internet, and so on) may also show typical signs of aging. As a trivial example, we all experience more frequent errors of our Windows program network when the system gets older